United States        Office of         EPA 520/6-85-011
            Environmental Protection    Radiation Programs      April 1985
            Agency          Washington, D.C. 20460

            Radiation
 o-EPA      An Engineering Assessment of
            the Potential Impact of
            Federal Radiation Protection
            Guidance on the AM, FM,
            and TV Broadcast Services
\

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                                  DISCLAIMER
     This report has  been  reviewed  by the  Office  of Radiation Programs,  U.S.
Environmental  Protection  Agency  and  approved  for  publication.   Mention  of
trade, names  or  commercial  products  does  not  constitute   endorsement  or
recommendations for use.

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An Engineering Assessment of the Potential Impact of Federal Radiation
     Protection Guidance on the AM, FM, and TV Broadcast Services
                            Paul C. Gailey

                                  and

                            Richard A.  Tell
                              April 1985
                 U.S.  Environmental  Protection Agency
                     Office of Radiation Programs
                     Nonionizing Radiation Branch
                            P.O. Box 18416
                       Las  Vegas, Nevada  89114
                                      u s  £nv»«mmentH Flection
                                              .
                                              Jackson Boulevard,
                                              IL 60604-3590

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                                   ABSTRACT
     This report describes  an  engineering analysis of the potential  impact of
proposed  EPA   Federal   Radiation   Protection   guidance  for   radiofrequency
radiation on  the broadcast  industry.   The study  was performed  by  developing
computer models  of  the radiofrequency  radiation  on the ground  near  broadcast
stations and  applying  the models to data bases of  the  stations.  The  models
were developed  using  theoretical  predictions,  empirical  data and an  existing
numerical electromagnetic  code,  and compared with  field study data  and  other
prediction techniques  to  Determine  their  accuracy.   Variations  of the  models
incorporating possible mitigation strategies  were  applied in conjunction  with
the  original  models  so  that  the  number of  effective  fixes   could  also  be
studied.   Descriptions  of  the  models  and  the  results  of  the  study  are
presented.

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                               ACKNOWLEDGMENTS
     We would  like  to  thank Michael Molony  for his assistance  in  programming
and organizing the data bases.  We  are also grateful to  Graciela Martucci  and
Lynne  Keeton  for   their  help  in   manually  augmenting  the  data  bases.
R. W. Adler and Edwin Mantiply provided many  helpful  suggestions  ana  editorial
comments.
                                      iv

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                                   CONTENTS
Abstract	iii
Acknowledgments 	  iv
Contents  	   v
List of Figures	vi
List of Tables	ix
1.  Introduction  	   1
2.  Data Bases	   2
3.  Guidance Levels 	   3
4.  Impact on FM Stations	   7
     Mitigation Strategies  	  14
     Operation of the FM Propagation Model	22
     Multiple Sites 	  30
     Building Mounted Towers  	  33
     Model Verification	36
     FM Modeling Results  	  37
5.  Impact on AM Stations	37
6.  Impact on TV Stations	65
References	77
Appendix A	79
  Section 1.  Pattern Measurements of FM Antenna Elements 	  79
  Section 2.  Pattern Reduction for Incorporation in the Model  	  86
  Section 3.  Arrays and Pattern Multiplication 	  99
  Section 4.  Array Nearfield Effects 	 104
  Section 5.  Mutual Coupling Effects 	 113
  Section 6.  Effect of Ground Reflections  	 116
Appendix B.  FM Model Verification  	 125
Appendix C.  Minimum Tower Heights for FM's 	 133
Appendix D.  Predicted Field Strengths for AM Stations  	 146
Appendix E.  Required Fencing Distances for Impacted FM Stations  	 156
Appendix F.  Preliminary Survey Results 	 167

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                                LIST  OF  FIGURES

Number                                                                     Page
   1.  Limiting values of field strength for guidance level 6	4
   2.  Hypothetical guidance shape to show application of impact results	6
   3.  Distribution of numbers of elements in FM antennas	8
   4.  Distribution of tower heights for single ground-based FM stations	9
   5.  Distribution of total ERP's for FM stations	11
   6.  Determination of compliance costs for FM stations	16
   7.  Antenna gain as a function of number of elements	18
   8.  Antenna gain as a function of number of elements	19
   9.  Antenna gain as a function of number of elements	20
  10.  Elevation angle to a field calculation point	22
  11.  Relative field strength pattern of a single element	23
  12.  Relative field strength array pattern for a 6 bay array	24
  13.  Power density near the ground for a 6 bay FM array	26
  14.  Total pattern of a 6 bay array	29
  15.  Effect of distance on radiation intensity	30
  16.  Total array pattern multiplied by the distance factor	31
  17.  Summation of power densities from two stations on the same tower	32
  18.  Distribution of building heights supporting FM towers	35
  19.  Distribution of physical electrical heights for AM stations	58
  20.  Distribution of tower heights for TV stations	67
  21.  NEC modeling results for a typical 6 bay TV antenna	71
  22.  NEC modeling results for a typical 6 bay TV antenna	72
  23.  The main beam of an FM broadcast antenna	80
  24.  Support configuration used to measure element patterns	81
  25.  Side view of a single element mounted on a tower	82
  26.  Top view of a single element mounted on a tower	83
  27.  Measured elevation pattern of a single element	85
  28.  Effect of element pattern on field strength	86
  29.  Effect of element pattern on field strength	87
  30.  Envelope of several element patterns	88

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31.   Envelope for a single direction away from the tower	89
32.   Final envelope for one polarization of a single element	90
33.   Elevation patterns for type 1 and type 2 elements	96
34.   Elevation patterns for type 3 and type 4 elements	97
35.   Elevation patterns for type 5 elements	98
36.   Array patterns for 2,4,6, and 12 bays	100
37.   Total patterns for type 1 and type 2 elements	101
38.   Total patterns for type 3 and type 4 elements	102
39.   Total patterns for type 5 elements	103
40.   Illustration of array near-field	104
41.   Calculation of the field produced by a two-bay array	105
42.   Comparison of far-field and array near-field calculations	109
43.   Comparison of far-field and array near-field calculations	110
44.   Comparison of far-field and array near-field calculations	Ill
45.   Construction of an array envelope model	112
46.   Geometry of direct and reflected rays	116
47.   Illustration of a vertically polarized signal	119
48.   Magnitude of the reflection coefficient for horizontally	
        polarized signals	121
49.   Phase shifts of reflected horizontally polarized signals	122
50.   Magnitude of the reflection coefficient for vertically	
        polarized signals	123
51.   Phase shifts of reflected vertically polarized signals	124
52-57 Calculated and measured power densities near FM stations	127-132
58.-  Minimum tower heights necessary to prevent creation of	
 60.    100^/cm2	134-136
61.-  Minimum tower heights necessary to prevent creation of	
 63.    200wW/cm2	137-139
64.-  Minimum tower heights necessary to prevent creation of	
 66.    500pW/cm2	140-142
67.-  Minimum tower heights necessary to prevent creation of	
 69.    1000wW/cm2	143-145
70.   Electric field strengths for 50 kw,  0.3 wavelength AM towers	147

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71.   Magnetic field strengths for 50 kW, 0.3 wavelength AM towers	148
72.   Electric and magnetic field strength for a 50 kW, 0.3	
        wavelength tower	149
73.   Electric and magnetic field strength for a 50 kW, 0.5	
        wavelength tower	150
74.   Wave impedance for several different electrical heights at 1 MHz	152
75.   Electric field strength for several different electrical heights	153
76.-  Percentages of SFMG exceeding guidance levels to specified	
 80.    di stances	157-161
81.-  Percentages of MFMG exceeding guidance levels to specified	
 85.    distances	162-166
86.   Distribution of distances from FM towers to furthest fence	172
87.   Distribution of distances from FM towers to property boundary	174
                                     viii

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                                LIST OF TABLES

Number                                                                     Page
   1.  Limiting values of the 18 guidance levels for AM,  FM,  and TV	
         frequencies	   5
   2.  Distribution of element types in the EPA FM data base	  14
   3.-  Interbay spacings to reduce downward radiation in  the  array pattern..  17
   4.  Numbers of bays used in one-half wavelength model	  21
   5.  Number of FM radio stations (from a sample of 878) having no CED's...
         within 0.5 to 5.0 km	  27
6-23.  Modeling results for single ground-mounted FM stations	38-46
24-41. Modeling results for multiple ground mounted stations	47-55
  42.  Summary of numbers of FM radio stations exceeding  power	
         density levels	  56
  43.  Summary of model results to evaluate different mitigation strategies.
         for FM Radio Stations	  57
  44.  Distribution of transmitter powers for  stations in the AM data  base..  59
  45.  Numbers of AM stations requiring fences at various distances to 	
         exclude areas in which field strengths exceed 18 possible guidance.
         levels.  Double entries in each row show whether the required	
         fencing distance is within or beyond  the extent  of the  ground	
         radials (estimated to be one-quarter  wavelength  long)	  64
  46.  Numbers of AM stations requiring fences at various distances to	
         exclude areas in which field strengths exceed 18 possible	
         guidance levels	  66
  47.  Numbers of TV stations predicted to be  impacted at 18  possible	
         guidance levels	  76
  48.  Data  points  for type 1 element model	  91
  49.  Data  points  for type 2 element model	  92
  50.  Data  points  for type 3 element model	  93
  51.  Data  points  for type 4 element model	  94
  52.  Data  points  for type 5 element model	  95
  53.  Distances (in meters) at which fields from a 1  MHz 0.2 electrical....
         height AM  station will  fall  below eighteen alternative	
         guidance levels	154

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54.  Distances (in meters) at which fields from AM stations will fall	
       below eighteen alternative guidance levels.  This table applies	
       to any frequency or electrical height	155
55.  Preliminary results for survey question 2	171
56.  Time frame for anticipated antenna replacement	175

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    An Engineering Assessment of the Potential Impact of Federal Radiation
         Protection Guidance on the AM, FM, and TV Broadcast Services
1.  Introduction

     This  report  describes  an  engineering  analysis  of  potential   impact  of
proposed   EPA   Federal  Radiation   Protection  Guidance   for   radiofrequency
radiation  on  the broadcast  industry.   The  task  of  assigning  costs  to  this
impact  has  been undertaken  by Lawrence  Livermore National  Laboratory (LLNL)
under an  interagency  agreement  with  EPA through the  Department  of  Energy.   It
was decided  at the  beginning  of  this  study  that  EPA was  best prepared  to
perform the engineering  analysis because of its knowledge  and  experience with
broadcast  radiating   systems.    EPA   has   examined  these  systems  through
measurements,   theoretical  predictions,  and  computer  modeling   for  over  ten
years.

     EPA's objective  in  this study was  to  develop the  most  accurate estimate
of  impact to  industry  practical  with  available  information.   A  completely
individualized  examination  of  each  broadcast  source was  not   possible  since
there are  currently  more that 10,000 such  sources in operation  in  the United
States.

     Limited information  about each source  is available in  computerized  data
bases maintained by the  Federal  Communications Commission  (FCC).  EPA obtained
these  data  bases  and  augmented   them  by  manually   extracting  additional
information from  the  FCC written  files  in  Washington, D.C.   Computer models
were  developed  which   combined  theoretical   methods   and  measured  antenna
patterns  to  accurately predict  the  fields  produced  near  broadcast  antennas.
The models were field tested  for  accuracy and  then  applied to  the  augmented
data bases.  The  results indicate,  for  eighteen hypothetical guidance  levels,
the numbers  of  stations predicted  to  exceed  the   guidance  as  well  as  the
numbers that  could  be  brought into compliance using various  "fixes".   These
numbers were provided to LLNL  for  determination  of  the total   societal  costs
and costs to  industry that  would  be  associated  with  implementation  of  the
proposed guidance [Ij.

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2.  Data Bases

     The data  on  each station  used  in this study  were  taken from  FCC  files.
The FCC maintains  records  of each station  on  magnetic tape which  is  provided
in updated form to EPA every six months.  These  computer  files  are  referred to
as the  AM,  FM, and TV  Engineering Data Bases.   The  tape records  contain  all
the required  information on  AM stations for EPA's  AM  model,  but only  part of
the  information  necessary  for the  FM  and TV  models.   Consequently,  manual
augmentation of these files was necessary.

     Because EPA's measurement  experience  indicated that the FM  radio  service
tends to contribute most to publicly accessible  high  intensity  exposures,  the
greatest effort  was  expended  treating  near-in  (close proximity)  propagation
models of FM radio stations.   The  FCC  FM automated records do not  contain  the
tower height  above ground,  type of antenna, or  number of bays in  the  antenna
used  to  transmit  the   signal.    These  parameters  are  critical  for  proper
modeling of  each  facility.  A  graduate  student  in  the  Washington, D.C.  area
was hired to  manually extract  this information  from  the  FCC files  during  the
summer  of  1980.   These  data were  later combined  with  the existing  magnetic
tape  records  to produce  an adequate  data  base for  FM  stations.   The  final
version of the data base contained a combination of 1980  and 1982 data.

     Although  there were  4,374 FM  stations in operation at  the  time  of  this
study,  the   student  was   only  able  to   extract  the   additional  required
information on 3,895  of these  facilities during  his appointment.   All  modeling
was performed  on  these  3,895  stations  with the  assumption  that  the  results
represented (3,895/4,374) X 100 per cent of the total  impact on FM stations.

     A  less detailed  propagation model was  used  for predicting  fields  produced
by TV  stations and  therefore  less  information  on each facility  was required.
The magnetic tape records from  FCC  contained all the  necessary  information  for
modeling  except  tower   height above  ground  and  aural  ERP.   This  missing
information  was  manually  extracted  from   the   1982-1983  TV Factbook  [2],  a
commercial publication containing  certain  information  about TV  stations  taken
from the FCC files.   The Factbook  information  was  merged  with  the January 1983
FCC automated  TV Engineering Data  Base  to  produce the final data base  used in

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modeling TV  stations.   The  automated FCC AM file  used  in this  study  was also
the January 1983 version.

3.  Guidance Levels

     Since the  final  values  at which the Guidance  will  be set  were  not known
at  the  time  of this  study,  all  analyses  were performed for  18  alternative
guidance levels.   This  approach  has the advantage  of  revealing  the variations
in impact as a function of guidance level.

     The  18  guidance  levels  each  differ  for AM  and  FM frequencies.   This
frequency  dependence   reflects   the   general  shape   assumed  by   existing
radiofrequency  standards in  the  United  States  and  other  countries and provides
an  approximation  to  the   shape  which will  probably  be  proposed  by  EPA.
Figure 1  shows  one  possible shape  and set of  limiting  values for  guidance
level 6.   Note  that the curve is flat from 30  MHz to  1  GHz.   Many  existing
standards begin an  upward ramp  at about 300 MHz.   EPA's  proposed guidance may
also incorporate a ramp,  but the exact  shape  was  not established  before this
study.    The  shape  which  was  chosen  for this study,  as  shown  in Figure  1,
represents the  most  conservative  approach which  might  be  chosen  by EPA.   If a
portion of the  flat  region  which extends from  30 MHz to 1 GHz  were changed  to
a  ramp  shape,  the  resulting  impact  of  the guidance on  UHF stations  woula  be
reduced  from the  values  predicted  in  this analysis.   The limiting  exposure
values  assigned to  the 18  alternative guidance  levels  for  AM,  FM,  and  TV
frequencies are shown in Table 1.

     The results of this impact  analysis can be  used even  if  a different shape
is  proposed.    Figure  2  shows  another possible  shape   and   set of  limiting
exposure values for the guidance.   The total  impact  for  this  case  could  be
found by combining the guidance  level 6 (see Table  1)  impact for FM and VHF-TV
stations, the guidance  level  9  impact  for  AM  stations,  and the  guidance level
6 or 7  impact  for  UHF-TV stations.  The UHF-TV  band extends  from  470-806 MHz
                                                            2
which would  correspond  to  guidance levels  of  157-269  wW/cm   for the  guidance
                                                                      2
curve  shown  in   Figure  2.   Thus,  guidance  level  6  (100  pW/cm )  would
                                                            2
overestimate  impact  while  guidance  level  7  (200  yW/cm )   would  probably
estimate the actual  impact more  accurately.  The range of alternative guidance

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FIGURE 1.    Limiting  values of  field stength for guidance  level 6

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TABLE 1.  LIMITING VALUES OF THE 18 GUIDANCE LEVELS FOR AM, FM,
                      AND TV FREQUENCIES
Guidance
Level No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Ib
17
18
Limiting Field Strength
at AM Frequencies
10.0 V/m
31.6
44.7
70.8
86.6
100.0
141.3
173.2
200.0
223.9
244.9
264.6
281.8
300.0
316.2
446.7
708.0
1,000.0
Limiting Power Densities
at FM and TV Frequencies
1
10
20
50
75
100
200
300
400
500
600
700
800
900
1,000
2,000
5,000
10,000
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                                                  Frequency
            FIGURE 2.     Hypothetical  guidance  shape to show application of  impact results

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levels examined in this report  should  allow combinations which may  be  used to
determine  the  impact  for  any  variations  with  frequency  in  the  limiting
exposure values which are finally proposed.

4.  Impact on FM Stations

     The  impact  of   proposed   EPA  Federal  guidance  on  the  FM  service  was
determined by  application  of a  computer  propagation model to  most  of  the FM
stations  in   the  U.S.   The  computer  model  was  developed  by  EPA  using  a
combination of  theoretical approximations  and  measured  data.   The  large number
of FM stations  precluded  the possibility of either  onsite  measurement  or very
detailed theoretical  predictions for each  source,  so the  model  was  designed to
estimate  the  maximum,  practically  expected  field  strengths  in  order  to
compensate for  the variety of conditions that may exist  near an FM broadcast
antenna.  This  means  that the  model  may  over-estimate  the field strength in
particular locations  and  thus  represents  a conservative  approach   to  dealing
with potential impact.

     Typical   FM broadcast antennas  consist of  one  to  sixteen elements  (see
Figure  3)  in  a vertically stacked  broadside array.  The  elements  are  fed in
phase and are  spaced  approximately  one wavelength apart.   Individual  elements
vary in shape  and radiation  pattern according to  model and manufacturer.   The
ideal is  an  antenna  that  is omnidirectional  in  the azimuth plane  (towara the
horizon)  and  has  a  cosine or  cosine  squared  pattern  in  any  elevation  plane.
Elements  are  usually  side  mounted on  a metallic tower  but  may also  be  center
mounted on top of a  tower.   Figure 4  shows  the distibution  of tower  heights
for ground mounted FM towers in the  EPA data base.

     The energy in the  antenna's  main  beam is specified  in terms of effective
radiated  power (ERP).   This  value  is  the  amount  of  power  which  must  be
radiated  from a  single  dipole  antenna  in  order  to  produce  field  strengths
equivalent to  those produced by the station at  the  same  distance in  the  main
beam.   ERP's  for  FM  stations  generally range from a  fraction  of   a  kilowatt
(kW) up to 100 kW.   A station  licensed for  100  kW  of ERP  will  generally have
100 kW  of horizontally polarized  signal   and  100  kW  of vertically  polarized
signal as permitted  by the FCC [3].

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00
          % of Total
       25
       20
       15
       10
            1     2     3    4     5    6    7     8    9    10   11    12    13   14  >14
                                            Number of Bays
          Figure 3.  Distribution of numbers of elements in antennas for stations in the FM data base.

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   % of  Total
20


18


16


14


12


10


 8


 6


 4
                                          I  Irir-innrnr-inr-il  I
    10-  50-  100- 150- 200- 250- 300- 350- 400- 450- 500- 550- 600- 650- 700- 750- 800- 850- 900- >950
    50   100  150 200  250  300  350  400  450 500 550 $00  650  700  750  BOO 850 SOO  950
                                Tower Height in feet
       Figure 4.   Distribution of tower  heights for single ground-based
                       FM  stations  in  the FM data base.

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     There  is  some  confusion over this point  because  the expression "circular
polarization" is used  in  the FCC  regulations  [3] regarding this subject.  True
circular polarization  is  best  described  as  a  horizontally polarized signal and
a  vertically  polarized  signal  of  equal  magnitude,   traveling  in the  same
direction but  90°  out  of phase.   In  such  a  case,  the  electric  field vector
will rotate once each cycle  and the point of this  vector  will  draw a circle in
a  plane  perpendicular  to the  direction  of  transmission.   Both  a  90°  phase
shift and a ratio  of one between  the  horizontal and  vertical  field strengths
are  necessary  for  true circular  polarization.   The  FCC regulations  on  this
subject specify  only that an equal amount  of  ERP of  vertical  polarization is
permitted as has been  licensed  for horizontal  polarization.   There is no phase
shift requirement.   Consequently,  most  FM  broadcast  antennas  do  not  radiate
true circularly  polarized signals, but  simply attempt to  achieve  a  ratio of
horizontal  to  vertical  field  strength of close to  one.   Although  the stated
ERP of a station may be 100  kW, any calculation  of  power  density at a distance
from the  station must consider both the  vertically  and horizontally polarized
signals.   A station's  "Total  ERP," the sum of the  horizontally and vertically
polarized ERP's is sometimes referred to in this report (see Figure 5).

     In  order  to  determine  some of  the  problems  involved   in  modeling  FM
antennas, broadside  arrays  of  half-wave  dipole  elements  were  studied.   These
arrays provide the closest  approximation  to actual  FM  antennas  while remaining
theoretically  tenable.   Predictions of  fields  on  the  ground   resulting  from
such  arrays   involves   coupling  equations   as   described   in   Kraus   |_4J,
non-parallel  ray  geometry,  vector  addition,  and  consideration  of  ground
reflections.

     Coupling  between  broadside  half-wave dipoles depends  on  the  distance
between elements and affects the  impedance of  the  elements  involved.  For  a
given transmitted power,  changes  in  impedance will  affect  the  current flowing
in each element  and  consequently  the  field produced by the  element.  Coupling
effects were found to  be  small  at one wavelength spacing between  elements but
very pronounced  at  half-wavelength spacing.  Since most  FM  broadcast antennas
use  approximately  one  wavelength  inter-bay spacing,  coupling  effects  can  be
ignored in the design of an approximate propagation model  (see Appendix A).
                                      10

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   5S of  Total
40
30
25
20
15
10
      o 0.1    0.1-1
1-3     3-10     10-20    20-50    50-100   100-200     >200
              ERP (kW)
        Figure 5.  Distribution of total ERP's (horizontal  and vertical)
                        for stations  in  the  FM data base.
                                        11

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     Proper  addition  of the component  fields  from each  element in  the  array
requires  knowledge  of  both  the  phase  and  magnitude  of  each  signal.   Simple
equations  have  been  derived  for  this  addition  at  distances far  from  the
antenna.   These  equations  require  that   the   rays   from  each   element  be
practically  parallel   at  the  measurement  point  as   is   the case  at  far
distances.   For  short distances,  however,  the  rays will  not be parallel  and
the  equations  do  not  accurately  predict  the  fields.   A  model  designed  to
predict  fields on  the  ground  near  an  FM  broadcast   antenna  must  therefore
consider non-parallel ray geometry,  especially  if the  antenna  is mounted on a
short tower.  The area  in which this effect  is important  can be referred  to as
the  array near-field  and  differs  from  the element  near-field which  extends
only a few feet from the antenna elements (in the case  of FM antennas).

     Examination  of  the fields  calculated  using  parallel  (far-field)  and
non-parallel  (array  near-field)   geometries  reveals   that  array  near-field
antenna  gains   are  generally  less  than  or equal  to  far-field   gains.   An
exception to this  rule  is that near-field  patterns often  do not  exhibit  the
same nulls  (or  have shallower  nulls) as the corresponding  far-field  patterns.
The position of the nulls may also shift.

     An  implicit  assumption  in the concept of  an  environmental guideline  is
that the restricted parameter can not exceed the  guideline  anywhere  within the
region of interest.   In other words, it  is  not the  typical  field level  that is
of concern,  but  the highest level  reached.  Thus  for modeling purposes,  the
conservative approach of  using  an envelope of the  far-field radiation  pattern
(all nulls  100  per  cent filled)  was  chosen.   This technique also  compensates
for deliberate null-fill by  some  stations.   The details of  this technique are
described in Appendix A.

     A single normal reflection from  a  perfectly  conducting plane  surface will
double  the   electric  field  strength at certain  locations  in  space.   While
electric field  strength  (E) and power density (S) are  not  easily related  under
                                                     p
these  conditions,   a  free  space  conversion  (S  =  E  1377)  can  be  used  for
modeling purposes since the  guidance is stated in  terms  of the maximum  E,  H,
or S at  FM  frequencies.  Thus the  reflection  described above  could  quadruple
the free space equivalent power density  at  a given  location.  Larger increases
                                      12

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in field are possible  if multiple  reflections  are  considered.   Under realistic
conditions, however, the ground  beneath an FM  broadcast  antenna has  a  finite
conductivity and dielectric constant.  Equations such  as  those  found in  Jordan
and Balmain [5] can be  used  to calculate the phase  and reflection  coefficient
for waves  reflected from  finite  conductivity  ground.   Examination of  these
equations  over the  typical  ground  conductivities  and  dielectric  constants
found in the United  States and over  the frequency  range  of FM  stations  shows
that the  magnitude of  the voltage reflection  coefficient  averages  less  than
0.6 under  the  tower.    In  general,  the resultant field will  be less  than  1.6
times the  incident field  since the  magnitude of  the reflection  coefficient
varies  with angle of  incidence,  polarization,  and  the  ground  constants.
However, 1.6 was  chosen as  a constant  multiplying  factor to  be  used in  the
model to cover the variable  height above ground of  the measurement  point  (the
guidance  may  limit  fields  at  any  height  above  ground  that   are  easily
accessible), the unknown angles of  nearby terrain,  and the  possibility of  more
reflective materials in the vicinity.  This multiplying factor  is  not valid at
far distances,  but  the primary area  of  concern for  this  analysis  is  within  a
few hundred feet of the tower (see Appendix A).

     FM  antenna  manufacturers  do  not  typically  provide  measured  elevation
patterns for their elements.  The  data they do  provide gives  information  about
the  main  beam  characteristics  of  their  antennas  and  is  not  useful   in
predicting the  fields   on  the ground near  the tower.  In  order to  determine
this information, EPA  obtained via  a  contract  [6]  measured  elevation radiation
patterns of  five  commonly  used FM broadcast  elements.  Elevation patterns  of
each element were measured  at four different azimuth angles with  the elements
mounted  on  a  dielectric  support   and  then  repeated with  the  elements  leg
mounted and  face  mounted on  a  metallic  tower  section.   The  final  report  for
EPA contract number 68-03-3054  [6] contains the results of these  measurements
along with an  explanation  of the measurement technique.  The twelve elevation
patterns  were  overlaid and  an  envelope  drawn  around  the  extremes  of  the
patterns   to   produce   a   single   worst-case  elevation   pattern   for   each
polarization of each element.   This worst-case envelope was used  to represent
the element  in the propagation  model.  This  approach helps  insure that  the
model will not underestimate  the  fields in any direction  away  from  the  tower
or for  any common antenna  mounting method.   The  resulting  envelope was  then
                                      13

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digitized at  five  degree intervals for  use in the  model  (see Appendix A  for
more details).

     Stations  in  EPA's  FM  data base  were  examined to  determine  how  many
stations actually  used  the  five element types characterized  for this  study.
The results  are  shown in Table  2 which indicates  that  the measured  elements
represent approximately 46 percent of the elements in use at the  time  the  data
base  was  assembled.   Another 25  percent   were  of  the  ring-stub  or  cycloid
design.  While elevation  patterns  for this type  of  antenna were not  measured
under  the  contract,  limited  measurement  data obtained  from one manufacturer
indicates that it has an  element pattern similar  to element type 1, which  was
measured under  the  contract.  The  remaining  approximately  28  percent of  the
elements  which  did   not  fall   into  any  measured  category  along  with   all
ring-stub antennas  were  modeled  as  type 1  elements since  these produce  the
highest field  levels  on  the ground of any  measured.   This  decision was based
on  the  desire   to  overestimate  rather  than   underestimate  impact   when
substantial  approximations are used.

        TABLE  2.  DISTRIBUTION OF ELEMENT TYPES IN THE EPA FM DATA BASE

Element Type
Type 1
Type 2
Type 3
Type 4
Type 5
Ring-Stub
Other
Number in Data Base
563
397
350
314
188
989
1,107
Percent of Data Base
14.41
10.16
8.96
8.03
4.81
25.3
28.33

Mitigation Strategies
     Modified  versions  of the  FM model  were developed  in  order  to  examine
possible mitigation strategies.   The  model  in its original form  can  determine
                                      14

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the number of stations  likely to  exceed  a given guidance level, but  only with
a  knowledge  of  the  corrective  measures  that  might  be   chosen   and  the
effectiveness of  these measures  can  impact costs be assigned.  EPA  explored
several  approaches  to  this  problem and  discussed these  ideas with  industry
consultants  and  antenna  manufacturers.    The  result  was  a  sequence   of
corrective measures or  "fixes"  that  would most  likely  be  chosen by  a station
in non-compliance (Figure 6).   The  sequence is ordered by  increasing  cost  and
it is assumed that  a  station would  choose the  least   expensive measure that is
effective in bringing their facility into compliance.

     Examination  of  measured  antenna  elevation  patterns  reveals  that  some
antennas  direct  much  less  energy towards  the  ground  than  others.    In  many
cases, a simple change to one of these "better"  antennas is all  that  is needed
to bring a  station into compliance.   This approach is the least  expensive
"fix" and  is  therefore  first  in the sequence  of corrective measures.   The FM
model  can check  the effectiveness  of  this  approach  by   simply  replacing  a
station's antenna with a "better" one if it is  not using one at present.

     Since  the  pattern  of  an  FM  antenna  is  a  combination of  the  element
pattern  and  the array  pattern,  another  approach  to  mitigation  is to  reduce
downward  radiation  in the array  pattern.   At  one wavelength  element  spacing,
the spacing typically used  for FM antennas, the array pattern  shows  downward
radiation equal  to that in the main beam.   This  effect  occurs because  the wave
from each  element  adds  in phase  with  all other elements in  the array  in  the
downward direction.   If the spacing  is reduced to one-half wavelength (for an
even number  of  bays  antenna),  each  wave has  an out-of-phase counterpart  and
downward  radiation  is eliminated.  Fields  on  the ground will  still   occur at
angles   slightly  different  than  directly  downward,  but   will   be   greatly
reduced.   The  drawback of using  this method  is  that  the  increased  coupling
that occurs at one-half wavelength reduces  the gain  of the antenna.   In order
to maintain  the  original  gain of  the  antenna,  the  number  of bays  must  be
approximately doubled.  Another way  to reduce  downward radiation is  to reduce
the  interbay  spacing  such that waves  from element (n) ana  element (N/2 + n)
are  exactly  out of  phase,  where n  indexes the elements  in  an  N  bay  array.
Thus, the required interbay spacing would vary  as shown  in Table 3:
                                      15

-------
Model  FM Focflity in
present configuration
Model with  "Better"
     Antenna
Model with 1/2 wave
  Spaced Antenna
     --------- AA ti_rtn-m
     fMQMMIry 10 Uf Mlty
  ftakte Mow
Exceed proposed
    guidance?
                                      YES
                                      YES
 Requires  no fix
Exceed  proposed
    guidance?
Exceed  proposed
    guidance?
Requires change
  of antenna
                             Itoqulrw change to 1/2
                             DETERMINE COST
             Figure 6.  Determination of compliance costs for FM stations.

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 TABLE  3.   INTERBAY  SPACINGS  TO  REDUCE  DOWNWARD RADIATION  IN THE ARRAY  PATTERN

            Number of bays     Inter-bay spacing in wavelength  units
                   2                           0.50
                   4                           0.75
                   6                           0.83
                   8                           0.88
                  10                           0.90
                  12                           0.92
                  16                           0.94
     A smaller  increase  in number  of  bays would be  required  to maintain  the
same  gain  for  this  method than  for  one-half  wave  spacing,  but feeding  the
array would  be  more difficult  since  the  length  of transmission line  between
bays  determines  phasing.   For  one-half  wave  spacing,  criss-crossing  the
transmission  line or  turning  alternate  elements  upside down  yields  proper
phasing.    Antenna  manufacturers  would  probably  achieve  decreased  downward
radiation  in a  variety  of  ways depending  on  the  characteristics  of  their
particular elements.

     Altered  inter-bay  spacing was chosen  as the  second probable  mitigation
method since the  cost  is higher  than  replacement  with  an  already  existing
"better"  antenna.  Exact modeling of this  fix  is  difficult because  the optimum
spacing may  differ for various  antennas.   Coupling  effects  at  less  than  one
wavelength  spacing are  prominent   and not  easily calculated  by  theoretical
means.  EPA  has explored this  problem through use of the Lawrence  Livermore
National   Laboratory  (LLNL)   numerical  electromagnetic   code   (NEC)   [7]   to
calculate  coupling effects and  the resulting  patterns  [8].   The  results  of
this study indicate that an increase in  the  number of bays would be  necessary
to maintain  the same gain.   Figures 7,  8,  and 9  show the effects of  altered
spacing  for  three  commercially   available  FM  antenna  elements.    As   an
approximate  solution,  EPA  modeled  this  fix  as  the  combination of  measured
element  patterns  and  the  far-field   array  patterns   for one-half  wavelength
spaced isotropic elements.   The array  patterns were for  an increased  number of
bays  to  replace the original  array in  order to  compensate  for  the  loss  in
                                      17

-------
             RING-STUB  TYPE  ELEMENT
                                     O one Wavelength spacing

                                     • one-half Wavelength spacing
                   6       8      IE     12
                     Number o-F  Bays
14
16
  Figure 7.  Antenna gain as a function of number of elements for
one-half and one wavelength spacing between ring-stub type elements
                             18

-------
 re
u

 E
 3
 E

 x
 re
Z
    IB
8
                       TYPE 2  ELEMENT
                                         O one Ksve'erigth spac-io

                                         • one-haif Keve'eioth sea:
                         6      10
                    Number of Bays
                                            12
   Figure 8.  Antenna gain as a function of number of elements for
    one-half  and one wavelength spacing between type 2  elements.
                               19

-------
                           TYPE  3  ELEMENT
       20r
        IB
        16
    cs

    T3   12
    ^_,   i t-
E

£

x
to
        8
                                             O one Wavelength spacing

                                             • one-haH Wavelength spacing
                         6       8      10     12

                            Number  of  Bays
                                                    14      IB
Figure 9.  Antenna  gain as a function of number of elements for one-half  and
          one wavelength spacing between type 3 elements.
                                   20

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gain at closer  spacings.   Table 4 shows the  increases  used for  various  sizes
of  antenna  arrays.   This  approach  tends  to  overestimate impact  since  the
greater than  half-wave  spacings shown  in Table  3  might  be  used  and  would
require a  smaller increase in number of bays.

          TABLE  4.   NUMBERS  OF  BAYS USED  IN  ONE-HALF WAVELENGTH MODEL
          Actual number of             Number of bays used in
           bays in array             1/2 x model to approximate
                                              same gain

                 1                                2
                 2                                4
                 3                                6
                 4                                8
                 5                                8
                 6                               10
                 7                               12
                 8                               14
                10                               16
                12                               18
                14                               20
                16                               24
     Stations which were not  in  compliance at any given guidance  level  either
in  their  present configuration  or  with  an  antenna change  were  then  modeled
with one-half wavelength spacing.   This  "fix" proved  to be very  effective  in
bringing stations into compliance.

     The third  mitigation  measure  examined  involved  raising the  tower  height
until  field  levels  on  the  ground  fell below  the  guidance  level.    Since
increasing tower height is expensive, it  was  assumed that  stations requiring a
height increase would also use altered  interbay spacing  to minimize the amount
of tower height  increase necessary.   In some  cases, tower  height increases  may
                                      21

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not  be possible  because  of FCC  regulations  limiting  maximum  height  above
average  terrain  (HAAT)   or  because  of  land  limitations  (for  guy  wires).
However, broadcast  consultants  have  indicated  that this  fix  is a  reasonable
third choice in situations where the first two approaches are not sufficient.

Operation of the FM Propagation  Model

     The following data for  an  FM  broadcast station are required to  apply  the
propagation model:

                   Horizontal ERP (Effective Radiated Power)
                   Vertical ERP
                   Antenna model and make
                   Height above  ground to center of radiation of the antenna
                   Number of bays in the antenna

     Beginning at one meter  from the  base  of the tower, and proceeding at  two
meter intervals,  the model  calculates the  elevation  angle of  each  point with
respect  to  the  antenna  center  of  radiation   (Figure  10).   Relative  field
strength  values  from  the  element  pattern  (Figure  11)  and  array  pattern
(Figure 12) are then found at this angle by interpolation.
          Center of.
          Radiation
                                         . Elevation Angle
                                                      P. Calculation Point
           Figure 10.  Elevation angle to a field calculation point.
                                       22

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                      ELEMENT  PRTTEKN
                                                  -20'
                                              -30'
                                                     Rtlative field
                                                     of eleoent pattern
                                                     at calculation
                                                     angle,9
                           -60*
                                         -40°



                                   "50*  Polarization: Horizontal

                                               * of Bays:  1
                   -70
          -80
Figure 11.  Relative field strength  pattern of a single element.
                           23

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                                RRRflY  PflTTERN
05
c
0)
tn
•o
0)
ll
Q)
Q!
                                                               -20*
                                                              o-f  Bays:  6
                                                Relative field of array pattern
                                                at calculation angle, 6
    Figure 12.   Relative field strength array  pattern for a 6 bay array.
                                  24

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     The  two  values are multiplied  to  give the total  relative field  for  the
direction to that point.  This total is squared to yield the relative power
and multiplied  by the ERP to provide  an  "adjusted ERP" corresponding  to this
direction from the antenna.-   The equation

   S UW/cm2)  =   (Adjusted ERP in watts} * 1.64 * 2.56 * 100   ^2    (1)
                               4 * TT * (Distance)
is then used  to  calculate  the  power density at the point.  The  factor  of 1.64
corrects  for  the  fact  that ERP's  as defined by  the  FCC are  relative to  a
one-half  wave  dipole element.   The  factor  of  2.56  is  the   square  of  the
reflection  factor,   1.6,  discussed  earlier  for  realistic  ground  conditions.
The  "distance"  in the equation  is  the distance  in  meters  from the  center  of
radiation to the calculation point.

     As the power density is calculated at each point,  it  is  compared to a  set
of eighteen  alternative guidance levels.   These  are 1,  10,  20, 50, 75,  100,
200,  300, 400,  500, 600,  700,  800,  900,  1,000,  2,000,  5,000,  and  10,000
                                         2
microwatts  per  square centimeter  (pW/cm ).    If the  calculated power  density
exceeds any of  these alternative guidance  levels,  the  distance from the base
of the  tower  to the calculation point is stored in the  corresponding  element
of an  eighteen  element mathematical  array.   This process  is repeated  as  the
model steps away  from the  tower  so  that  the final numbers  stored  in  the array
are the farthest  distances  away  from the tower at which  the  eighteen guidance
levels  are  exceeded.  The  highest  power  density  reached  at  any  point  along
with the  distance at which  it  occurs is  also  stored.   This peak power  density
or S    .  typically  does not occur  directly  underneath  the antenna.  A sample
output from the model is shown in Figure 13.
I/  This "adjusted ERP" differs from the ERP specified by the FCC which
      refers to the power in the main beam.
                                      25

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     ftrit e
                TYPE 2
     Tower Height:   10.6&8 m
     Tot») ERF :   200 kW
     Distance from
     Tower 

          2497
           602
           556
           345
           261
           220
            39
            25
            16
            13
            13
            10
            10
             5
             5
             5
             3

     PEflK POWER  DENSITY  •
     PERK FIELD  STRENGTH
    D«nsity (uW/CTTl2)
         1
        10
        20
        50
        75
       100
       200
       300
       400
       see
       £00
       700
       800
       900
      1000
      2000
      5600

 6185.32 uW/c«2  flT  3.20  METERS FROM TOWER BRSE
• 152.70 V'M
       ie00o
    ru
    <
    E
    u
    \
    Z
    L.
    i
    o
    Q.
         100

             Distance  from  tower

                   (Meters)
                          s
                          &
G>
S>
                                     s>
                                     6
Figure 13.   Power density near the ground  as  a function of distance
from  a 6-bay FM array with the  lowest element 10.7 m above ground.
                                  26

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      The  model  output  was  designed to  facilitate  a  more  detailed  impact
 analysis  using  information  on  land  ownership  and  fencing.   It  was  intended
 that  this  information be  obtained  through  surveys  for comparison  with  the
 distances  to each  guidance  level  predicted  by the  model.   If a  station  was
 already  fenced to  a  distance  of ten  meters  from the tower,  only  those  power
 densities predicted to occur outside the fenced  areas would be considered  for
 impact.   Similarly, if the  station  owned  property around the  tower which  was
 not   fenced,   fencing  would  be   considered   as   an  alternative  mitigation
 strategy.   The  survey  results   would  also  indicate  how  many stations  are
 located  in  remote  areas so that posting radiation  hazard signs might  serve as
 an adequate  "fix".

      A  statistically  based   questionnaire  survey  of  FM  radio stations  was
 accomplished  in  early  1984  after  most  of  this  impact  analysis  had  been
 completed.    Preliminary   results   are  shown  in  Appendix  F.   As   a   rough
 indication  of the  possibility  of posting,  a computer automated population data
 base  of  the  1980 United States  census  [9]  was  employed  to  examine population
 densities around a  sample  of 878 FM broadcast antennas having predicted ground
                                         p
 level  fields  in  excess  of  100  yW/cm .    Using  the   coordinates  of   these
 transmitters  from  the  FCC  data   base,  the  1980 population   data  base  was
 examined  to see how  many  of the station  locations  showed  zero  population  in
 circles  of  0.5, 1,  2, 3,  4,  and  5 km  radius centered  on the towers.   The
 results  (Table  5)   actually represent  whether or  not  a  census  enumeration
 district  (CED)  occurs  within  the  radius,  since the  data  base  is structured
 only  by CED's.   However,  the  density  of  CED's  is   directly  related  to the
 population  density  and provides  a  reasonable indication  of the remoteness  of
 the station.

 TABLE 5.  NUMBER OF FM RADIO STATIONS (FROM A SAMPLE OF 878) HAVING NO CED's
	WITHIN 0.5 to 5.0 km	

                    Radius (km)          Number of stations
                                            with no CED's
                         0.5                      713
                         1.0                      529
                         2.0                      325
                         3.0                      196
                         4.0                      122
                         5.0                       83

                                      27

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     In order to  obtain  better coverage, many  FM  transmitters are  located  on
remote mountain tops.  Many  of these mountain  top stations have  short  towers
and produce relatively high field  strengths  on  the ground near the  tower.   It
is likely that  these  stations comprise  a  large percentage of those predicted
to be  impacted  by various proposed  alternative guidance exposure  levels.   If
so,  actual   impact  would  be  significantly  less  than  predicted  here  since
posting and fencing are  generally less  expensive  than  the other  "fixes"  used
in the model.   Thus,  until  such time as a  detailed  survey of land  use  in  the
vicinity of  FM towers  is completed,  it must  be  emphasized  that  the  impact
estimates reported  here  should  be  interpreted  as upper  limits;   in  reality,
actual impact should be less  and may be significantly less.

     The increase in  tower height  "fix"  was calculated  using a  variation  of
equation (1) along  with  a distance factor.  First,  the  total  pattern for  the
station is found by multiplying the  station's element and  array  patterns.   The
total pattern shown in  Figure 14, for example,  is the  product of  the  element
and  array patterns  shown in  Figures 11 and  12.  Next,  the  total  pattern  is
multiplied by  (sin  e)  to correct  for  the variation  in  distance which  the
radiation must  travel as  a  function of  angle before reaching the  ground  (see
Figure 15).

     The total  pattern multiplied  by the distance factor  (sin e)   is shown  in
Figure  16.   The  angle  at which  maximum  field strengths  will  occur  on  the
ground (e ) is  equal  to the  angle  at which a  maximum  occurs in  this  pattern
regardless of tower height.   Once an "adjusted  ERP"  is  found for  this  angle,
the minimum tower height necessary to bring the  station  into compliance  can be
found using equation 2.
          MTH = | /(Adjusted ERP in watts) * 1.64 * 2.56 * 100 * sin2 (e )
                                                      2
                         4 * » * (guidance level  yW/cm )
          MTH » minimum tower height necessary to bring station
                into compliance in meters
            e  = angle at which maximum radiation reaches the ground
                                      28

-------
                     TOTRL  PRTTERN
      e.e
O)
c
O
in
•o
c
L.
O
V
0
u
                                                            -20°
                                                              -10°
                                                        -30*
                                                  -40*
                                           -50*
                                   -60*
                               Polarization:  Horizontal
                                     *  of  Bays: 6
 Figure 14.  Total pattern of a 6 bay array; this is the product of the element
           pattern (Figure 11) and the array pattern (Figure 12).
                              29

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                             *xT7
                              A
Figure 15.   Radiation  traveling  path  B will  travel  further than  radiation
traveling  path A.   If  the antenna radiates equally  in  all  directions,  the
field strength at ?2 will  equal the field strength at P-j times sin e.
     Appendix  C illustrates the application of  this  simple methodology for
performing a  preliminary analysis of guidance compliance.   Equation 2 is used
to plot minimum tower height required  to comply with  a given  guidance  level
vs. the ERP of  the station.
Multiple Sites

     In  many  cases, more than one FM station locates  its  broadcast antenna on
the same tower.  The FCC automated data base does not indicate  which  stations
are co-located, but it  does contain the longitude and latitude  coordinates of
each station's  tower.   By computer searching for matched  coordinates, EPA was
able to  determine which  stations  were co-located.   This technique  does not
distinguish between antennas which are on the same tower and towers  that are
separated by  less than about 100 feet  due  to  the  resolution of the coordinates
as recorded on  FCC forms  by each station,  but for modeling  purposes, matched

                                    30

-------
                           Type:  1     # Bays:  6
          0.0
+*
Ol
c
t>
L.
4-*
T3
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0)
4-1

U.
4J
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d
          -20°
      -30'
-40'
                                                  -50'
                                         -60«
                                                    Polarization: Horizontal
                                -70
    Figure 16.  Total  array pattern multiplied by the distance factor  (sin e).
                                     31

-------
coordinates  were  assumed  to  indicate  antennas  on  the  same   tower.   This
assumption is  reasonable  since  fields from nearby antennas  will  add much the
same as fields from antennas on the same tower.                    ...

     Modeling multiple  station  sites required a more  involved  technique than
the  treatment  of  single  sites  because  of  the  large   number  of  possible
modifications which could bring the  site  into compliance.   It is assumed that
the least expensive fix is  the one that will be chosen  regardless  of whether
the total cost is borne by one or several entities.  This may be a combination
of fixes for several antennas at the site or simply a modification of only one
of  the  antennas.   The  modeling technique  described below  examines  possible
solutions to determine which one is effective  and least expensive.

     The model described for single station sites calculates the power density
at points on the ground extending  away from the tower.   The same model is used
for each antenna at a  multiple site but in this case  the power density at each
distance point  is  stored in  a large  mathematical  array.   This process  is
repeated  using   the  change   of  antenna  and  altered  spacing fixes  described
earlier.  Thus, three arrays are generated  for each  antenna  on  the  tower, one
for the  original configuration, one  with  a change  of  antenna,  and  one with
altered interbay  spacing.  The  various  possible fix  configurations can  now be
examined by  simply  adding corresponding elements  of the  proper  arrays.   This
addition is  possible  because each  station  operates  at a  different  frequency
preventing  coherent wave  addition.   On  a time  averaged  basis,  the  power
densities from each station  can  be  added directly (Figure 17).
                          103   98   90   85   100
                           50   45   35   25   40
                          153   143   125   110  140 To.tffe~.D~.tv
                           20   40   60   80   100 Dwttnca fram T«M> inktonn


Figure 17.   Summation  of power densities from two stations on the same tower.


                                     32

-------
     The first step in analyzing a multiple  site  is  to  add  the arrays for each
station  in its  present  antenna  configuration.   The resulting  array  is  then
checked  to see  if a  given  alternative  guidance   level  is  exceeded  at  any
point.   If  not,  the site is considered  a non-problem at that  guidance level.
If the site does exceed  the  alternative  guidance  level,  the distance points at
which the  alternative  level is  exceeded are identified.   The  power densities
from  each  antenna are  then  examined  at  those points  to  determine  which
antennas are creating more  than  some specified fraction of  the  guidance  level
under consideration.  For  purposes of this  analysis, this  fraction  (1/n)  was
arbitrarily defined to be the  reciprocal  of  the number of  stations  (n)  at  the
site.  It  is  assumed  that  only those  stations  exceeding  (1/n * 100) per cent
of the guidance  level  would be required  to  make  changes in  their facilities.
These stations are considered for changes to bring the site into compliance.

     The next step  is to subtract  the  power density  array  for the station  (in
the  subset  exceeding  1/n *  100  per cent)  with  the  lowest number  of  antenna
bays  from  the total power  density array and replace  it  with  that  station's
"change  of  antenna"  array.  The new   total  array consists  of  the  power
densities predicted to result  if  the  above specified station changes to  a  new
antenna  and all  others  remain  the same.  This  total  array  is then  checked to
see  if  it  still  exceeds  the   guidance  level.   If   so,  the  next   lowest
number-of-bays station in the subset is  changed to a  new antenna and the  total
checked again.  If the power densities still  exceed  the  guidance after  all  the
stations  in the  subset  are changed  to  a  new antenna,  then the replacement
process  is  repeated using  altered interbay  spaced  antennas.   Once  the  power
density  at  the  site falls  below the  guidance  level, the  changes made  up  to
that point are recorded   and the replacement  process  is ended.  The output  is  a
table for   each  alternative guidance  level   showing  the numbers  of  stations
requiring each  kind of   fix  grouped  by the  number of bays  in their  antennas.
The output format contains no information about the  number  of  stations  at each
specific site requiring  a fix, but  does  contain the  total  numbers of stations
at all sites  in the data  base requiring each  kind  of  a fix.   The  latter  is
easier to work with and  is  adequate for impact analysis  costing.

Building Mounted Towers

     Approximately  ten per  cent  of  all   FM  stations  (licensed  American,  1980
data) are located on top  of buildings.   Typically, they are mounted  on  a  short

                                      33

-------
tower  which is  secured to  the  building  rooftop.   In  nearly  all  cases,  the
ground  around  the  building  is  shielded  from  the  downward  beams  or  grating
lobes  by  the building  rooftop.   The  height of the  building also  reduces  the
intensity of any radiation reaching the  ground  (see Figure  18).   Areas  which
must  be considered in  terms of  guidance  levels  are  the  rooftop  itself,  the
interiors of adjacent buildings, and  the  top  floor of  the building on  which
the tower is mounted.

     High field  levels  are often found on rooftops supporting  FM  towers.   The
low  towers  and  metal  roofs  frequently  used  for  such  buildings   account  for
these  levels.   Aside  from the field  level  hazard,  there  may also  be  a  shock
(RF  burn) hazard when the bottom element is  within  reach.   However,  for  the
purposes  of this  study,  it was  assumed  that  very  few  such  rooftops  are
accessible  to  the  public.  It  is realized  that  in  certain  high-rise  city
environments, this assumption may be invalid.

     Locations  on the  top floors of these buildings are  not  usually exposed to
high levels of  RF  radiation  because of the  shielding  provided  by  the  rooftop
building  materials.    A  metal   rooftop,  while   greatly   increasing   field
intensities   on  the   roof  due  to  reflections,  will  effectively  shield  the
interior of the  building.  Other materials  are less effective,  but the simple
application   of  metal  screen   to  the  rooftop  surface  will   eliminate  any
significant  field levels in the unusual case that  such  are  present.

     Finally,  an  issue of some  concern has been  the creation  of  high  field
levels  in adjacent buildings by exposure to  an  antenna's  main beam  through
windows or walls.  Such  a  situation occurs when new  buildings constructed  near
a building mounted  station are  higher than the broadcast  antenna  or at  least
high enough  to  intercept the  antenna's main beam.  This  presents a  problem  for
the  station  as  they  have  now  lost part  of their coverage  by  obstruction  of
their beam.

     These  situations  were  not  treated  in the  impact  analysis   for  several
reasons.  First, broadcast consultants  indicated  that  these cases  are  usually
self-correcting.   In  other  words,  the station chooses  to  move to a  higher
building in order  to  regain lost coverage.   Such  a move  is  not  dictated  by
                                      34

-------
               % of Total
en
           25
           20
           15
10
-
•

MMMMI

••MMH

••^•M


n n n n n n m
0- 20- 40- 60- 80- 100- 120- 140- 160- 180- 200- 220- 240- 260- 280- >300
20 40 60 80 100 120 140 160 180 200 220 240 260 280 300
                                             Building Height in feet
                             Figure 18.   Distribution of Building Heights  Supporting FM Towers.

-------
Federal  guidance  and  thus  cannot  be  included  as an  impact.   Second,  the
building materials  can typically  attenuate the  fields  by  about  6  dB  [10],
reducing  exposures  below  the  levels  currently  being  considered   for  the
guidance.  This concept has  been  supported by EPA  surveys  of field  levels  in
buildings [11].   Thus,  an accurate knowledge  of the  fields  created   in  these
situations  would  increase  impact  costs  at  the   lower  alternative   guidance
levels,  but  would  not  affect costs  at  the  guidance levels currently  being
considered or at  higher levels.   Finally, accurate modeling  of these  cases  is
impossible without  information  about  the  proximity and  heights of all  nearby
buildings.  EPA was unable to obtain  this information for the  large  number  of
stations  involved  (over  400).    If  a  problem  did  occur  in  a case  where  a
station was unable  to  move,  a likely mitigation  strategy  would be to install
solar reflective film on the windows of  the affected  building [11].   This film
very effectively shields RF signals and would  probably eliminate the problem.

Model Verification

     EPA conducted  a field study  in August, 1982,  to perform  measurements near
a sample  of  FM stations for comparison with FM  modeling results for  the same
stations.  Most of  the  measurements were  performed with broadband,  isotropic,
electric  field  strength probes which  had been  calibrated  in  the  laboratory.
Measurements  were made at two  to five  foot  intervals  along  a  radial  line
extending away from the  base of the  tower.   At  each distance,  the  electric
fields  were  examined  from the ground  up  to about  eight feet and  the maximum
value  was  recorded.    The particular  radial   chosen was  often  dictated  by
accessibility,  but when   several  radials were  available,  the one   with  the
highest fields was  chosen.

     The modeled  and measured  curves  show good agreement in  nearly all  cases.
Typically, the  model  draws an envelope  above  the  measured data  following the
general  trends.  In  two  cases,  the  model underestimated  the  fields over a
limited  area.  This  is not  considered to be  a  serious  problem  because the
model overestimates the maximum fields  in all  cases and  the impact  analysis is
based  on maximum  fields.  The  figures  in  Appendix  B  show the modeled and
measured curves for each station plotted  on the same graphs for comparison.
                                      36

-------
FM Modeling Results

     The FM model  was  applied to  approximately  3,300 FM stations  with  ground
mounted  towers  for  which EPA  had  complete  data.   Single FM  stations  with
ground mounted  towers  (SFMti)  accounted  for  2,908  of the  stations while  the
remaining 357 belonged to multiple FM broadcast locations  with  ground mounted
towers (MFMG).   The  results are presented  in Tables 6  through  23 for  the  18
exposure  levels  studied.  Table  11, for  example,  gives  the  number  of  SFMG
stations  (by number of  bays)  exceeding  the   given guidance  level   (column
labeled # Stations > S)  and the number requiring an  antenna fix,  or an altered
                                                                2
interbay  spacing  fix,   in  order  to  comply  with   a  100 yW/cm   level.   The
"Antenna and 1/2 Wave Fix" column  shows  the additional number  of stations that
could be fixed by combining these two approaches.  A similar set  of tables  are
presented  for the  MFMG  stations   (Tables  24  through   41).   The  "Unfixable"
stations  in these  tables  were  further analyzed  to determine  tower  height
increases  necessary  to  bring  these  stations  into  compliance.    Table   42
summarizes the impact for all  18 power  density levels and  Table  43 summarizes
the effect  of  the mitigation  strategies for  single  FM's on the ground.   Bar
graphs showing distances  at  which  stations exceed  the  18 exposure  levels  are
presented in Appendix E.

     These  results  represent the  predicted  impact  to  Fto   broadcast  stations
which would result from  18  alternative  guidance levels.   At the  lowest  level,
        2
1  yW/cm ,   over  94  per   cent  of  the  stations would   be   affected.   At  the
                                 2
highest  level  studied,   10  mW/cm ,  less than  1  per cent  would be  affected.
Assignments  of  cost to   these  impact levels are  discussed  in  the  Economic
Impact report from Lawrence Livermore National Laboratory [1].

5.  Impact on AM Stations

     An AM  broadcast antenna  consists of one or more monopoles  above ground.
The  ground  plane  is made  more  conductive  by  burying  metal  ground  radials
around the  tower.  The electrical  heights of the  towers may range from about
0.1 wavelength to one wavelength, the majority being less than 0.30 wavelength
tall (see Figure 19).  Multiple towers are  sometimes used to  produce  nulls  in
the direction of other stations.   The transmitted power  may  be 0.1,  0.25, 0.5,
1.0, 2.5, 5.0,  10.0,  25.0,  or 50.0  kW   (see  Table  44)  in accordance  with  FCC
regulations [12].

                                      37

-------
  TflBLE  6.   FM  Modeling  results
                        S FIX
66 25
295 113
626 229
515 132
162 19
369 26
92 15
176 11
14 6
221 0
20 e
311 2
3 1
31 0
5 1
be 1 ou
WflVE
FIX
0
32
197
223
37
111
47
ee
ie
76
12
222
2
26
4
guidance level with:
flNTENHft fiHD 1/2
WflVE FIX
0
27
69
42
14
43
5
23
1
59
5
48
e
i
e
                                                          UNFIXfiBLE

                                                              41
                                                             123
                                                             133
                                                             116
                                                              92
                                                             195
                                                              25
                                                              62
                                                               3
                                                              66
                                                               3
                                                              39
                                                               0
                                                               2
                                                               0
TOTflLS
2988
568
1081
337
922
              TftBLE 7.   FM Modeling results for Guidance  Level  2
                                  
« BAYS

    1
    2
    3
    4
    5
    £
    7
    e
    9
   ie
   ii
   12
   13
   14
   16
St*t ions
> S
41
223
513
436
142
341
' 62
166
14
204
20
270
3
21
2
• stations brought
RNTENNR 1/2
FIX
12
76
369
267
28
56
39
30
1
9
1
35
2
10
2
F

1
1
1

2

1

1

2



below
WflVE
IX
14
18
22
48
63
41
36
20
11
60
19
30
1
11
0
guidance level with:
RNTENNfl ftND 1/2
WflVE FIX
3
5
1
1
4
1
0
0
0
2
0
2
0
0
0
                                                          UNFIXRBLE
                                                              24
                                                              21
                                                              20
                                                              27
                                                              43
                                                               7
                                                              10
                                                               2
                                                              13
                                                               0
                                                               3
                                                               0
                                                               0
                                                               0
TOTflLS
2472
937
1334
 19
                                                                         182
                                    38

-------
              TABLE 6.  FM Modeling results for- Guidance Level
                                  (S • 20 uW/c**>
* BflYS

    1
    2
    3
    4
    5
    6
    7
    e
    9
   ie
   n
   12
   13
   14
• stations brought
Stations ftUTEHNfi 1/2
> S FIX
29 13
146 €2
316 241
311 216
123 36
see 93
63 34
141 39
13 4
192 43
28 9
239 103
2 2
28 13
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bel out
WfiVE
FIX
9
72
€4
79
€6
iei
26
95
7
144
11
134
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guidance level with:
RUTENNR ONE 1/2
WRVE FIX
1
i
3
3
3
6
e
e
i
2
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e
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o
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                                                          UMFIXPiELE

                                                               6
                                                               ie
                                                               10
                                                               11
                                                               is
                                                               20
                                                               3
                                                               7
                                                               1
                                                               3
                                                               e
                                                               2
                                                               e
                                                               e
TOTfiLS
1917
910
695
21
91
              TRBLE 9.  FM Modeling results for Guidance Level 4
                                 
• BAYS

    1
    2
    3
    4
    5
    €
    7
    a
    9
   ie
   11
   12
   13
   14
   16
* stations brought
Stations flNTENNR 1/2
> S FIX
11 5
69 33
122 82
153 106
90 36
227 126
41 25
105 61
7 4
163 65
ie is
167 141
1 1
12 9
e e
bel ou
WflVE
FIX
1
35
32
41
44
93
16
42
3
77
3
46
e
3
e
guidance level with:
fiNTENNfl flND 1/2
WftVE FIX
2
0
2
2
2
1
0
1
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e
0
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0
                                                          UNFIXfiBLE

                                                               3
                                                               1
                                                               €
                                                               <
                                                               6
                                                               7
                                                               0
                                                               1
                                                               0
                                                               1
                                                               0
                                                               0
                                                               0
                                                               0
                                                               e
TOTflLS
1206
729
436
10
31
                                    39

-------
              TRBLE 10.   FM
                 Model ing
                       S

    7
   41
   72
  103
   63
  198
   32
   94
    7
  151
   17
                        stations brought below guidance level with:
flNTEHHfl
  FIX
1x2 MOVE
   FIX
RNTEMNfl RNE 1x2
   WRVE FIX
    1
    7
    e
    1
   13
   44
   69
   43
  126
   21
   63
    4
   99
   17
  144
    1
    6
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     3
    27
    22
    31
    35
    64
    11
    38
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TOTflLS
  963
  653
   385
                          22
              TRBLE 11.   FM Modeling results for Guidance Level 6
                                 (S • 180 uUxcm2>
• BflYS

    1
    2
    3
    4
    5
    6
    7
    e
    9
   ie
   11
   12
   13
   14
   16
Stat ions
  > S

    6
   35
   59
   61
   72
  172
   38
   69
    7
  145
   16
  159
    1
    6
    8
                      * stations brought below guidance level with:
RNTENNR
  FIX

    1
   13
   36
   55
   38
  113
   19
   63
    4
  182
   16
  143
    1
    5
    8
1x2 WflVE
   FIX

     4
    22
    16
    24
    29
    56
    11
    26
     3
    43
     0
    16
     8
     1
     e
RNTENNR RND 1x2
   WRVE FIX

        8
        8
        1
        8
        1
        2
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        8
        0
        0
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        8
        8
L'NFIXRBLE

     1
     8
     4
     2
     4
     1
     8
     8
     8
     8
     8
     8
     8
     8
     8
TOTRLS
  678
  611
   251
                          12
                                   AO

-------
TABLE 12.   FM
Modeling
      S FIX
5 3
19 10
30 19
43 26
48 26
111 76
23 15
68 32
3 1
107 84
9 9
90 85
e 0
4 4
0 0
bel ou
URVE
FIX
2
9
10
16
22
34
8
16
2
23
0
5
0
0
0
guidance level with:
RNTENNR flMD 1'2
WflVE FIX
0
0
0
0
0
1
0
0
e
0
0
0
0
e
0
                                                         UHFIXRBLE

                                                              0
                                                              0
                                                              1
                                                              1
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
TOTflLS
560
 410
                          147
tt BRYS

    1
    2
    3
    4
    5
    6
    7
    8
    9
   10
   11
   12
   13
   14
   16
TflBLE

Stations
> S
4
10
22
28
40
75
15
44
2
83
6
68
0
3
0
13. FM Model

ing results
 300 uWx
for Guidance Level 8
c«>2>
i stations brought below guidance level with:
flNTENNfl 1x2 WflVE flNTENNfl flND 1x2
fix
3
6
13
16
27
54
9
35
0
66
6
65
0
3
0
FIX
1
4
8
12
13
21
6
9
2
17
0
3
0
0
0
WflVE FIX
0
0
1
0
0
0
0
0
0
0
0
0
0
0
0
                                                         UNFIXRBLE

                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
TOTflLS
400
 303
                          96
0
                                    41

-------
              TflBLE 14.  FH Modeling results for Guidance Level 9
                                (S • 400 uU/cn>2>
« BflYS

    I
    2
    3
    4
    5
    6
    7
    8
    9
   10
   11
   12
   13
   14
   16
• stations brought
Stations RNTEHHR 1/2
> S FIX
1 1
4 2
18 12
20 11
34 23
61 45
12 7
32 23
2 1
53 44
3 3
36 34
e 0
2 2
0 0
bel ou
WflVE
FIX
0
2
6
9
11
16
5
9
1
11
0
2
0
0
0
guidance level with:
fiMTEHHfl fiMD 1/2
WflVE FIX
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
                                                         UHFIXflBLE

                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
TOTflLS
280
               208
72
0
                                                                           0
TflBLE 15.   FM
               Modeling results
                    S FIX
0 0
3 2
IS 9
16 9
28 17
50 38
9 6
24 17
2 1
47 48
2 2
27 25
0 0
2 2
e 0
bel ou
WflVE
FIX
0
1
6
7
11
12 '
3
7
1
7
0
2
0
0
0
guidance level uith:
flHTENNfl flND 1/2
WflVE FIX
0
0
0
0
0
0
0
0
0
0
Q
0
0
0
0
                                                         UNFIXflBLE


                                                              O
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
TOTflLS
223
               168
37
                   0
                                    42

-------
              TftBUE 16.   FM Modeling results for Guidance Level  11
                                (S • 680 uM/cm2>
tt BOYS

    1
    2
    3
    4
    5
    6
    7
    8
    9
   10
   11
   12
   13
   14
   16
« stations brought
Stations flNTEHNfl 1/2
> S FIX
0 0
3 3
13 8
13 8
25 17
39 31
6 4
28 13
2 1
39 34
2 2
24 22
8 8
2 2
8 8
below
UflVE
FIX
8
8
5
5
8
8
2
7
1
3
8
2
8
8
8
guidance level with:
RNTENHfl FIND 1/2
WflVE FIX
8
0
0
0
0
0
0
0
0
0
0
8
8
0
8
                                                         UHFIXflBLE

                                                              8
                                                              0
                                                              0
                                                              0
                                                              8
                                                              0
                                                              0
                                                              8
                                                              8
                                                              8
                                                              8
                                                              8
                                                              8
                                                              8
                                                              8
TOTflLS
188
143
43
8
8
tt BflYS

    1
    2
    3
    4
    5
    6
    7
    8
    9
   10
   11
   12
   13
   14
   16
TRBLE 17. FM Modeling results for Guidance Level 12
 S FIX
8 0
i 1
12 7
13 8
22 14
37 29
6 4
16 9
2 1
30 29
0 8
18 17
0 0
1 1
0 8
uM/cm2)
below guidance
WflVE flNTENNfl
FIX WflVE
8
0
5
3
8
8
2
7
1
1
0
1
0
O
0

level with:
fiND 1/2
FIX
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
                                                         UNFIXflBLE

                                                              0
                                                              8
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
TOTflLS
158
120
38
0

-------
              TflBLE 18.  FM
               Model i ng
                   
Guidance Level 13
« BflYS

    1
    2
    3
    4
    5
    6
    7
    8
    9
   10
   11
   12
   13
   14
   16
TOTflLS
* stations brought below
* Stations fiNTENNfl 1/2 WflVE
> S FIX FIX
0 00
1 1 0
12 34
12 75
21 14 7
31 25 6
6 3 1
14 86
2 1 1
27 26 1
e 00
15 14 1
e e e
i i e
e e e
guidance level with:
flNTENNfl flHD 1/2
WftVE FIX
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
                                                         UNFIXflBLE

                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
142
118
                                         32
                                           0
                                              0
tt BflYS

    1
    2
    3
    4
    3
    6
    7
    8
    9
   10
   11
   12
   13
   14
   16
TflBLE

Stations
> S
0
1
10
11
19
28
6
11
2
23
0
12
0
1
e
19. FM Modeling results for
2>
Guidance Level 14

below guidance level with:
WflVE flNTENNfl flND 1/2
FIX
0
0
4
4
6
6
1
4
1
1
0
I
0
0
0
WflVE FIX
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
                                                         UNFIXflBLE

                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
                                                              0
TOTflLS
124
 96
                                                        0
                                   44

-------
TflBLE 20.   FM
                 Modeling results
                      S FIX
0 0
1 1
9 7
10 7
19 14
23 22
3 5
8 5
2 1
21 20
0 0
12 11
0 0
1 1
0 0
bel ou
WflVE
FIX
0
0
2
3
5
6
0
3
1
1
0
1
0
0
0
guidance level with:
RNTENNfl flND 1/2
WflVE FIX
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
                                                           UHFIXRBLE

                                                                0
                                                                0
                                                                0
                                                                0
                                                                0
                                                                0
                                                                0
                                                                0
                                                                e
                                                                0
                                                                e
                                                                e
                                                                0
                                                                0
                                                                0
TOTflLS
  116
                94
    22
        0
     0
              TABLE 21.  FM Modeling results for Guidance Level 16
                                
II BRYS

    1
    2
    3
    4
    5
    6
    7
    8
    9
   10
   11
   12
   13
   14
   16
Stations
  > S

    0
    0
    8
    4
   11
   17
    3
    3
    1
    8
    0
    3
    0
    1
    0
                      • stations brought belou guidance level with:
             flNTENNfl
               FIX

                 0
                 0
                 7
                 2
                 8
                16
                 3
                 2
                 1
                 7
                 0
                 3
                 0
                 1
                 0
1/2 WflVE
   FIX

     0
     0
     1
     2
     3
     1
     0
     1
     0
     1
     0
     0
     0
     0
     0
flNTENNfl flND 1/2
   WflVE FIX

        0
        0
        0
        0
        0
        0
        0
        0
        0
        0
        0
        0
        0
        0
UNFIXflBLE

     0
     0
     0
     0
     0
     0
     0
     0
     0
     0
     0
     0
     0
     0
     0
TOTflLS
   59
                30
                   0
                           0
                                    45

-------
TflBLE 22.   FM
              Modeling result*
                  S FIX
0 0
0 0
4 4
0 0
2 2
4 4
1 1
2 2
0 0
2 2
0 0
0 0
0 0
0 0
0 0
bel ou
WflVE
FIX
6
0
0
0
0
0
0
0
0
0
0
0
0
0
0
guidance level with:
flNTENNfl FIND 1/2
WflVE FIX
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
                                                        UHFIXflBLE

                                                             0
                                                             0
                                                             0
                                                             0
                                                             0
                                                             0
                                                             0
                                                             0
                                                             0
                                                             0
                                                             0
                                                             0
                                                             0
                                                             0
                                                             0
TOTflLS
15
                15
0
0
              TflBLE 23.  FM Modeling results for Guidance Level 18
                               2>
tt BflYS

    1
    2
    3
    4
    5
    6
    7
    8
    9
   10
   11
   12
   13
   14
   16
# stations brought
Stations flNTENNR 1/2
> S FIX
0 0
0 0
1 1
0 0
0 0
1 1
0 0
1 1
0 0
0 0
0 0
0 0
0 0
0 0
0 0
bel ou
WflVE
FIX
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
guidance level with:
flNTENNfl flND 1/2
UflVE FIX
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
                                                        UNFIXflBLE

                                                             0
                                                             0
                                                             0
                                                             0
                                                             0
                                                             0
                                                             0
                                                             0
                                                             0
                                                             0
                                                             0
                                                             0
                                                             0
                                                             0
                                                             0
TOTflLS
                                          0
                                                                           e

-------
TABLE 24.   Ffl Modeling
               result*
               (S • I
                                             for Guidance Level 1
       » BRYS

            1
            2
            3
            4
            5
            6
            7
            8
            9
           10
           11
           12
           13
           14
           15
           16
TOTflLS

SITES

St «t
>

4
4
4
2
5
1
^

2

3





i ons
S
8
8
^
&.
7
6
3
0
9
4
5
1
1
0
9
0
5
* Stations
gui d*nc
fiNTEHNR
FIX
0
ie
2
2
1
3
0
0
e
e
e
e
0
0
0
0
brought below
• level with:
l/Z UflVE
FIX
0
19
9
7
1
5
4
13
4
5
1
22
0
9
0
4
348
148
                        15
103
                                         UHFIXfiBLE

                                               3
                                              19
                                              31
                                              38
                                              24
                                              48
                                               6
                                              26
                                               0
                                              20
                                               0
                                               9
                                               0
                                               0
                                               0
                                               1
                              51
230

 97
              TfiBLE 25.  FM Modeling results for Guidance Level
                                     S
6
24
35
39
25
51
9
35
3
23
1
29
0
4
0
3
* Stations brought
guidance level
fiNTENNR 1 '2
FIX F
0
8
8
7
1
9
2
5
0
1
1
8
0
3
0
1

be 1 ow
with:
WOVE
IX

1
1
1
15

1
9
0
18

3
19

3
13

1




0
9
0
1
0
2
287
133
                                                 UNFIXflBLE

                                                       5
                                                       5
                                                      12
                                                      23
                                                      14
                                                      24
                                                       4
                                                      11
                                                       0
                                                       9
                                                       0
                                                       2
                                                       0
                                                       0
                                                       0
                                                       0

                                                     109

                                                      41
                                     47

-------
              TABLE 26.  FM Mod*ling  results for Guidance  Level  3
                                     S
6
17
29
35
24
49
6
26
2
22
1
26
0
3
0
2
250
117
# Stations
gui dance
flMTENUft
FIX
0
8
7
6
4
10
2
3
0
3
1
12
0
2
0
0
58
brought below
1 eve 1 with:
1/2 WAVE
FIX
2
5
10
3
3
22
2
13
2
13
0
13
0
1
0
2
101
83
UMFIXflBLE

      4
      4
     12
     21
     12
     17
      4
     10
      0
      6
      0
      1
      0
      0
      0
      0
     91

     34
              TflBLE 27.   FM Modeling results for Guidance Level 4
                                   
       « BRYS

           1
           2
           3
           4
           5
           6
           7
           8
           9
          10
          11
          12
          13
          14
          15
          16
TOTflLS

SITES
St at i ons
> S
5
9
22
31
23
44
7
19
0
18
1
20
0
2
0
1
202
105
ft Stations
gui dance
flNTENNfl
FIX
2
5
5
6
7
16
3
5
0
7
1
14
0
2
0
1
74
brought below
level with:
l-'2 WflVE
FIX
3
1
9
10
9
21
2
10
0
6
0
6
0
0
0
0
77
39
UNFIXflBLE

      0
      3
      8
      15
      7
      7
      2
      4
      0
      5
      0
      0
      0
      0
      0
      0

      51

      16
                                     48

-------
TfiBLE 28.   FM Modeling results
                     


1
2
2
3

1

1

1




17
e
i ons
S
4
S
8
7
2
9
6
8
0
7
0
4
8
1
0
1
'2
18
tt St at i ons
gui dance
flHTENMR
FIX
1
1
5
5
6
13
2
4
e
7
0
10
e
i
0
i
56

brought be low
level with:
1/2 WflVE
FIX
3
1
8
3
9
20
3
14
0
7
0
4
0
0
0
0
77
77
                                                 UHFIXflBLE

                                                       0
                                                       3
                                                       5
                                                      14
                                                       7
                                                       6
                                                       1
                                                       0
                                                       0
                                                       3
                                                       0
                                                       0
                                                       0
                                                       0
                                                       0
                                                       0
                                                      39

                                                      11
TflBLE 29.   FM
              Modeling results
                       S

           3
           4
         17
         26
         22
         36
           5
         17
           0
         16
           0
         11
           0
           0
           0
           1

         158

         92
tt Stations
gui dance
RNTENNR
FIX
1
1
6
4
6
10
1
5
0
7
0
7
0
0
Q
1
brought below
1 eve 1 wi t h!
I/- 2 WflVE
FIX
2
3
11
16
10
25
3
12
0
7
0
4
0
0
0
0
                        49
UNFIXRBLE

      0
      0
      0
      6
      6
      1
      1
      0
      0
      2
      O
      0
      0
      0
      0
      0

     16

      5
                                    49

-------
TABLE 30.   FM
Modeling results
        S
1
2
13
20
14
27
3
12
0
15
0
7
0
e
0
i
115
57
i Stat i ons
gui dance
ANTENNA
FIX
1
I
3
'i
1
10
1
4
0
8
e
6
0
0
0
i
39
brought belou
1 eve 1 with:
1/2 HAVE
FIX
0
1
10
13
10
17
2
8
0
7
e
i
0
0
0
0
69
55
                                   UNFIXfiBLE

                                         0
                                         0
                                         0
                                         4
                                         3
                                         0
                                         0
                                         0
                                         0
                                         0
                                         0
                                         0
                                         0
                                         0
                                         0
                                         0
                                         7

                                         2
TABLE 31.  FM
Modeling results
        S
0
1
13
20
13
22
2
10
0
13
0
6
0
0
0
1
101
53
* Stat ions
gui dance
ANTENNA
FIX
0
0
3
5
3
9
O
4
0
9
0
5
0
0
0
1
3?
brought belou
1 evel with:
1x2 WAVE
FIX
0
1
10
15
10
13
2
6
0
4
0
1
0
0
0
0
62
50
                                   UNFIXABLE

                                          0
                                          0
                                          0
                                          0
                                          0
                                          0
                                          0
                                          0
                                          0
                                          0
                                          0
                                          0
                                          0
                                          0
                                          O
                                          0

                                          0

                                          0
                                    50

-------
              TABLE 32.  FM Modeling results for Guidance Level  9
                                  (S • 400 uU/cm*>
       tt BAYS

           1
           2
           3
           4
           5
           6
           7
           8
           ^
          18
          11
          12
          13
          14
          15
          16
TOTALS

SITES
Stat i ons
  > S

    0
    1
   11
   £0
   12
   18
    2
    7
    0
   12
    0
    6
    0
    0
    0
    1
   90
   46
« Stations
gui dance
ANTENNA
FIX
0
0
3
6
3
9
0
2
a
9
0
5
0
0
0
1
brought below
level with:
1/2 HAVE
FIX
0
1
8
14
9
9
2
5
0
3
0
1
e
0
0
0
38
52
                         UNFIXABLE

                               0
                               0
                               0
                               0
                               0
                               0
                               0
                               0
                               0
                               0
                               0
                               0
                               0
                               8
                               8
                               0
0
              TABLE 33.  FM Modeling results for Guidance Level 10
                                   S

    0
    1
   11
   16
   12
   16
    2
    7
    0
   12
    0
    6
    0
    0
    0
    1

   84

   43
41 Stations
gui dance
ANTENNA
FIX
0
0
4
3
4
9
0
2
0
9
0
3
0
0
0
1
brought below
1 evel with:
1/2 WAVE
FIX
0
1
7
11
8
7
2
5
0
3
0
1
0
0
0
0
             45
                                     51
            UNFIXABLE

                  0
                  0
                  0
                  0
                  0
                  8
                  0
                  0
                  0
                  0
                  0
                  0
                  0
                  0
                  0
                  0

                  0

                  0

-------
              TRBLE 34.  FM Modeling results for Guidance Level 11
                                  
       i BRYS

           1
           2
           3
           4
           5
           6
           7
           8
           9
          13
          11
          12
          13
          14
          15
          16
TOTflLS

SITES
Stat
>


1
1
1
I



1






7
3
i ons
S
0
1
0
5
2
4
2
5
0
0
0
4
8
e
0
0
3
6
• Stations
gui danc
flNTENHfl
FIX
0
8
5
5
4
3
e
i
e
3
6
4
0
0
0
0
33

brought below
9 level with:
1/2 UfiVE
FIX
0
t
S
13
3
6
2
4
0
2
3
3
3
0
3
3
38
36
                          UMFIXfiBLE

                                0
                                3
                                8
                                3
                                3
                                3
                                0
                                3
                                3
                                3
                                3
                                8
                                3
                                0
                                3
                                3
                                8

                                8
TflBLE 33.  FM
Modeling
       S
3
1
9
12
11
11
2
3
0
8
3
3
0
3
Q
3
62
33
* Stations
gui dance
fiNTENNfl
FIX
3
1
5
5
4
7
1
i
3
7
3
3
0
0
3
0
34
brought below
leu*l uith:
l-'2 URVE
FIX
0
3
4
7
7
4
1
4
0
1
3
3
0
0
3
8
28
33
                                    52
                          UNFIXflBLE

                                8
                                8
                                e
                                8
                                0
                                8
                                8
                                8
                                8
                                3
                                8
                                8
                                8
                                8
                                8
                                8

                                Q

                                8

-------
TRBLE 36.   FN
        Modeling
               S
0
1
9
12
11
11
2
4
0
8
0
3
0
0
8
0
61
33
tt Stations
gui dance
flNTENNft
FIX
Q
1
6
5
4
7
1
1
0
7
0
3
0
0
0
0
35
brought below
level uith:
1/2 WflVE
FIX
0
0
3
7
7
4
1
3
0
1
0
0
0
0
0
0
26
33
                                           UNFIXflBLE

                                                 0
                                                 0
                                                 0
                                                 0
                                                 0
                                                 0
                                                 0
                                                 0
                                                 0
                                                 0
                                                 0
                                                 0
                                                 0
                                                 0
                                                 0
                                                 0
                                                 e
                                                 0
TfiBLE 37.   FM
              Modeling
                     S

    0
    1
    7
   12
   10
   11
    2
    4
    0
    7
    0
    3
    0
    0
    0
    0

   57

   29
                                 tt Stations brought below
                                    guidance level  uith:
                     fiNTENNfi
                       FIX

                         0
                         1
                         5
                         5
                         5
                         7
                         1
                         1
                         0
                         6
                         0
                         3
                         0
                         0
                         0
                         0

                        34
                                    53
1- 2 WflVE
   FIX

     0
     0
     2
     7
     5
     4
     1
     3
     0
     1
     0
     0
     0
     0
     0
     0

    23
                                                 UNFIXflBLE

                                                       0
                                                       0
                                                       0
                                                       0
                                                       0
                                                       0
                                                       0
                                                       0
                                                       e
                                                       o
                                                       0
                                                       0
                                                       0
                                                       0
                                                       0
                                                       0

                                                       0

                                                       0

-------
              TABLE  38.   FM  Modeling results  for  Guidance  Level  13
                                  
       i BAYS

           1
           2
           3
           4
           5
           6
           7
           8
           9
          10
          11
          12
          13
          14
          15
          16
TOTflLS

SITES
St at i ens
> S
0
1
€
12
10
16
2
3
0
6
0
3
0
e
0
0
53
28
tt Stations
guidance
flNTEMNfl
FIX
0
1
5
6
5
7
1
2
0
5
0
3
0
0
0
0
35
brought belou
1 eve 1 with:
U2 WflVE
FIX
0
0
1
6
5
3
1
1
0
1
0
0
0
0
0
0
18
28
                          UNFIXflBLE

                                0
                                0
                                0
                                0
                                0
                                0
                                0
                                0
                                0
                                0
                                0
                                0
                                0
                                0
                                0
                                0
                                0

                                0
TflBLE 39.  FM
Modeling
       S
0
1
1
5
6
6
2
3
0
2
0
0
0
0
0
0
26
19
# Stations
gui dance
flNTEHNfl
FIX
0
1
1
2
5
5
1
3
0
2
0
0
0
0
0
0
20
brought belou
level with:
1x2 WflVE
FIX
0
0
0
3
1
1
1
0
0
0
0
0
0
0
0
0
6
19
                          UNFIXflBLE

                                0
                                0
                                0
                                0
                                0
                                0
                                0
                                0
                                0
                                0
                                0
                                e
                                o
                                0
                                0
                                0

                                0

                                0
                                     54

-------
              TABLE 40.   FM Modeling results  for  Guidance  Level  17
                                 
       41 BftYS

           1
           2
           3
           4
           5
           6
           7
           8
           9
          16
          11
          12
          13
          14
          15
          16
tt Stations
    X S

      0
      0
      O
      1
      1
      2
      0
      0
      0
      e
      8
      0
      e
      0
i Stations
gui dance
flHTEHNfl
fix.
0
0
0
2
2
1
1
2
0
0
0
e
0
0
0
0
brought below
level with:
i'2 WflVE
FIX
0
0
0
0
0
0
0
0
0
e
0
0
0
0
0
0
                                      UHFIXflBLE

                                            0
                                            0
                                            0
                                            0
                                            e
                                            0
                                            0
                                            0
                                            e
                                            o
                                            0
                                            e
                                            e
                                            a
                                            0
                                            0
TOTflLS

SITES
      8
                           0
TflBLE 41.
FM Modeling
         S

      0
      e
      o
      0
      0
      e
      0
      0
      0
      0
      0
      0
      0
      0
      0
      O
                                 ft Stations brought belou
                                    guidance level with:
                     flNTENNfl
                       FIX
                         0
                         0
                         0
                         0
                         0
                         0
                         e
                         0
                         0
                         0
                         0
                         0
                         0
                         0
                         0
                         0
1/2 WflVE
   FIX
     0
     0
     0
     0
     0
     0
     0
     0
     0
     0
     0
     0
     0
     0
     0
     0
                                      UNFIXflBLE

                                            0
                                            0
                                            0
                                            0
                                            0
                                            0
                                            0
                                            e
                                            e
                                            0
                                            0
                                            0
                                            0
                                            0
                                            0
                                            0
                                  0

-------
                     TABLE  42.   SUMMARY  OF  NUMBERS  OF  FM RADIO STATIONS EXCEEDING POWER DENSITY LEVELS
en
CD
Power Density Single
Level uW/cm2 on
10,000
5,000
2,000
1,000
900
800
700
600
500
400
300
200
100
75
50
20
10
1
3
15
59
116
124
142
158
188
225
280
400
560
878
983
1206
1917
2472
2908
Stations
Ground
0.1
0.5
1.9
3.7
4.0
4.6
5.1
6.1
7.3
9.0
12.9
18.1
28.4
31.8
39.0
61.9
79.9
94.0
Multiple Sites
on Ground
0
6
19
28
29
33
33
36
43
46
50
57
82
88
105
117
133
148
0.0
4.0
12.7
18.7
19.3
22.0
22.0
24.0
28.7
30.7
33.3
38.0
54.7
58.7
70.0
78.0
88.7
98.7
Single Stations
on Buildings
14
29
51
76
83
88
99
107
116
134
154
173
195
211
227
275
325
389
3.5
7.2
12.7
18.9
20.6
21.9
24.6
26.6
28.9
33.3
38.3
43.0
48.5
52.5
56.5
68.4
80.8
96.8
Multiple Sites
on Buildings
6
9
11
13
14
14
15
15
15
15
15
15
15
15
15
15
15
15
37.5
56.3
68.8
81.3
87.5
87.5
93.8
93.8
93.8
93.8
93.8
93.8
93.8
93.8
93.8
93.8
93.8
93.8
All Sites
23
59
140
233
250
277
304
345
399
475
619
805
1170
1297
1553
2324
2945
3460
0.6
1.6
3.8
6.4
6.8
7.6
8.3
9.4
10.8
13.0
16.9
22.0
31.9
35.4
42.4
63.4
80.4
94.5
    Totals
3095
150
402
16
3663

-------
TABLE 43.  SUMMARY OF MODEL RESULTS TO EVALUATE DIFFERENT MITIGATION STRATEGIES
                             FOR FM  RADIO STATIONS

                          Numbers  of Stations Exceeding Power Density Levels
Power Density ~
Level in uW/cm
20,000
10,000
5,000
2,000
1,000
900
800
700
600
500
400
300
200
100
75
50
20
10
1
Without
Modification
1
3
15
59
116
124
142
158
188
225
280
400
560
878
983
1206
1917
2472
2908
With Change
of Antenna
0
0
0
9
22
28
32
38
43
57
72
97
150
267
330
477
1007
1535
2340
With one-half
Wavelength Spacing
0
0
0
0
0
0
0
0
0
0
0
1
3
16
25
41
112
201
1259
                                      57

-------
          % of Total
en
00
40



35




30




25




20




15




10
            30-50  50-70  70-90  90-110110-130130-150150-170170-190190-210210-240240-260  >260

                                        Electrical Height  in  degrees


   Figure   19.    Distribution  of   physical   electrical   heights  for  stations   In   the  AH  data  base.

-------
          TABLE 44.  DISTRIBUTION OF TRANSMITTER POWERS FOR STATIONS
	IN THE AM DATA BASE	

Transmitter Power            Number of Stations             Percent of Total
0.25
0.50
1.00
2.50
5.00
10.00
25.00
50.00
286
447
2,332
65
1,190
149
2
149
6.2
9.7
50.5
1.4
25.8
3.2
< 1.0
3.2

   Three methods were examined for  predicting  fields  around AM stations to use
as a possible  basis  for  an  AM model.  These three were  a textbook theoretical
approach  [5],  the LLNL  Numerical  Electromagnetic Code  [7],  and  the "RADIAT"
program developed by the FCC  [13].   The  requirements  for the method chosen are
that  it  accurately  predict electric  and  magnetic fields  in  the near-field,
properly  add  the  component fields,  and be relatively  easy  to  apply  to any
power, electrical  height,  and  frequency.    The  region  in  which  the possible
guidance levels might be exceeded extends  to about 300  meters from the tower,
much of which is within the near-field of the antenna.

   The FCC "RADIAT"  computer program  is   used  to  predict  fields  and  other
characteristics of  any  AM  station  in the  FCC data  base.   It is  designed  to
automatically retrieve the  necessary data  from  the  FCC's  AM  Engineering  Data
Base  to   be used  in  the  calculations.   Because  of  Radiat's  availability,
connection  with  the  FCC AM  data  base,  and  its  stated  ability to  predict
near-fields,  it  was  considered  as  a   possible  basis   for  an  AM  model.
Examination  of  the  output  from  RADIAT,  however,   revealed  that  it  uses
far-field equations to predict the  fields  no matter  how  close the calculation
point  is  chosen  to  the  tower.   It is   therefore  inadequate  for  accurate
modeling  in the area of interest.
                                      59

-------
   Theoretical   approaches,   such   as  the   one  described   by  Jordan   and
Balmain [5], assume  a  current  distribution  and  then  develop  equations  to
predict the  fields.   These equations were  automated in order  to examine  the
results  as  a  function  of  electrical height.   Because a  sinusoidal  current
distribution is assumed,  this method predicts  low  electric fields  around  the
base  of  the tower when  the  electrical  height  is  an  odd multiple  of  0.25
wavelengths.  At these  electrical  heights,  the  current  is  a maximum  (and  the
voltage  a   minimum)  at  the  base  of  the  tower,  resulting  in  low  electric
fields.  Limited  measurements around  0.25  wavelength  tall AM  towers do  not
show  these  low field   levels.   It  is  apparent  that  this  idealized  current
distribution does  not occur in typical AM broadcast  systems.

   The LLNL  Numerical  Electromagnetic Code  (NEC)  [7]  was  studied  as  a  third
approach to modeling AM transmitting  antennas.   It  can be  operated  easily for
AM  towers  since the geometry of  the  antenna is simple.   NEC  offers  several
advantages over the other  techniques.  It  is structured  to  calculate fields at
any  point   or  set of  points  chosen  and  can be  directed   to   use  near-field
equations when  necessary.   The output consists  of electric  and  magnetic  field
components  as  well as  the  properly  summed total fields  at each point.   This
last  feature  is particularly  important  since the orthogonal  field  components
in the near-field  may differ  in  both  magnitude  and  phase relationship and thus
require complex techniques for determining the resultant fields.

   NEC  was  found to   agree  quite  well  with   the  theoretical  approach  [5]
discussed earlier for most  cases.  A  notable exception  is that the  NEC results
do  not show the  greatly  reduced electric  fields  for 0.25  wavelength towers.
Two   possible   reasons  for  this  lack  of   agreement  are  that  the  current
distribution predicted  by  NEC  is calculated over  each segment  (20 segments
were  used  to model AM  towers) and the  feed point was chosen  at the bottom of
the  tower  preventing zero voltage from  occuring at  this  point.   In reality
most  AM  towers have  an elevated feed  point, sometimes  several  feet above the
ground.

    The NEC  code was  chosen to be used as the basis  for  an  AM  model  because of
its  ease of use and  other advantages  discussed  above.   Fields  as a function of
distance  were  plotted  from  NEC  runs  for  various  electrical  heights  and
frequencies in order to  study trends.   Several  important  characteristics were
noticed.   If the  electrical height is held  constant but the frequency varied,

                                      60

-------
the  electric fields  will  be  higher  over a  certain  range  of  distances  for
higher frequencies.  This effect  can be explained by noticing  that ten meters
at 600  kHz  and 10 meters  at  800  kHz are  different  relative  distances.  Since
the  towers  are shorter  at higher frequencies, the  fields  are expected  to be
higher.   Another  trend  is  that  magnetic  fields  are typically but  not always
higher  than  electric fields  in  the near-field  if  a  free-space  comparison is
used.   In other  words,   the  magnetic  field  can be  converted to  "free-space
equivalent"  electric  field  using  E     =  H*377  for  comparison.   Magnetic
fields  must therefore  be  considered  as  a possible limiting  factor from  a
guidance point of  view.   When  fields from  towers of  various electrical heights
were  compared,  it was  obvious that a simple trend  could  not be  established
with  regard  to  electrical  height.   Fields  may   increase  or  decrease  as
electrical  height is  increased.   All   of  the  comparison  runs were  performed
holding the  input power constant.

   The  AM model  was  developed using  the  considerations discussed  above.   In
summary,  fields   may be   higher  for  higher   frequencies  (holding  electrical
height  constant),  magnetic fields may  be  higher than electric fields from  a
guidance  viewpoint,  and  no simple  trend  can  be  established  as a  function of
electrical  height.   The  variety  of   parameters  for  a  given  station  are
frequency (540 to 1,600  kHz  in  10 kHz  increments),  electrical height (<  0.1
wavelength to  1.0 wavelength),  power  (nine discrete values  listed  earlier),
feed  design,  and  array  factors.   We  simplified the last  two parameters  by
assuming  a  base  feed and a  single tower  in all  cases.    The  single  tower
assumption  is  reasonable  since   feeding  all   the power  into  a  single  tower
generally results in higher fields  immediately  adjacent  to  the  tower (within  a
few meters).  The large number of AM stations  considered and  the  time and cost
involved  in  running  NEC,  eliminated  the  possibility of  performing  an  exact
modeling using NEC  in  each case.   Instead, NEC was  used on a  set  of discrete
values comprising 60 possible configurations.

   6 frequencies              0.6,  0.8,  1.0, 1.2, 1.4, 1.6 MHz
   10 electrical  heights      0.1,  0.2,  0.3, ...  1.0 wavelengths
   50 kW power was used in all  cases
                                      61

-------
   In each case, the total electric  and  magnetic fields were computed  at  four
meter intervals ranging from 2 to 298 meters from the tower at a  height of two
meters above ground.   Fields  from AM stations  do  not vary significantly  from
the ground up to a few meters  above  ground.   Data  from each of these runs was
stored for future use.

   A computer program  was  written to find the  farthest  distance from  each  of
the 60 configurations  at which the  eighteen  alternative guidance  levels  were
exceeded.  The  program  functioned by stepping  toward the tower  and  comparing
the  higher  of  the  electric  or  magnetic  field to   the  alternative guidance
levels.    This  process  was  repeated  with  the  field  levels  scaled for  lower
power stations.  The fields  at 100  meters from a 5  kW station, for example,
would be M-JTtimes  the fields from a 10 kW station at  100  meters assuming the
same tower configuration.  In general:

              i*

   where:
   E^ = field from station 1  at a given  distance
   E£ = field from station 2  at the same distance
   ?l = broadcast power of station 1
   ?2 = broadcast power of station 2

   These distances were  stored  in  a  large, four dimensional mathematical array
for easy access.  The dimensions of this array are  as  follows:

   Frequency               6
   Electrical Height      10
   Output Power            9
   Guidance level         18

   Thus  the  array  consists  of  9,720  distances  corresponding   to the  above
parameters.   For example,  the array point  (1,  1,  1,  1)  is the  distance  away
from a 600 kHz, 0.1 wavelength, 0.1  kW  station  at  which the  fields drop below
10 V/m (E < 10 V/m and  H*377  < 10 V/m).
                                      62

-------
    Impact of the various guidance  levels  on  the  AM service was found using the
 above array.  Each station  in  the  AM  data base was considered individually and
 its  power,  frequency,  and  electrical  height used  to  choose  distances from the
 array.   In  cases  where the frequency was  not  one of the  modeled  values (0.6,
 0.8, 1.0, 1.2,  1.4,  1.6  MHz),  the  next highest of the  modeled values was used
 since field  levels may increase  at higher frequencies.   For electrical heights
 other  than  those modeled,  the distances  for  the next  lower and  next higher
 electrical  heights were  compared and the  largest value was  used.   The result
 was  that eighteen distances corresponding  to  the alternative guidance levels
 were  assigned  to  each  station,  and  then summarized  in  a  table  (Table  45)
 showing  the numbers  of  stations  requiring various  property  restrictions  at
 each guidance level.   The  table  also  shows how many  of  these restricted areas
 are within the ground  radials  of the  stations  (estimated to be 0.25 wavelength
 long).

   The results  of  the AM modeling are  shown  in  Table 45.   It  is  important  to
 note that the 18  field strength  levels  in the row headings  are different from
 the  18 alternative guidance levels examined for  FM stations.   The  reason  for
 this difference is that  the proposed  guidance levels for this  frequency band
 are  given  in field  strength  units  rather  than  power  density units  and  are
 likely to be higher than the levels applicable to FM frequencies where maximum
 energy absorption  rates in the human  body occur.   These  AM  field  strength
 values were chosen to  be a factor  of  five greater than those  used  for the  VHF
 spectrum on  account  of these  absorption  differences.   Distances  shown  in  the
 table are in meters  and the double entries in  each row  show the  numbers  of
 stations requiring fences  to that  distance and guidance level:   1)  within  the
 ground radials (estimated to be 0.25 wavelengths  in  length),  and 2)  beyond  the
 ground radials.   This table was provided to LLNL  for economic analysis.

   Examination of Table 45  shows that only at  the lowest guidance  levels do  AM
 stations  present a significant problem.   Some  stations  would  exceed  the lowest
 level,  10 V/m,  to distances of  280 meters  from the tower.   It  is  unlikely,
 however,  that guidance levels  this  low would be recommended  in  the AM  band
 since the body  absorbs  energy inefficiently  at  these  frequencies.   At  field
strength  limits  of 173 V/m  and  above,  only a few  stations  can  exceed  the limit
at distances greater than 20 meters.   It  should  be possible to  exclude  public
access  to these  areas with  fences in most  cases.

                                      63

-------
TABLE 4b.  NUMBEKS UF AM bTAliUNi KEgUlKINb FENCES  Al  VAKlUUb DISTANCED TO EXLLUUE  AKEAS  IN WHICH FIELD SIKENbTHS EXCEEl)
     18 POSSIBLE GUIDANCE LEVELS.  DUUBLE  ENTRIES  IN EACH KUW SHOW WHETHER THE  KEUUIKEU FENCING DISTANCE IS WITHIN
                OK BEYOUU THE EXTENT  OF  THE MOUND  KAOIALS (ESTIMATED TO BE ONE-QOAKTEK WAVELENGTH LONU)
Distance from
tower (meters)
2-20
20-40
40-60
60-80
80-100
100-120
120-140
140-160
160-180
180-200
200-220
220-240
240-260
260-280

within
beyond
within
beyond
within
beyond
within
beyond
within
beyond
within
beyond
within
beyond
within
beyond
within
beyond
within
beyond
within
beyond
within
beyond
within
beyond
within
beyond
10
155
0
2744
0
342
57
319
651
60
17
10
lib
0
0
0
1
0
)
0
u
0
7
0
102
0
30
U
10
Field strength limits (V/m)
31.6 44.7 70.8 8b.b 100.0 141.3 173.2 200.0 223.9 244.9 264.6 2«1.8 300.0 316.2 446.7 70b.O 1000.0
3249 3631 4389 4465 45U2 4566 4614 4619 4619 4619 4619 4619 4620 4620 4621 4622 4622
00000000000000000
1215 902 231 157 120 56 3 3 3 3 3 3 2 2 1
000000000000000
68 71 2
0 17 0
45
44
1
0



•The field limits shown in the top row are those values which would correspond to the example radiation
protection guidance frequency response curve illustrated in Figure 1 for frequencies less than 6 MHz
(page 4).

-------
     Table 46 presents  some  of the same data  as  shown  in Table 45 but  with  a
finer resolution for distances close to the tower.  This  table  provides  a more
detailed look at  the  fencing distances which  would be  required at the  higher
guidance levels.   Entries in  the  "0 meters"  row are  stations  which did  not
reach the  field  strength levels shown  in  the column  headings  at the  closest
calculation  point  (2   meters).   Higher  fields  are  possible  closer  to  the
tower.   More  information about  the  AM  modeling results  can  be  found  in
Appendix D.

6.  Impact on TV Stations

     Television broadcast  antenna  systems  are similar to  FM  systems  in that
they typically consist  of an array of  radiating  elements mounted on  a  tower.
The elements  of  TV antennas,  however,  tend to be more complex in design  and
direct  less  energy towards  the ground.   The  towers  for  these antennas  are
generally  higher  than  FM towers,  further  reducing the  net fields produced  at
ground  level  (see  Figure 20).   There  are approximately  1,100 VHF  and  UHF
licensed American TV stations  in the FCC's  TV  Engineering Data  Base,  excluding
low power  stations.   It was  not possible  to  use  the same  modeling  techniques
for TV's  that were used  for FM stations  because measured elevation  patterns
throughout 360 degrees  of elevation  for TV's  were not  available.  Measurements
of TV elevation patterns  could  not  be  performed within the time frame  of this
project.   Instead,   available  information   was  examined  to  identify   an
alternative approach.

     VHF and  UHF  antennas must  be  considered  separately because of  differences
in their design and radiation patterns.  Manual examination by  EPA  of a sample
(approximately  10 percent)   of  the  FCC TV physical  files maintained  at  FCC
headquarters  revealed  that   the  batwing  element  is  most   common  for  VHF
broadcast.   In  the interest  of  time  and  simplicity,  it  was  assumed  for
purposes of  this  study that  all VHF TV antennas  were  of the  batwing  design.
One  reference by  the   inventor  of  this  antenna  contains some  measured  and
calculated elevation  patterns for  a single element  [14].   We  compared these
data to EPA field  study data and  a single  measured elevation pattern obtained
from one antenna manufacturer.  These data  indicated that batwing  elements  may
radiate approximately 20 per  cent  as much  in  the downward direction  as  in  the
                                      65

-------
                            TABLE 46.  NUMbtKS OF AM STATIONS REQUIRING FENCES AT VAK10US DISTANCES TO EXCLUDE AKEAS
                                         IN WHICH FIELD STRENGTHS EXCEED 18 POSSIBLE GUIDANCE LEVELS.
Distance from
tower (meters)

 0-2
 2-6
 6 - 10
10 - 14
14 - 18
18 - 22
'ft - 26
26 - 30
30 - 34
34 - 38
3d - 42
42 - 46
46 - 50
10.0
0
0
0
70
85
473
382
293
1150
446
219
40
38
31.6
0
109
i093
1799
248
204
684
135
50
142
54
5
1
44.7
1
846
1932
591
261
625
130
130
11
6
1
15
8
70.8
7b
2680
533
962
138
95
59
13
11
53
1
0
1
86.6
129
2949
834
448
105
91
0
63
1
1
1


100.
431
2673
1125
194
79
54
48
16
1
1



Field
.0 141.
909
2372
1087
140
58
54
1
1





strength
3 173.2
1222
2947
299
84
67
2
0
1





limits
200.0
1241
2988
259
131
1
1
1
0





IV/m)
223.9 244.
2933 2960
1440 1454
135 135
111 70
2 2
0 1
1
0





,9 264.6 281.8 300.0 316.2 446.7 708.0 100C
3070 3075 3076 3098 3202 4251 4447
1362 1374 1375 1354 1305 368 0173
119 166 164 Ib7 112 2 2
68 4 5 1 2 1 0
22111
1111







•The field strength  limits  shown  in  the  top  row  are those values which would correspond  to  the example radiation  protection  guidance frequency
response curve illustrated  in Figure 1 for frequencies  less than 6 MHz (page 4).
"The numbers  shown  in the U-2 meters row  represent  the  number of AM stations  not  exceeding the specified  field  strength levels shown  in  the
column headings, for distances up to 2 meters.

-------
               LOM VHT TEUviaioN
II
        I    If
                                                                            UHT TELEVISION 8THTION9
                                                           II
          I        I       I        !        *       i
                          »....«
                   TOME* HEIGHT (FT)
'«?!?!
          TOME* HEIGHT (FT)
               HIGH VHT TELEVISION STATIONS
IS
                             lir
                                                                           1M3 TELEVISION 8THTIONB
                                                           18
         *       «       •       !       ?       5
                   TOMER HEIGHT CFT>
2       I       S       I       J

         TOMER HEIGHT CFT>
      Figure 20.   Distributions  of tower  heights for stations  in  the TV data  base.

-------
main beam  in terms  of relative  field strength.   As  a  more thorough  check,
extensive modeling  of typical  batwing elements  when  grouped  in a  broadcast
array was accomplished using the LLNL NEC code [8].  An  individual  channel  2-3
antenna element was modeled  at the channel  2  frequency and  additionally  when
used in 4,  6, and 8 bay configurations.  Similarly, a channel  7-13  antenna was
modeled at channel 10  in  the  same  configurations.  The results agree  with  the
other studies indicating  downward  electric  field of approximately 20  per  cent
of main beam values.   Variability in the  amount of downward  radiation  occurs
because of increased coupling  as  the  number of  elements  increases  and because
the  same   physical   interbay  spacing   is   used  for   several   channels.
Consequently, the relative spacing for a channel  7-13 antenna used  for channel
7  will  be  different  than  when  the  antenna  is  used  for  channel   13.   The
relative  size of  each  element also  varies  when different  frequencies  are
broadcast.

     The FCC automated TV Engineering Data Base  contains  no information  on the
type of  antenna  or number of  bays.   Thus, detailed modeling is not  possible
even when elevation patterns  are  available.   It was considered  sufficient  for
this study  to use the typical  values  of downward  relative field strength  at
-90° elevation or directly down, which represents  the shortest distance  to the
ground.  Other directions would involve a  greater transit  distance  and predict
lower fields on the ground.  The following equation was  used  to  predict  fields
at the base of a TV broadcast antenna:

          S = [(0.4 * VERP) + AERP] * F2 * 2.56 * 1.64  * 100           (4)
                          4 * Bl * D2
                              V                                  2
S = highest power density likely to occur near the ground in pW/cm
VERP = ERP of the video signal in watts
AERP = ERP of the aural signal in watts
F = typical relative field in the downward direction (-60° to -90° elev.)
D = the distance from the ground to the center of radiation in meters
1.64 corrects for the gain with respect to an isotrope
2.56  is the  possible increase in power  density  due  to ground  reflections
(assumes a field reflection coefficient of 1.6)
                                      68

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     The factor of 0.4  appearing  with the VERP corrects for  the fact that  TV
stations video  power  is specified  in terms  of peak  visual  ERP  and the  0.4
factor converts this  to an RMS value for most practical  conditions of  video
programming.

     The aural ERP and tower height were  added  to  the data base manually from
the TV  Factbook  [2].   Tower heights  listed  in the  Factbook  (and FCC  written
files) are the height  to the top of the tower and not  to the  antenna center of
radiation.    Examination of  diagrams  accompanying  applications  in  the  FCC
written files  showed  the range of  differences between  these heights and  the
heights to the centers of radiation.  Averaging these  differences  for low VHF,
high VHF,  and UHF  stations  gave the following correction factors:

         D = T - 50 (ft)      (Low VHF)
         D = T - 70           (High VHF)
         D = T - 40           (UHF)

where:  D = the height above ground of the center  of radiation in feet
        T = the overall  height of  the tower in feet

     When utilizing these corrections  in  the model, a minimum tower height of
less than 30 ft. was  never  permitted.  This  assumption was based  on EPA field
experience.   In  some  cases,  antennas may be  mounted  at  other  places  on  the
tower instead of  the  tower top especially if  several  TV  antennas  are  mounted
on a  single  tower.  However,  this  was  impossible  to  determine  for  each case
with  available data.    Experience  has  shown  that TV  antennas  are  usually
located near,  if  not  at  the  top  of the  tower.   When  FM  antennas   and  TV
antennas are  mounted  on the  same  tower,  the TV antenna  is  normally found  at
the top.

     The model  uses  a   value  of   18  percent  relative field strength   in  the
downward direction as  compared to  the  main  beam   value.    It  also uses  the
shortest  distance  (straight   down)  and  makes no   allowance for  fencing  or
exclusion of the area around the base of the tower.  The presence  of the tower
itself will further reduce fields below the  predicted  values.  In  general,  the
                                      69

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model should tend to overestimate the  impact  of  the guidance due to  the above
factors.

     Various mitigation  strategies  were examined in  order  to determine which
were the most practical  and economical.  After evaluating the effects of these
possibilities and  discussing  them  with  industry consultants,  it was  decided
that a  change of  antenna  and/or an  increase in  tower height  were the  best
choices  of  those which  could be  evaluated  through  modeling.   Other  methods
such  as  fencing the  area  which  exceeds  the  guidance  may  often  be  more
economical,   but  are not amenable  to  modeling  with  available  data.   It  was
assumed  that the  least  expensive,  effective  mitigation  strategy  would  be
chosen in each case.  Thus, other alternatives such as  fencing will  reduce the
impact of the guidance  when they are feasible.

     The concept of antenna change for TV's was  not as  straight  forward as the
similar case for FM stations  since  exact patterns  for the various types of TV
antennas were not  available.   As an alternative approach,  the results  of the
NEC  modeling of arrays  of batwing  elements   were  reviewed.  At  an  interbay
spacing of 0.833 for a six  bay  array these  results show downward radiation as
low  as  10 percent  of  the main beam  value on  a  relative field basis  with very
little  reduction  in main  beam  gain (Figures  21 and 22).   Although a single
element  produces  about  20  percent  relative  field  in the downward  direction,
array coupling  and interference  effects  can   significantly  reduce  this value
depending on the relative interbay spacing.

     Since a single array with the  same absolute interbay spacing can  be used
for  any of several channels,  the relative  interbay spacing will depend on the
frequency or channel at  which the station  is operating.  The  implication here
is that  an array can be  custom  designed to  minimize  downward radiation at any
single channel.   In order to  verify the validity of this concept, the idea was
discussed with  a major  TV antenna  manufacturer.   Engineers  at this  company
indicated that  they had  in fact designed  arrays  as described  above  for the
purpose  of  minimizing  interference  between  their   antenna  and other antennas
located below it.  These custom  antennas cost  about 2.0 to 2.25  times the cost
of a standard antenna.   Using the above considerations, it  was  estimated that
downward radiation of 7 per cent (field) of  the main beam could be obtained.
                                      70

-------
         CETEC JAT TURNSTILE ANTENNA  6 BAYS  CHANNEL 2

              SPACING OF .8333 LAMBDA BETWEEN BAYS
                 120
       ISO
           PATTtRHGAIM IH OBI
         ........ HORIZONTAL
         — •     VIHTICAl
                 TOTAL

         CLCVATION ANCLC
                                90
                                               60
                                                          -30
                                                -60
-90
Figure 21.  NEC model  results for a typical  6-bay batwing  TV antenna.
                                71

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               ©Off7*
C JAT TURNSTILE ANTENNA  6 SAYS  CHANNEL 2



SPACING OF .8333 LAMBDA  BETWEEN BAYS
               li
             -*-   •„;  '  o.o*
                                                     \
             --•   .„  "AW      .».... ^  ~8).0«      ... .:... ^,
             -»r                -«»?                -»Y

                                                     ».
                           "I
                                -».    ^.   iSb.O'     .^    -n   i&.o
                                  •*•            ,      •*•
Figure 22.   NEC model results  for a typical  6-bay batwing TV antenna.
                                  72

-------
     The  second  mitigation strategy  was an  increase in  tower  height.    The
minimum tower height necessary to bring  a  station below a given power  density
level was found using the following  equation:

        MTU   1 /[(.4 * VERP)  + AERP] * F2  * 2.56 * 1.64 K u>o           m
        MTH = l/u	   4 * n * S(b)

where MTH - minimum tower  height  (ground to center of radiation)  necessary to
bring the station below a power density, S  (same units as equation  4).

     Prediction of the potential impact  on  VHF  TV stations began with  equation
(4) to determine which stations would  be likely to exceed  a  given alternative
guidance  level  in their  present  configuration.  The  18 guidance  levels  used
for  television  stations  were  the  same  as  those used for FM radio  stations.
This assumes that  the frequency dependence  of the proposed guidance is  of the
shape  shown  in  Figure  1,  i.e.,   a  constant  exposure  limit   from  30  to
1,000 MHz.  Application  of a  ramp  function  for  the  guidance for frequencies
greater  than  300  MHz,  similar to  that used in  the ANSI  guide   [15],  would
result in reduced  impact  compared  to the results obtained  with this  approach.
Fields  from these  stations were then re-calculated   using  equation  (4)  with
F = 0.07  representing a  change  of  antenna.   A  notation was  made  on  each
station  file  indicating  for  each  alternative  guidance  level   whether  the
station was already  in compliance or could be brought into compliance  with an
antenna change.  All stations  predicted  to  exceed the guidance level  were also
subjected to equation  (5)  to  determine the required tower  height  to  bring the
station into compliance.   Equation  (5) was  then  re-calculated with F  = 0.07 to
determine the  increase  in tower height  required  if the  station employed both
fixes.   In  other  words,  if a  station required  500  feet of  additional  tower
height to come  into compliance with their  present  antenna, they  may  elect to
purchase  a  new  antenna  and increase  their tower height  by  a lesser  amount.
For  the  economic  analysis,  each  TV  station was  analyzed  to  determine  the
minimum-cost compliance  measure that  would  achieve  the  required  reduction in
field  strength  levels.   Results of  the TV analysis  were provided to  LLNL on
magnetic tape in the form of tables.  An example  is shown below.
                                      73

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                                                            2
                 Sample of VHP TV stations exceeding 1 mW/cm

  Present          Compliance        New Tower Height        New Tower Height
Tower Height     with Change of      Required without          Required with
	      Antenna Only       Change of Antenna       Change of Antenna

     54 ft.           Yes                   255 ft.                 54 ft.
     90               No                    322                    170

     UHF  stations  were modeled  with  the  same  equations  as  for  VHP stations
described  above  but  using different values  of  F, the  relative  field strength
in   the   downward   direction.   Values   of   F   are   not  available   in  the
manufacturer's literature at  the large  depression angles needed  for this study
and  cannot be  determined  using  wire  codes such  as the  LLNL NEC  because of the
large  surfaces involved  in  the antenna  design.   Slotted  waveguide  antennas,
for  example,   cannot  be  accurately  modeled  using  NEC.  As  an  alternative
approach,  field  study  data  were reviewed and this  question was  discussed with
a major  UHF antenna  manufacturer.   The  manufacturer's engineers  stated that
typical values of F are about 10 percent  and that some more expensive antennas
have an F  of about 5 percent.   These  values  agreed with EPA's own measurements
underneath  operating  UHF  stations  which  indicated  an  F  of   less  than  10
percent.   Although  the above information  provides a  limited  basis  for  F for
UHF  antennas,  it is  reasonable that F  should  be  small  for these antennas for
two  reasons.   First,  UHF  antennas  have very   high  gain  in   the   main beam
indicating that  a large portion  of  the  transmitted energy is  contained in this
beam  rather  than   other   directions.    Second,   the   large  vertical  surfaces
incorporated in  these antennas tend to eliminate  downward radiation.

     UHF  stations  were  thus modeled  using  an  F  value  of  10  per  cent  for
stations  in their present configuration and assuming  that  this  value could be
reduced to 5  percent by  a  change  to an  antenna  of different design.   As for
VHF  stations,  the  power density values  at  ground level  were predicted at the
present tower  height  with and without  a  change of antenna.  The increases in
tower  height necessary  to bring the stations into  compliance at each guidance
level were also  calculated with and without a change of  antenna.
                                      74

-------
     The TV modeling results  are  shown  in Table 47.  The  eighteen  alternative
power density levels are the  same  as  those  used  for FM stations, but  fewer TV
stations are  impacted  than FM's at  all  power density  levels.   The number of
potentially  impacted  TV  stations  drops off rapidly  with  increasing  power
density  limits  until   zero  impact  is  predicted  for   levels   above  1,000
     2
pW/cm .  Cost  analysis results  are  discussed  in  the  economic impact  report
[1J.
                                     75

-------
TABLE 47.  NUMBERS OF TV STATIONS PREDICTED TO BE IMPACTED
              AT  18  POSSIBLE  GUIDANCE  LEVELS
Number of Stations Predicted to
Exceed Power Density Levels
Power Density
Levels pW/cm<-

1
10
20
50
75
100
200
300
400
500
600
700
800
900
1,000
2,000
5,000
10,000

VHP
390
117
96
47
29
25
16
9
6
5
3
2
1
1
1
0
0
0

UHF
429
129
87
55
34
35
14
10
7
5
2
2
2
2
1
0
0
0

Total
819
246
183
102
73
60
30
19
13
10
5
4
3
3
2
0
0
0
Percentage of
All TV's

75.8
22.8
16.9
9.4
6.8
5.6
2.8
1.8
1.2
0.9
0.5
0.4
0.3
0.3
0,2
0.0
0.0
0.0
                             76

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                                  REFERENCES
 1.   Hall,  C.  H.  (1985).   An Estimate  of  the  Potential Costs  of  Guidlines
     Limiting  Public   Exposure  to  Radiofrequency  Radiation   from  Broadcast
     Sources, Lawrence Livermore  National  Laboratory,  Livermore,  CA.

 2.   Television and   Cable  Factbook  (1982-1983).   Television  Digest,  Inc.,
     Washington D.C.

 3.   Federal   Communications   Commission  (1980).    Rules   and   Regulations,
     Volume III,   Part 73-Radio  Broadcast  Services,  subpart   B-FM  broadcast
     stations.

 4.   Kraus, 0. 0.  (1950).   Antennas.  McGraw-Hill  Book Co., New  York,  NY.

 5.   Jordan,   E.  C.,  and  K.  G.  Balmain  (1968).   Electromagnetic  Waves  and
     Radiating Systems.   Prentice-Hall,  Inc.,  Englewood Cliffs,  New Jersey.

 6.   Micro  Communications,  Inc.  (1983).   Element Pattern Measurements on  FM
     Antennas for  EPA Contract  No.  68-03-3054.    Micro  Communications,  Inc.,
     Manchester, NH.

 7.   Burke, G. J.,  and  A. J.  Poggio (1981).   Numerical  Electromagnetics  Code
     (NEC)  - Method of Moments,  NOSC Technical Document  116  (TD 116)  Vols.  1
     and 2.  Naval Ocean  Systems  Center,  San  Diego, CA, January.

 8.   Adler, R. W., and S. Lament (1984).  Numerical Modeling Study  of  Gain  and
     Downward Radiation for Selected  FM  and  VHF-TV Broadcast Antenna  Systems.
     AGL Inc., Pacific Grove,  CA.

 9.   Donnelly  Marketing   Information  Services   (1980).    1980  Master   Area
     Reference  File  with  Propriatory  Geographic  Coordinates.    Information
     Services Division,  Advanced  Demographic  Systems,  Standford,  CT.

10.   Snider,   J.   B.   (1965).   A  statistical   approach to  measurement  of  RF
     attenuation by building material.   NBS  report 8863, July.

                                      77

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11.  Tell, R.  A.,  and  N.  N.  Hankin  (1978).   Measurements of  radiofrequency
     field intensity in  buildings  with close proximity to  broadcast  stations.
     Technical   Note  ORP/EAD  78-3,   U.S.   Environmental   Protection   Agency,
     Las Vegas, NV,  August,  (NTIS order no.  PB 290 944).

12.  Federal    Communications   Commission   (1980).    Rules  and   Regulations,
     Volume III, Part 73 - Radio  Broadcast  Services,  subpart B -  AM  broadcast
     stations.

13.  FCC. RADIAT  computer  program  for  computing  near-field and  re-radiation
     patterns  for  AM  radio  stations.   Written  by Phillip  Tremper,  Federal
     Communications  Commission,  and Elton Davis.   Date unknown.

14.  Masters,  R.  W., 6.  Sato,   H.  Kawkami,  and M.  Umeda  (1979).   Study  of
     Batwing  Radiator of the Superturnstile  Antenna for TV  Broadcasting.   IEEE
     Annual  International Symposium on Antennas and  Propagation.

15.  ANSI (1982).  Safety  level  of electromagnetic radiation  with respect  to
     personnel, American National  Standards  Institute,  C95.1-1982.
                                      78

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                                  Appendix A
                         Development of the FM Model
Section 1.  Pattern measurements of FM antenna elements.

    Complete elevation patterns  for commercial FM  broadcast  antennas  are  not
generally available.  Only a few degrees of  elevation  pattern  illustrating  the
shape  of  an  antenna's  main  beam  can  be  obtained from  most  manufacturers.
Broadcasters have  little  interest  in the rest  of  the elevation  pattern  since
the  energy  transmitted  outside  of  the  main  beam  is seldom  involved in  the
station's  coverage.   Strictly  speaking, the  complete  elevation  pattern  is
important in terms of efficiency.  Energy that  is directed  outside  of the main
beam is wasted and may present a potential  exposure  problem.

    The main  beams of most FM  antennas  subtend less  than  30 degrees  and  are
directed  approximately  in the horizontal plane.  Consequently,  the  energy  in
this beam  intercepts  the  ground at  distances  ranging from several  hundred  to
several thousand  feet from the antenna.   The exact  distance  will  depend  on
beam width,  tower  height,  beam  tilt,   and  terrain.  By  the  time  this  beam
                                                                             2
reaches the  ground  its  power  density  is  low,  usually less  than  10  uW/cm .
Thus,  in assessing the impact of alternative  guidance  levels,  the main beam is
not  of major concern except  at very  low  levels.    It  is  worthwhile  to  note,
                                                                        2
however,  that enforcement  of  alternative  levels   less  than  10 yW/cm  would
require a  departure from the methods  of FM  broadcasting  currently  in use  in
the  United States  unless  many stations  were able to relocate  to remote sites.
Otherwise, the energy in  the  main beam  of  many broadcast  stations  would have
to be  reduced and  radio coverage would be affected.
                                     2
    Guidance  levels  above  10  yW/cm  are  generally  only exceeded  in  areas
near the  broadcast tower  and  by energy  outside  of  the main beam.   Bringing a
station  into compliance  with  these guidance  levels  can  be accomplished  by
changing  factors   other  than  the  main  beam.   The  audience   coverage of  the
station can  be  maintained using the proper mitigation  strategy.   Figure (23)
below  shows  the pattern for an  array of one-half wave dipole  elements.  These
                                      79

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elements are similar to those used  In  FM broadcasting, and help to  Illustrate
the points discussed above.
                              Main Beam
                                                •*• Main beam
                                                  intercepts ground
                                                  far from tower.
                              Other beams
                              not involved in
                              audience coverage.
Figure 23.  The main beam of  an  FM broadcast antenna typically intercepts  the
ground at distances far from the  tower.

    The pattern of a real broadcast antenna consists of two components.  These
are an element pattern or the pattern of  a  single element when it is  isolated
from other elements, and an array pattern which results from addition  of waves
from  an  array  of  point  sources.   These  two  patterns  must be  multiplied
together to obtain the total pattern of  the antenna.   Array patterns  are easy
to generate using geometry and phase considerations  and are available in many
textbooks.  Element patterns,  however cannot be predicted  in any simple way.

    Element  patterns  for  five   commonly  used  FM  broadcast elements  were
measured  via  contract [6].   Measurements of  the elements were  performed  to
determine   their   elevation  patterns   In   several   configurations.   These
configurations  were   free  space,  side-mounted   on   a  tower  section,   and
leg-mounted on a  tower  section.   The  pattern  of  an  element  Is   partially
dictated  by the way  it 1s  mounted because  of  interactions  with  supporting
metallic  structures.   By  measuring  the patterns in  these  three   different
configurations, it  was possible  to obtain  some  understanding  of the pattern
                                      80

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 variations that may occur  due to the  variety of mounting  configurations used
 by broadcasters.


     The patterns  were determined by rotating  the elements  and tower section on
 a   25  ft.  dielectric  support  and   recording  the   element's  output  while
 irradiating  it with  a reference antenna.  This received  pattern is the same as
 the element's  transmitting  pattern.    The  direction  of  rotation  determined
 which pattern  was  recorded  as shown  in figure  (24).  Efforts  were  made  to

 minimize  ground  reflections  which  can  affect  measurements  and  obscure  the
 pattern.
                               Test Antenna
                Head
               Rotation
                 for
               Azimuth
               Pattern
                                              Tower
   Up
 Direction
for Normal
 Broadcast
 Position
                                         I Turntable Rotation
                                         I  for Elevation
Figure 24.  Support configuration used to measure element patterns.


    Patterns were measured in four elevation  planes for each  element in  each

configuration.  Figure  (25)  below shows  an elevation pattern superimposed  on  a
sketch of a tower and  single element.   An elevation  pattern can be thought of

as a polar plot of field intensity in a vertical plane.
                                      81

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                                              — 0°
Figure 25.   Side view of single element mounted on tower.  The curved line is
the elevation pattern of the element in the plane of the page.

    The pattern  in  Figure (25)  shows that more  radiation is  emitted  at o!
elevation  then  at  -90" for  this elevation  cut.  The  four  elevation cuts
measured are illustrated in Figure (26) which  shows a top view of a  broadcast
element when mounted on a tower for operation.

    The dashed lines in Figure (26)  represent  edge views of the planes  of  the
four elevation patterns measured.  The  0*  -  180° elevation pattern  (or cut),
for example, shows  the  radiation  emitted directly  in  front of and in back of
the element.  This is the pattern illustrated  in the previous  Figure.  The  90!
-  270!  elevation pattern  shows  the  radiation emitted  to the  sides   of  the
element.   Although  the  total  pattern  of  an   element  is  a  three-dimensional
solid  angle plot of the element's radiation  in  all  directions,  these  four
elevation slices provide a good indication  of the shape of the total  pattern.
                                     82

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                                    90°
                     135°
               180°
                        \
                          \
                           Tower  ,'
                     225°
                                                  45°
                                          Element
                                                 S315°
                                   270°
Figure 26.  Top  view of a  single element mounted on  a tower  for broadcast.
Dashed lines  represent  edge views of  the elevation planes  discussed  in  the
text.

    Both horizontally and vertically  polarized  signals were  measured  in each
plane to  fully  characterize  the elements.   Thus, a  total   of 24  elevation
patterns were  found for  each element as shown below.
                   Vertical  polarization
                        free space
                        leg-mounted
0 - 180°,  45° - 225°
90* -270", 135° - 315'

0 - 180°,  45" - 225°
90° -270°, 135° - 315'
                                     83

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                        face mounted      0 - 180°,  45°  - 225°
                                         90° - 270°,  135°  - 315°

                   Horizontal  Polarization

                        free space       0 - 180°,  45°  - 225°
                                         90° - 270°,  135°  - 315°

                        leg-mounted      0 - 180°,  45°  - 225°
                                         90° - 270°,  135°  - 315°

                        face-mounted      0 - 180°,  45°  - 225°
                                         90° - 270°,  135°  - 315°

    An example pattern  is  shown  in Figure  (27).   See  the final  report to  EPA
contract number 68-03-3054  for  the complete  set of  patterns  and more  details
on the measurement methods [6].
                                      84

-------
PATTERN: ElEVATIOM

ANTENNA LOCATION
ON TOWER: fRgE SPACE

TRANSMITTING ANTENNA
POLARIZATION:  VERT.
                                +45'
  0'.
                                        +90"
.180*
HEIGHT OF TRANSMITTING
ANTENNA:       Ifi.Sf^

DIST. FROM TRANS. ANT.
TO TEST ANT. :  I3/ FT.
-45'
    ELEVATION CUT
                 RING RADIUS:
                            :   21.5"
         Figure 27.   Measured elevation  pattern of a  single element.
                                       85

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Section 2 - Pattern Reduction for  Incorporation in the Model

     The electric and magnetic fields created by  an  FM antenna vary from point
to point around the tower.  The total pattern of the  antenna  is generally not
symmetrical  around the  tower.   Consequently,   the   four  measured  elevation
slices  for a  single element  in  one configuration  are  shaped  differently.
Fields produced 10 feet  from the  tower in one direction  will  differ from those
produced ten  feet  from the  tower  in  another  direction.   From  a  Guidance
standpoint,  the  highest  field  produced  anywhere  near  the   ground  is  the
limiting quantity  since Guidance  levels dictate maximum  permissible  limits
rather than typical  values.

     Prediction of the highest fields produced near  the  ground requires use of
the worst elevation patterns.  The pattern showing  the  highest relative field
strength at a given angle will produce the highest field at the distance from
the  tower  corresponding to  that angle.   Figure (28)  below  shows  a  single
non-symmetric elevation  pattern and antenna to illustrate this concept.
Figure  28.    Although  Pj  and  P2  are  the  same  distance  from  the  tower,
fields at P  are more  intense because of the shape of the element pattern.
                                     86

-------
     At an angle QA  from the horizon, the  right-hand  half  of the pattern 1s
more Intense.   Points  P-, and  P£  are  the  same distance  from the  tower but
the field strength produced  at PI  is  higher  than that produced  at P2.  The
right-hand half  of the  pattern is  the  important  one at  angle  eA  from an
impact point of view.  At angle 8g  in  Figure  (29),  the left hand half of the
same pattern  produces a greater field strength on the ground.
Figure 29.  Fields are  more  intense at P^ than at P-j.

     Thus, for impact analysis,  a model  must use the highest value of relative
field  strength  found among  the  available  elevation  patterns  at each  angle.
These worst case values can be found by overlaying  the  elevation  patterns  and
drawing an envelope as  shown in Figure (30).

     A single envelope  pattern was constructed using the 12  elevation patterns
for  each  polarization  of  each   element.   By  combining  these  patterns  for
different directions and  configurations, an approximation  to  the worst case
fields likely to occur  under actual  broadcasting conditions  was obtained.   The
final  step in  reducing these patterns was  to combine  the  two halves of  the
envelope to produce an envelope  pattern for  a single direction away  from  the
tower as shown in Figure  (31).
                                     87

-------
        180
                                    270
Figure  30.   The dotted  line illustrates  the  envelope of  several  elevation
patterns of the same element.

     Only the bottom half of this  envelope  was  used in modeling  since  the top
half is  not  involved  in  field levels on the ground.   After  the impact study
was completed, it was  found that the  top half of the pattern  can be important
if the element is mounted upside down as is sometimes the case.   However, the
top and  bottom halves of  the  final  envelopes  are  similar  in  shape  and the
above oversight does not  introduce a significant error.
                                     83

-------
Figure 31.  Envelope for a  single direction away from the tower.

     A  single   quadrant  envelope   (Fig.  32)   was   constructed   for   both
polarizations of the five  antenna elements  in the study.   These  ten envelope
patterns were normalized to unity at the  horizon  and digitized at five degree
intervals for use in the model.   Tables  48 through 51  show the data points for
both polarizations of each  element.

     The  digitized  patterns  as they  were used  in the  model  are  plotted  in
Figures 33-35.  Some of the patterns will  be  noted to  have values  greater than
unity  at  certain  angles.   This  indicates  that,  when  mounted  singly,  some
elements  emit more  radiation  at  these  angles  than  in   the  main  beam  or
horizontal direction.   When the  elements are  mounted  in arrays, this effect is
usually obscured by the array  factor.
                                     89

-------
                                   90°
                                              Normalized to
                                               unity at 0°
                                   -90°
                                    or
                                   270°
Figure 32.  Final envelope for one polarization of  a  single element.
                                       90

-------
               TABLE 48.  DATA POINTS FOR TYPE 1 ELEMENT MODEL
         Angle              Vertical  Polarization      Horizontal  Polarization
(Degrees Below Horizon)    (Relative Field  Strength)    (Relative  Field  Strength
0
-5
-10
-15
-20
-25
-30
-35
-40
-45
-50
-55
-60
-65
-70
-75
-80
-85
-90
1.00
1.00
1.00
1.02
1.12
1.20
1.23
1.23
1.02
1.12
1.15
1.18
1.12
1.12
1.05
1.02
0.98
0.85
0.81
1.00
0.98
0.95
0.85
0.79
0.76
0.65
0.62
0.55
0.47
0.42
0.39
0.37
0.33
0.30
0.27
0.24
0.21
0.19
                                     91

-------
               TABLE 49.  DATA POINTS FOR TYPE 2 ELEMENT MODEL
         Angle              Vertical  Polarization      Horizontal  Polarization
(Degrees Below Horizon)    (Relative Field Strength)    (Relative Field Strength)
0
-5
-10
-15
-20
-25
-30
-35
-40
-45
-50
-55
-60
-65
-70
-75
-80
-85
-90
1.00
0.98
0.85
0.81
0.78
0.65
0.55
0.49
0.44
0.42
0.38
0.35
0.32
0.28
0.25
0.21
0.17
0.13
0.11
1.00
1.10
1.12
1.23
1.23
1.20
1.12
1.00
0.87
0.68
0.50
0.40
0.28
0.20
0.11
0.06
0.03
0.02
0.03
                                      92

-------
               TABLE 50.  DATA POINTS FOR TYPE 3 ELEMENT MODEL
       Angle                Vertical  Polarization       Horizontal  Polarization
(Degrees Below Horizon)    (Relative Field  Strength)    (Relative Field Strength)
0
-5
-10
-15
-20
-25
-30
-35
-40
-45
-50
-55
-60
-65
-70
-75
-80
-85
-90
1.00
1.05
1.02
0.98
0.91
0.89
0.72
0.60
0.48
0.39
0.34
0.28
0.21
0.16
0.11
0.07
0.05
0.03
0.03
1.00
1.00
0.93
0.89
0.81
0.71
0.65
0.63
0.56
0.50
0.40
0.32
0.23
0.16
0.12
0.09
0.05
0.03
0.03
                                     93

-------
               TABLE 51.  DATA POINTS FOR TYPE 4 ELEMENT MODEL
       Angle                Vertical  Polarization       Horizontal  Polarization
(Degrees Below Horizon)    (Relative Field  Strength)    (Relative  Field  Strength)
0
-5
-10
-15
-20
-25
-30
-35
-40
-45
-50
-55
-60
-65
-70
-75
-80
-85
-90
1.00
0.98
0.95
0.91
0.89
0.89
0.89
0.81
0.74
0.63
0.51
0.41
0.33
0.25
0.19
0.14
0.10
0.08
0.07
1.00
0.91
0.93
0.91
0.91
0.93
0.91
0.83
0.66
0.51
0.42
0.39
0.32
0.28
0.19
0.14
0.10
0.06
0.06
                                     94

-------
               TABLE 52.  DATA POINTS FOR TYPE 5 ELEMENT MODEL
       Angle                Vertical  Polarization       Horizontal  Polarization
(Degrees Below Horizon)    (Relative  Field  Strength)    (Relative  Field  Strength)
0
-5
-10
-15
-20
-25
-30
-35
-40
-45
-50
-55
-60
-65
-70
-75
-80
-85
-90
1.00
1.00
0.89
0.81
0.63
0.60
0.52
0.51
0.46
0.41
0.33
0.29
0.22
0.16
- 0.14
0.13
0.11
0.10
0.09
1.00
0.91
0.87
0.83
0.81
0.76
0.74
0.79
0.74
0.58
0.39'
0.30
0.28
0.26
0.23
0.19
0.14
0.09
0.07
                                     95

-------
Horizontal  Polarization

          • Beye:  I   Type: I
0.0 •—. -i	1	1	1	1  0*
                                        0.0
                                          Vertical  Polarization

                                                  • B«y»: 1   Type: 1
                           -20*
                         -30*
                       -40»
 -90*
      -B0»
0.0
           -70
           Beys:  1   Type: 2
                                                                      0*
                                                                     -10*
                                                                    -80*
                                                        •=•60*
                                               -B0*
                                                   -70
                                                    B*ye: 1   Type: ?
                                                                     -10*
                                                            -SB*
                                                        -60'
                                          -90.  -80*
                                                   -70»
      Figure  33,  Elevation patterns for Type 1 and Type 2  elements.
                                96

-------
 Horizontal  Polarization


          * Bays:  1   Type: 3
•9. D
                                           Vertical Polarization

                                                   • Bays:  1   Type: 3
                                         0.0
                                                                      -10*
                            -20*
1.0
  _90. -80«
           -70*
                                                                -40*
                                                             -50*
                                                         -60*
                                           -90*
                                                -80*
                                                     -70
0.0
          • Bay*:  I   Type: 4
                                          0.0
                                                   * Bays:  I   Type: 4
                                                                      -IB*
                      -40»
      -B0
           -70*
                                          1.0
                                           -30*
                                                -B0*
    Figure 34.   Elevation  patterns  for Type 3 and Type 4 elements.
                              97

-------
 Horizontal  Polarization

         • B»ys:  t   Type: S
0.0
Vertical  Polarization


        * Beys: 1   Type: 5
                                      e.e
1.0
 -90*
      -80*
          -70»
                                                     -60*
                                        -90*
                                            -80»
                                                 -70«
           Figure  35.  Elevation patterns for Type 5 elements.
                                98

-------
Section 3.  Arrays and Pattern Multiplication

     FM  broadcast  antennas normally  consist of  arrays  of  up  to  16  elements
stacked  vertically on  a  tower.   The  elements   are  spaced  approximately  one
wavelength  apart  and  are  usually  fed in  phase  with  equal  power  division
between  the  elements.   The relative  field  strength pattern  of  these  antennas
is  the  product of the  element  and  array  patterns.  Far-field  array  patterns
for  in  phase  point sources can be generated in  a number of  ways.   The simple
formula below is one method.

         E  = Sin n W2
              Sin  */2

where EA = Electric field strength from the array
      ED = Electric field strength which would result from a single element
             radiating the same total power as the array
        ^ = 2wd/x cos 4
        d = separation between elements
        x = wavelength
        d = angle of measurement direction with respect to the horizontal
        n = the number of elements

     Polar plots  of  EA/(n  *  EQ)  are  shown  in Figure  36 for  n =  2, 4,  6,
and 12  with  d = one  wavelength.   The  plots  are  normalized at  0°  and  do  not
illustrate the increase in gain as n is increased.  They  do show that  the  beam
narrows at higher  values  of n.  The total  patterns  (element  times array)  for
the five elements  used  in this study  grouped in six  bay arrays are  shown  in
Figures 37-39.
                                      99

-------
     HRRHY PflTTERN TOR 2 BRYS RT
         1 HHVELENGTH SPHCING
                 90*
        128*      •       68*
  150*
168*
  218*
                               338*
        248*
                         388*
                278*
     RRRflY PHTTERN FOR 6 BHYS HT
         I HRVELENGTH SPBCING
                 98*
        12B»      .       SB*
  158*
                               38*
168*
  218*
 HRRRY PflTTERN FOR 4 BRYS HT
     1 MHVELENGTH SPHCING
             98*
    I2B«    	i_    68*
                                                     158*
                                                  IBB*
                                                    218*
                                                                                  38*
                                                                                   i- e»
                                                                                 338*
    248*
                                                                           388*
            278*
RRRHY PHTTERN FOR 12 BHYS HT
     1 HHVELENGTH SPRCING
             SB*
    128*       .       68*
                                                    158*
                                                  188*
                               338*
                                                    218*
                                                                                 38*
                          338*
             Figure  36.  Array patterns  for 2,  4, 6,  and 12  bays.
                                          100

-------
Horizontal  Polarization
         t B»y«i 6  Typa: I
B.0
                            -20*
                   -50*
               -60*
           -70*
      -B0'
0.0
          * B»ys: 6   Type: Z
                             -10*
                            -20*
                          -30*
               -60*
      -60
           -70
                                         Vertical  Polarization
                                                  • B»ys: 6
                                        0.0
                                                           -50'
                                                       -60'
                                                   -70
                                              -80
                                        B.0
                                                 t B«y»: 6   Type: 2
                                                                     -10'
                                                           -50*
                                                       -60'
                                         -se*
                                              -60-
                                                   -70
    Figure 37.   Total  patterns for Type 1 and Type 2 elements.
                               101

-------
 Horizontal  Polarization

          • Bays:  6   Type: 3
0.B •fe-^, i	1	1	1	^a 0*


                             -10*


                            -20*


                          -30*
                       -40»
           -70
            Bays:  6   Type: 4
                             -10*
                            -20'
                          -30'
1.0
                -60*
  _9a.  -80*
           -70*
0.0
  Vertical Polarization

          • Bays:  6   Typo: 3
                             -10*
                            -20*
                                                                     -30*
                                                                  -40*
                                                               -50*
            Bays:  6   Type: 4
                                                                        -10*
                                                                       -20*
                                                                     -30*
                                                          -60*
                                                      -70*
       Figure 38.   Total  patterns for Type 3 and Type 4 elements.
                                   102

-------
                                                                              Relative fl«ld Strength
O
co
                                                                  
-------
Section 4 - Array Near-field Effects

     The  array  patterns  discussed  in Section  3 are  far-field patterns  which
means  that  the  rays  from  each  element  are  practically  parallel  at  the
measurement point.   Close to the array,  the rays can  no longer  be  considered
parallel as illustrated in Figure 40.
                            Parallel rays
                           in direction of
                      distant measurement point
                        Far-field Case
                                                        Non-parallel rays
                                                        Array Near-field Case
Figure  40.   Rays  from  each element  are  nearly parallel  for  points  in  the
far-field but not for points in  the  array near-field.

     The  region  near  the  antenna  where this  effect  is  significant  can  be
termed  the  array  near-field.   It  differs  from the  element  near-field which
extends only a few feet  from each element.  The  array  near-field region for FM
broadcast can extend several hundred meters from the  antenna.   In this  region,
the  far-field  pattern does not  accurately represent  the  radiation occurring
around the antenna.

     Array near-field  effects must be considered in  impact  modeling since they
occur  in  the  region near  the antenna where the  Guidance is most  likely to be
exceeded.   The  array  near-field  region  can  be  examined  by  calculating  the
phase  and magnitude of the electric fields produced  by each element and adding
them  vectorially.  Calculation  of  the  field  produced by  a two-bay  array is
shown  in Figure  41.
                                       104

-------
                              Element 1
                                        "--^ X,
                                                       Calculation
                                                     "--,. Point
                                     ^-~'~~ X,
                              Element 2
Figure 41.  Calculation of the field produced  by a two-bay array.

     e = Phase difference =  (X] -  X2) *  2ir/x
     d = separation between  elements
     x = wavelength
     E, = rms electric field produced by element 1
     Ep = rms electric field produced by element 2
                     ]/377*
                     y 4 * ir
     pi =
     P2 =
     En, =
power radiated by element  1
power radiated by element  2
rms resultant electric field  (superposition of E,  and E~
  at the measurement point)
        2     2
          + F
       I    b2
                            2E]E2  Cos  e
     A more general  method  for combining waves  from arrays with  any number of
elements was  developed for  this  study.  Assuming  that N  sinusoidally varying
waves  of  the  same  frequency  (u>/2n)  but  different  peak  amplitudes  (E  )  and
phases  (en)   combine,  the   principle  of   superposition  states   that  the
resultant wave will  be specified  by:
                                      105

-------
             N
     E(t) »^En sin  (cut + en)
            n=l
Using sin(x + y) = sin  x cos y +  cos  (y)  sin  (y)
             N
     E(t) =  /^En sin  («t) cos (en) + En  cos  (ut)  sin  (en)
            n=l
                    N                           N
            sin uit / ^ En cos  (en) * cos  (u>t) 2J En  sin  (en)
                   n=l                        n=l
             N
Setting A =     En cos  (en)
            n=l

            and

             N
        8 " £]En sin  ^en)
            n=l
        E(t) = A sin (u>t) + B cos («t)

The instantaneous power is given by
     P     _
      inst ~ 37
and integrating over one cycle
                  1/f
     P    = -3-77-     I   (A sin o>t + B cos ut)2 dt
                   o
                                      106

-------
where P    = the average power
       cLVQ

and f = frequency.

The integrand can be expanded and P    expressed  as three  integrals.


                  1/f

     Pavg - 377   /  A' ^ <»*> dt
                   o

                  1/f
             jj    I   2 AB Sin  (cot) cos  («t) dt
                   0

                  1/f
             f      f   2   2,   ,
       +    ->-?->    /   B Cos  (ut) dt
            377   J
                   o

The second integral equals zero  and the  first  and  last  simplify to:
     p
                2    2
                  +  d
          _
      avg ~    2 * 377


The magnitude of the peak or rms  resultant  electric  field  can  then be found
     E            2 *  2
V
      -R-rms    V    2

The resultant wave is


      E(t) = ER sin («t + OR)


     where SR is the resultant phase  angle.


The phase angle e^ can be found by  noting  that  E(t)  =  0 when t

The earlier expression for E(t),



      E(t) = A sin («t) + B cos (ut)


                                      107

-------
is set to zero at t = -eR/u.

     A sin (-OR) + B cos (-OR)  = 0

     -A sin (OR) = -B cos eR

     tan (eR) = B/A

     6R = tan-1 (B/A)

Thus, the  resulting  wave  is  completely identified assuming that  all  component
waves are of the same polarization.

     The above  technique  was  used to  study  the  importance of  array  nearfield
effects  in  modeling.  A  computer program was written  which calculated  field
levels near  the ground using  far-field  (parallel-ray)  calculations  and  array
near-field (non-parallel ray) calculations.  Results from  both  techniques  were
plotted on the  same  graph for comparison.   Figures  42-44  are examples of  the
output of this  program.  The value for height  above ground in these  graphs is
the  height  of  the  lowest element.   The  program  does  not  consider  coupling
between elements or ground reflections.

     Examination of  Figures  42 through 44 reveals two differences  between  the
results of the  two calculational  methods.  Nulls  in  the  array near-field  plots
tend  to  be shallower and  shifted in position when  compared  to  the  far-field
plots.  These  effects  are most  prominent  when the  array  is  mounted on a  low
tower (Figure 42).   For  high towers,  as  in Figure 43,  the far-field  plot  is a
good approximation of the near-field  plot.

     The  implication  of these results  is  that array  pattern nulls  should be
ignored  in  impact modeling.   Further support for  this concept  is  that  many
stations  deliberately  fill  the  nulls  through phasing  techniques  to  improve
coverage.   To  avoid  under-predicting the fields  at  null  locations,  the FM
model  uses  far-field  array  patterns with  100  per  cent null  fill.   These
patterns  are constructed  by drawing  an   envelope  around  far-field  patterns.
Figure  45  shows  a  far-field  array  pattern and   the  constructed  envelope.
Envelopes  of 1,2,  3, 4,  5,  6, 7, 8,  10,  12, 14,  and  16 bay  far-field  array
patterns were digitized and stored in files for use  in the FM model.
                                      108

-------
                         Comparison of  Far Field  and fir ray Near Field Calculations
o
vo
                Z
                \
                >
                 a.
                CQ
                T3
                     160  r-
     140
                M   120
                     100
C3

U
Of.
r-
0)

Q
_J
U
!-«    80
                      60
                              Element Pattern:     Dipole
                              Number of Elements:   6
                              Height Rbove Ground:  10 Meters
                              Height of Subject:    2 Meters
                                               Frequency:
                                               SpacIng:
                                               Input Power;
                                     Fir Field Calculation

                                     •Rrray Near Field Calculation
                                          I I I I
100  MHz
1  Wavelengths
1  kW
                                               10                   100

                                    DISTRNCE FROM  BRSE OF  TOWER  (Meters)
                                                                         1000
                         Figure 42.  Comparison of  far-field and array near-field calculations.

-------
         Comparison of Far field and Rrray Near Field Calculations
z
>
3.
m
•o
z
M

I
     160 r-
              Element Pattern:      Oipole
              Number of Elements:   6
              Height Rbove Ground:  20 Meters
              Height of Subject:    2 Meters
                                               Frequency:
                                               Spncing:
                                               Input  Power:
M    120
     100
u
a:
(/)
a
u
•-«    80
U.
     60
                    > Fir Field Calculation
                    •Rrray Near Field Calculation
100  MHz
1  Wavelengths
1  kW
                                                                I   I  I
                               10                   100

                    DISTRNCE FROM  BflSE  OF  TONER (Meters)
                                                                         1000
         Figure 43.  Comparison of far-field and array  near-field calculations.

-------
         Comparison of Far Field and Rrray Near  Field Calculations
3.
m
•o
z
I
u
     160 r-
     140
M    120
     100
in
a
u
C    80
     60
              Element Pattern:     Dlpote
              Number of Clements:  6
              Height Rbove Ground: 30 Maters
              Height of Subject:   2 Meters
                                               Frequency:
                                               SpacIng:
                                               Input Power:
                     Far Field Calculation
                    •Rrray Near Field Calculation
100 MHz
1  Wavelengths
1  kW
                                            I  I  I i I I
                                                               I  I  I J  I I I I
                              10                    100

                    DISTflNCE FROM BflSE OF TOWER  (Meters)
                                                                         1000
          Figure 44.  Comparison of far-field and array near-field calculations.

-------
                           6 Elements,  1  Wavelength Spacing
ro
         CD
         C
         o
tn
T»
o
iZ

>
4*
*
                                  -30°               -60°
                                    Depression  flngle
                                                                -90'
                       Figure 45.  Construction of an array envelope model.

-------
Section 5 - Mutual Coupling Effects

     The technique  of pattern multiplication described  in  Section  3,  Appendix
A,  ignores mutual  coupling effects which  can be important  in  certain antenna
configurations.  Each  element  of  an  array  interacts with other nearby elements
changing  its  net impedance.   The net  impedance for the  first element  in  an
N-bay array, for example,  is:

                                    N
          zl(net) - Z](self) +     / v zl,n
where zwnet\ - the net impedance of the first element
      Z.gjf = the self impedance of the first element
      Z,   = the mutual impedance between elements 1 and n
        i ,n

     For a  given  input power, changes in  impedance  change the current  in  the
element  and consequently  the  radiated  field.   As  an  example,  the  electric
field at a point produced by a one-half wave dipole can be expressed as:

     ED = kI0

where EQ = the electric field produced by the dipole at a given point
      IQ = the current in the dipole
      k = a constant involving the distance between the dipole and
          measurement point

If the same element is placed in an array:
Assuming that power is held constant
where R denotes the real part of the antenna's impedance
                                     113

-------
Rearranging terms,

                   I
                    D  =


                        or

                   ED  =
                        and
                 EE-

Thus, when  an  element  is placed  in an  array, the  resulting electric  field
changes by the square root of the ratio of resistances,
     Exact  calculations  of mutual  impedances have  been worked  out only  for
simple geometries  such as  broadside  or colinear  dipole antennas.  Actual  FM
broadcast antennas are far from dipoles  in  shape  and  radiation  patterns making
theoretical impedance  calculations  impractical.   In  order to get  some  idea of
mutual coupling  effects,   broadside  arrays  of  one-half  wave  dipole  elements
were modeled using equations  from Kraus [4].  Results  of the  modeling showed
that coupling  effects  can significantly alter the predicted field  levels  for
certain  interbay  spacings, but  are minimal  when the  spacings  are near  one
wavelength.

     The  above results  are not  directly  applicable  to  actual   FM  broadcast
antennas  for  two  reasons.  First,  the  broadcast  elements  have  a substantial
vertical   height such that  not  all points on the  element  are the  same  distance
from  adjacent elements.    Second,  broadcast  arrays  typically  use  spacings
slightly  less  than one wavelength.   Coupling is  reduced, however,  by  the fact
                                      114

-------
that broadcast  elements radiate  less  energy  up  and  down  (towards the  other
elements)  than  dipoles.   Without  a  very  extensive  numerical  analysis  and
knowledge  of  the feed  systems  used,  it  is impossible to  predict the  exact
effects  of mutual  coupling.   As a  first  approximation,  the  above  factors
indicate that coupling effects  in FM antennas  can  be ignored  without seriously
affecting the accuracy of the model.
                                     115

-------
Section 6 - Effect of Ground Reflections

     Electromagnetic waves  striking the  ground  from  an  FM broadcast  antenna
are reflected  and add  to or  subtract  from  direct  waves  to  alter  the  total
field (See Figure 46).
                                             Direct Ray

                                        \       ,
                                                    / Reflected Ray
Figure 46.  Field strength at a point is the result of the direct and
            reflected wave.

     Consideration  of  ground   reflections   is  important  in  impact  modeling
since field  strengths  can be  significantly  increased.   Field  enhancement  by
reflected waves  can  result in  increases  in field strength  which may  not,  in
some circumstances,  correspond to  a similar increase  in power  density.   The
worst  case   increase  from a  reflection  as  shown  in  Figure  46  would  be  a
doubling  of  field   strength.   For  free  space  waves,   a  doubling  in  field
strength  corresponds   to  a   quadrupling  in  power   density.    Reflections,
however, create  standing  waves.    In  a   standing   wave  the  power  density
could be zero  if,  for  example,  the magnetic field  is zero  but  the  electric
field  is  large.   Nevertheless,  field  enchancement  due to reflection  must  be
considered  in impact modeling  because proposed  Federal   Guidance  would  most
likely  be  stated  in   terms  of  electric  field,  magnetic  field,  and  power
density.  Any of these  three  quantities can  be  the  limiting parameter in  a
given  situation.  Where wavelengths are less than  the height of  the subject,
calculation   or   measurement  of   either   electric   or   magnetic   field   is
satisfactory.   In  these  cases,   the  value   of  either  field  maxima  will
                                      116

-------
correspond to the free  space  equivalent  (E * 377H) of  the  other field maxima
which will also occur near the ground.

     The  actual  position  and  intensity  of the field  maximas depends  on the
factors listed below:

     1.  polarization of the signal
     2.  frequency of the signal  (f)
     3.  ground conductivity (o)
     4.  ground dielectric constant  (c)
     5.  angle that wave makes with  the earth  ()  (see  Figure  47)
     6.  roughness of terrain

     Equations for calculating the magnitude and phase  of the reflected signal
are given in Jordan and Balmain [3].
    R  =   Ur " JX)sin (l° "^(e-~ JX)  "C0$2(
           (er - jX)sin ( * )  +(e- JX)  - cos2(*)
    Rh
Sin (*) -y(cr - jX)  -  cos2U)

Sin U ) +t/Ur - JX)  - cos2(iJO
     These equations  express the  reflection coefficients  for  vertically  and
horizontally   polarized   signals   as   complex   numbers.    After  extensive
manipulation, they can be expressed as a real and imaginary part and then  used
to calculate the  magnitude  and  phase of the reflected signal.   Techniques of
vector addition such as those described  in  Section  4,  Appendix  A,  can be  used
to  sum the  direct  and  reflected  rays at a  given  point  to  determine  the
resultant field.   The final form  of the  equations for  R.  and RV  are  given
below using intermediate variables  to reduce the  size of  the expressions.
                                     117

-------
        Set X =
                    a x 1.8 x  10
                                10
                    /((«  - COS2  (* ))2+  X2)1/2 +  (er - COS2( * ))
                    	

                                         2
                     ((e   - COS2(*))2  +  X2)1/2  -
                                               (er - COI  (*))
     The  reflection  coefficients  can then  be  expressed  as  a  real  and   an

imaginary part.
           2(e F - F + X G)sin ( *)  + (c -1)  cos 2 ( *)
R.(real) =	^	                  r
                                                               -  e2  -  X2
                             (1  - er)2  + X2
   R.  (imaginary)
2(XF + G -
                             6 er)  sin ( * )
                                                 x  cos  2  ( * ) -  X
                                (1  - er)2  + X2
     The magnitude of R.  is:
     and the phase is:
     Rh (phase) = Atn
                        (imaginary)

                       Rh (real)
     For vertically polarized signals:
                        (e 2 + X2)  sin2 U)  - F2 - G2
                                     (2erF + 2GX)sin
                                      118

-------
                                  (2Ge   - 2XF)  sin  (*  )
     R (imaginary) =  —~	~	~	*	 ?
      v              (e^ + X^)sin*( * )  + (2erF + 2GX)sin( * ) + r + G^

     The magnitude and phase  of  R  may be found using  expressions  similar to
those  for  R..   Care  must  be exercised  when  using  the  expressions for  R .
Vertical  polarization in  this  context  means  vertical  with  respect  to  an
observer at the reflection point  looking towards the transmitter.

                                 xr             -
                                      •X /
                                             Observer
Figure  47.    Vertical  polarization  means  that  the  E-field  vector  appears
vertical to an observer looking  towards the  source.

     Figure  47  illustrates  direct  and  reflected  rays  emanating  from  a
broadcast antenna.   In both cases,  the rays  are vertically  polarized,  but  one
is not  perpendicular  to  the ground.  Directly  beneath  the  antenna  (if>=  90°),
the electric  field  of a vertically polarized  signal  is  actually  parallel  to
the ground and equivalent to a horizontally  polarized signal.

     Plots of the magnitude and  phase  of the reflection  coefficients are  shown
in Figure 48-51.  These were generated using the above  equations  at  100 MHz,
relative dielectric constants of  7-30,  and conductivities from 0.001  to 0.03
mho/m.  Examination of these curves reveals that directly beneath  the antenna,
the magnitude of the reflection  coefficient  ranges from  about  0.45  to 0.70  for
the range  of dielectric constants  and conductivities  commonly  found in  the
                                     119

-------
United States [5].  It was felt that  the  lack  of knowledge concerning terrain,
buildings, and electrical properties  of the  soil  around each station precluded
the  possibility  of  calculating  accurate  reflection  coefficients  at  each
point.  Thus  a  constant  value  of 0.6 was  chosen as  an approximation  to the
actual reflection coefficients  for  use in the model.   Although  the horizontal
reflection coefficient  increases  with distance  from  the  tower  a  decrease in
vertical   reflection coefficient  also  occurs.  Thus, multiplying  all  predicted
fields by a constant 1.6  appears  to be a  reasonable  approach to  modeling field
enhancement by ground reflections.
                                      120

-------
                    HORIZONTflL POLHRIZRTION
                                                                  Cr-3B.
                                                                  Cr-IS. 8lfl-.«IZ
                                                                  Cr-7.
                                                                  Cr-7. SlQ-.mi
   0      10    20    30     40     50     60     70     80
                  DEGREES  RBOVE HORIZON  (Ps1)
90
Figure 48. Magnitude of the reflection coefficient for horizontally polarized signals.

-------
                                        HORIZONTRL  POLRRIZRTION
ro
                  -195
                LJ
                >
                tE
                2


                a
                u


                tj-198

                _J
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                QL


                U.
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                u
                (/)
                cr
                i
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                   -IBB
                                                                                       Er-7.
         Cr-7. Sl|-.ltt
         Cr-IS. 8lg-.«II

         Cr-38. Slg-.B3
                                                                                       Cr-7. Slf-.MI
                       0     10     20     30     40     50    60    70

                                     DEGREES  RBOVE  HORIZON  (Psl)
80
90
                          Figure 49.  Phase shifts of reflected horizontally polarized signals.

-------
                                    VERTICRL POLRRIZRTION
CO
                   0     10    20     30    40     50    60     70
                                DEGREES RBOVE HORIZON  (Psi)
80    90
                Figure 50.  Magnitude of the reflection coefficient for vertically polarized signals.

-------
                                     VERTICRL POLRRIZRTION
rs>
              Ul
              a
              u
                  e
                 -28
                 -40
              a  —
              _J
              u.
              LJ  -80
              o:

              u.
              0-100
I

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U
                -14B
              (E

              £-160
                -180
                   0
                                                                                 Er-7. Slf.BI?
            10    20     30     40    50    60     70


                    DEGREES RBOVE  HORIZON  (Psl)
80
90
                     Figure 51.  Phase shifts of reflected vertically polarized signals.

-------
Appendix B - FM Model Verification

      In  order  to  verify  the accuracy  of  the  FM  model,  a  field  study was
conducted in August  1982.   Measurements  were made around six FM stations  which
represented a  variety of  antenna types,  ERP's,  and terrains.   After the study,
the measured field  strength values  were  plotted as free-space equivalent  power
densities for  comparison with the FM model output for those stations.

      Holaday Industries Model 3001  electric  field strength meters were used to
make   the  measurements.    These  meters  were   calibrated   beforehand  in  a
transverse electromagnetic  cell  (TEM) at  EPA which has  been characterized to
better  than  ^ 1  dB  accuracy.   The  meters  were  found to  accurately measure
field  with errors  less  than ^ 2  dB.  As mentioned in Section  6  of Appendix A,
measurement of electric field alone  is  sufficient for  FM  broadcast  stations.
The electric field  maxima  will  always occur at heights  above ground which can
be reached with  a hand-held probe  (typically  less  than 3 ft.).   These maxima
will  be similar in  intensity to  the magnetic field  maxima and greater than the
true  power  density  if  a  free-space  conversion  (based on the  square  of the
field  strength and free-space impedance) is  used.

     Since the FM model predicts the  highest equivalent power density expected
at each radial distance from the base of the tower,  measurements were taken to
reflect the  same concept.   The  ideal measurement  method would be  to choose
about  eight or more equally spaced radial directions  away from the tower and
take  measurements  along  each  at three  foot  intervals.   At  each measurement
distance,  the  probe  is raised  slowly  from the  ground to  eight  feet  while
watching the meter  for a  maximum  value.  Once the  location  of the maxima is
found,  the  region   is  carefully  probed  to  determine the  maximum  reading.
Values obtained  along  the  various  radials  are then  compared and  the highest
value for each distance from the tower is used.

     It was not  possible  to follow the  above  protocol  exactly at most  of the
measurement  sites.  Buildings and terrain  features  often prevented measurement
along  all radial  directions.  However,  after  measuring field strengths  in as
many  locations  as   possible,  it   was   often  found  that  the  highest  field
strengths  occurred along a  single radial.   In all cases, efforts  were made to
                                      125

-------
duplicate the  results  of the ideal  method  within the physical  constraints of
the location.

     Figures 52  through 56  show  the measured  values plotted  along  with  the
curve  predicted  by  the  EPA  FM   model   for   each   station   in  the  study.
Examination  of  these   graphs  show  that  the  predicted  curves  are  in  good
agreement with the  measured  values.   The  intention  of the  FM  model  is  to
predict  an   envelope  or  upper  bound  of  the  actual  values  occurring  at  a
station.  This  goal  appears to have been met  to  a  reasonable degree for  the
six  stations  measured.   In  some   cases   the  measured  values  exceeded  the
predicted  curve  at  certain  points,  but   in  all  cases  the  highest   value
predicted by the model  was  not exceeded  by the  measurements.   Since  impact
predictions  were  based on  the  highest   values  predicted by  the model,  these
results add to the credibility of  the impact analysis.
                                     126

-------
  10000
                     Rntenna:  Type  1      B  bays
                     Tower  Height:  22.86m    (75
                     Total  ERP CH+V):  34 kW
                                                      ft)
pj  1000
                                                   Measured Data
                                                   Calculated Curve
E
u
N
2
3

C


X
C
u
Q

i.
V
3
o
Q.
    100
        Distance  from tower

        (Meters)


            Figure 52.  Calculated and measured power densities
             (free-space equivalent)  for an actual FM station.
                                127

-------
  10000 r
                     Rntennai  Type  3      2  bays
                     Tower Height:  13.1064  m    (43  ft)
                     Total ERP CH+V):  4.6 kH
OJ  1000 •
                                                   Measured  Data
                                                   Calculated Curve
E
O
\

3

C
M
C
o
n
v
3
O
Q.
    100
        Distance  from tower

        (Meters)


              Figure 53.  Calculated and measured power densities
               (free-space equivalent) for an  actual FM station.
                                  128

-------
  10000 r
tvj  1000

E
u


3
     100
v>
c
I)
1=1
U
3
O
Q.
                     Rntcnna: Type  2      6  bays
                     Tower  Height:  46.6344  m    (153
                     Total  ERP  (H+V):  158 kW
                                                            ft)
                                                    Measured Data
                                                    Calculated Curve
                     OJ
                            CO
                            n
                                  Q
                                         in
(S
u>
eg
eg
GO
eg
en
        Distance from tower

        (Meters)


               Figure 54.  Calculated and measured power densities
                (free-space equivalent) for an actual  FM station.
                                   129

-------
  10000 r
                     Rntenna:  Type 1      6  bays
                     Tomer Height: 50.5968  m    (166
                     Total ERP (H+V):  100 kW
                                                           ft)
ru  1000
E
u
\
                                                   Measured Data
                                                   Calculated Curve
3

C
V)
c
I)
Q

1.
1)
3
O
CL
    100
        Distance  from tower

        (Meters)
             Figure 55.  Calculated and measured power densities
              (free-space equivalent) for an actual FM station.
                                  130

-------
  10000 r
rv,  1000
E
U

JC
3

C


X
M
C
t>
a

t.
t>

o
a.
    100
     10
                    Rntenna:  Type 1      5 bays
                    Tower Height: 21.336 m    (70
                    Total ERP 
-------
  10000
                     Rntenna:  Type  2      6  bays
                     Tower  Height:  45.72 m     (150
                     Total  ERP CH+V):  100 kW
                                                         ft)
OJ  1000
                                                   Measured Data
                                                   Calculated Curve
E
u
\
Z
3

C
C
V
n

t.
u
3
o
Q.
    100
        Distance  from tower

        (Meters)


              Figure 57.  Calculated and measured power densities
               (free-space equivalent) for an actual FM station.
                                  132

-------
Appendix C - Minimum Tower Heights for FM's

     The  FM model  was designed  to  predict  field  strengths  (as  free  space
equivalent  power  densities)  on  the ground near  FM broadcast  facilities  when
given values  for  ERP, antenna type,  tower height, and  number of bays.   This
process can  be inverted  so  that for a given  antenna  type,  the model  draws
curves of the minimum tower heights necessary to  prevent the  creation  of  power
densities exceeding an established  limit.  The x-axis  is ERP  ranging from  0 to
100 kW  and  it is assumed  that  this value occurs  in  both polarizations as is
usually the case.

     These  graphs  (Figure  58-69)  are  useful  in  making estimates  of  tower
heights necessary to  stay  below a given power  density.  The graph  labeled
                              2
Type 1  antenna at  200  pW/cm ,  (Figure  60)  for  example,  can  be  used  by
finding the station ERP on the x-axis and using  the  proper curve to  find  the
corresponding  tower  height on  the  y-axis.   If   the  station  tower  height  is
signficantly  less  than  the  height  found on  the  graph,  there  is   a   good
                                                                        2
probability that  equivalent  power  densities  of  greater  than 200 yW/cm   will
occur near the tower.   Many assumptions were used  in the formulation of the FM
model,  as  described  in  this  report,  and there  can   be no  guarantee of  the
accuracy  of  these  graphs.   However,  the field  study data  in  Appendix  B
indicates that the  model  is a  good approximation to  the upper bounds of  the
equivalent power  densities occurring near the  tower.
                                     133

-------
   700
   600
*>  500
   400
Q)


13
L.
V
3
   300
   200
   100
                          100 uW/cm~2

                        RNTENNR TYPE  1
                <\J    D
                           r    in


                           ERP (kW)
                                              Si
                                              CD
                    IS
                    ft
   780
   600
   500
 Ol
 t.
 i)
   400
   300
   200
   100
                E3
                rvj
                          100

                        RNTENNR TYPE  2
E
in
E3
U)
s
O)
                            ERP
Figure 58.  Minimum tower heights necessary to prevent  creation
                  of 100 viW/crn^ on the ground.
                               134

-------
                         100
                       RNTENNR  TYPE 3
                                                       t ••>»
                           ERP (kW)
                         100 uW/cm~2
                       RNTENNR TYPE 4
   70B
   60Z
                                                         ••/•
                           ERP
Figure 59.   Minimum tower  heights  necessary to prevent creation
                 of 100 yW/cm2 on the ground.
                              135

-------
                          100
                         RNTENNR TYPE 5
 X
 O)
    700 r
    600
    see
    400
 1  300
 L.
 u
 3
 c  200
    100
                                                         1 l«r«
                 S)
                 (VI
                           r    in    (

                            ERP  (kW)
                                               as    tn
Figure 60.
Minimum tower heights necessary  to  prevent  creation
     of  100  pW/cm2  on  the ground.
                              136

-------
                           200
                         RNTENNR TYPE 1
    500
    40E
O)

QJ
I

i.
QJ
3
O
    30E
    200
    100
                  S>
                  C\J
                            r    in

                             ERP  (kW)
                                               S3
                                               CD
E>
en
    500
    400
    300
 X
  05
    Z00
    100
                           200
                         RNTENNR TYPE 2
                                                           t »•/•
                  ts
                  
-------
O)

V


i.
V
3
O
   500
   400
   300
     200
                          £00

                        RNTENNfl TYPE 3
      8    8
                           S    G3    IS    S
                           v    in    us    r\.
                                                 G3    ^3    03
                                                 tD    S)    S
                            ERP  CkW)
   500
   400
^  300
X
D)
V
3
O
   200
   100
     0
                          200
                        RNTENNR TYPE 4
                ru
                           r    in    io

                            ERP  (kW)
                                          63
                                               C3    ^3    CS
                                               (D    CD    O
Figure 62.
          Minimum tower heights necessary to prevent creation
                of 200  pW/cnv2  on the ground.
                             138

-------
                            200 uW/cm~2

                          RNTENNR TYPE  5
en


u
I


L.

V


o
      508
      400
      300
      200
      100
                                                            ••/•
                   63
                   01
                     IS
                     n
                             r    in


                             ERP CkW)
8
0
ca
01
ca
ca
Figure 63.  Minimum tower heights necessary to prevent  creation

                  of  200  uW/cni^  on  the ground.
                              139

-------
                             500 uW/cm~2
                           flNTENNR  TYPE 1
       300
       200
    Ol

    e



    §  100
    o
          sssssssssss
               —    M<*>»inu>rv.s

                              ERP  (kW)
                             500 uN/ctri"?
                           RNTENNR  TYPE 2
       300 r
    i  zzz
     O)

     o



     1  100
     o
    I-
          Q    ^Q    0   Q    ^3    Q    Q)    ^3    (Q   Q    CD
               —    (umvintof^acns

                              ERP  CkW)
Figure 64.  Minimum tower heights  necessary to prevent creation
                  of 500  pW/crn^ on the ground.
                                140

-------
                           500 uW/cm~2

                          RNTENNR TYPE 3
      380
   i  200
    O)

    u
    §  100
    O
                                                         I §«r»
                  S
                  M
D   S
r   in


 ERP CkW)
s
(A
s
a
B
at
§
                           500 uW/cm~2

                          flNTENNfl TYPE 4
      380 r
                                                         I i«r»
                             ERP CkW)
Figure 65.  Minimum tower heights necessary  to  prevent creation

                  of  500 pW/cm"2 on the ground.
                              141

-------
                           500
                          RNTENNR TYPE
      300 r
   i  230
   .C
    O)
    u
    I.
    0)
    o
                             ERP CkW)
Figure 66.  Minimum tower heights necessary to prevent creation
                  of  500  yW/cni' on the ground.
                               142

-------
                          1000 uW/cm~2
                         RNTENNH TYPE  1
     250
     200
  v  150
  I)
  I

  k
  I)

  o
   100
      50
        SSSSS8SBS8B
            —    (vinvinu>N.(B(n8

                            ERP CkW)
O)

I)

i.
V

o
     250
     200
     150
     100
     50
       888
            —    ru
                          1B00
                         RNTENNR TYPE 2
                        8888888
                        »    in    ID    P>-   o    «n    8

                         ERP  (kW)
Figure 67.  Minimum  tower heights necessary to prevent creation
                 of  1000 yW/cm2 on the ground.
                               143

-------
     250
     200
  v  150

  £.
  at
     100
      50
                          1000

                         RNTENNR TYPE  3
        tSSSCBSQSOSCDO
             •"•ojniYintfir^onQ


                            ERP  (kW)
     250
     200
  B)

  c
  0


  o
     150
     100
      50
                 O
                 (VI
                          1000 uW/cm~2

                         RNTENNR TYPE 4
B   S    (
r   in    i


 ERP CkW)
B
CD
S
tr>
CO
s
Figure 68.  Minimum  tower heights necessary to prevent  creation

                  of  1000 vW/cm^ on the ground.
                               144

-------
                           1000 uW/cm-2
                          RNTENNP, TYPE 5
      250
                             ERP CkW)
Figure 69.  Minimum tower heights  necessary  to  prevent creation
                 of 1000 uW/cm^ on the  ground.
                              145

-------
Appendix D - Predicted Field Strengths for AM stations

     The modeling procedures for AM  stations  described  in this report computed
field strength values in the vicinity of  single  tower stations.   Some of these
results  are  shown  in   the following  figures  to   illustrate  typical  field
strength values  found near  AM  transmitters  and  the trends  described  in  the
text.

     Figure 70 shows  electric  field strength plots  for 50 kW,  0.3 wavelength
electrical  height  towers operating at 0.6,  0.8,  1.0,  1.2,  1.4, and  1.6  MHz.
The curves  coincide out to about 15 meters from the  tower and then  split apart
with  higher  frequencies  producing higher  field  strengths.   Figure  71 is  a
similar  plot  showing   magnetic   field   strengths   produced   under  the  same
conditions  as above.

     Figure 72 shows the electric and magnetic field strengths for a 1 MHz,
50 kW, 0.3 wavelength electrical height  tower plotted on  the  same  graph.   The
electric and magnetic field strength scales  of  the vertical  axes  are related
by  the  free-space  condition   E   =  377H.    The  magnetic  field  strength  is
consistently higher  than the electric  field strength  when they are  compared
using  free  space  equivalence.    This   is  a relevant  comparison  since  the
limiting values for E and H  specified in  the  proposed Guidance will be related
by  E     =  377  H   .   When   the  electrical   height   is   changed   to  0.5
     Illu/x           illuA
wavelength, neither field is consistently greater  as  shown in Figure 73.  Thus
both fields must be considered in impact modeling.
                                      146

-------
     NEC flM  Model  for  50 kW,  0.3  Wavelength Towers
  100 r
   60
£
\
TJ

  60
O

I.

O

- 40
u
0
*>
O
  20
             Fields are computed at 2 meters
             above ground.
               25
50        75         100

    Distance  (Meters)
                                                        125
150
        Figure 70.   Electric field strengths for 50 kW, 0.3 wavelength
        electric  height towers operating at 0.6, 0.8,  1.0,  1.2, 1.4 and
         1.6 MHz.  Higher frequencies produce higher field strengths.
                                  147

-------
      NEC RM  Mode)  for  50 kW,  0.3 Wavelength  Towers
   0.3 r
E

5 9.2
u.
u
c
O)
  e.0
Fields are computed at 2 meters
above ground.
                          50        75         100

                              Distance  (Meters)
                   125
150
       Figure 71.  Magnetic field strengths for 50 kW, 0.3 wavelength
      electric height towers operating at  0.6, 0.8,  1.0, 1.2,  1.4, and
        1.6 MHz.  Higher frequencies produce higher field strengths.
                                  148

-------
     NEC flM  Model  for  50  kW,  0.3  Wavelength  Tower
  iee
  80
E

>
•o
•^  60
L.
*>
O

f  40
Ul
0

o
t-


   20
                                            EUctrlc n»ld
                                            • M«gn«tlc Flald
                                      Fields are computed at 2 meters
                                      above ground.
                                             0.28


                                             0.26


                                             0.24


                                             0.22


                                             0.20


                                             0.18


                                             0. 16


                                             0. 14


                                             0. 12


                                             0.10


                                             0.08


                                             0.06


                                             0.04


                                             0.02
                                                                            O
                                                                            «*
                                                                            0
                                                                            a
                                                                            (O
                                                                            3
                                                                            O
                                                                            O

                                                                            •n
                                                                            N,
                                                                            3
               25
50         75         100

    Distance (Meters)
                                                          125
150
                                                                       0.00
       Figure  72.  Electric and magnetic field strengths for a 50 kW,
                          0.3 wavelength tower.
                                  149

-------
     NEC flM Model  for  50 kW,  0.5  Wavelength  Tower
  100
  B0
E

>
1 60
L.

*>
(J
LJ


a
*»
o
   20
   0
     0
25
                                            CUctrtc Ft«ld
                                      - — - - — Htgnct le F"1 • I d
                       Fields  are computed at 2  meters
                       above ground.
50         75         100

    Distance (Meters)
125
                                            0.28


                                            0.2E


                                            0.24


                                            0.22


                                            0.20


                                            0.18


                                            0. 16


                                            0.14


                                            0.12


                                            0. 10


                                            0.08


                                            0.06


                                            0.04


                                            0.02
                                                            3
                                                            0
                                                            10
                                                            3
                                                            9
                                                                           a

                                                                           a
                                                                           3
150
                                                       0.00
            Figure 73.   Electric and magnetic field strengths  for a 50 kW,
                              0.5 wavelength tower.
                                       150

-------
     Although  the curves  in  Figure 70  through 73  represent the  fields from
50 kW  stations,  they can be used to  predict fields from  lower  power stations
as well.   Figure 72, for example,  shows  that  a 50 kW,  1  MHz,  0.30 wavelength
electrical  height  transmitter  produces  about  a  20  V/m  electric   field  at
100 meters.  A 10 kW station would produce an electric field of:

     E =   J]Q  kW   x   20 V/m   =   8.9 V/m
             50  kW

     Figure  74  shows  wave impedance  (E/H)  for  several  different electrical
heights  at  1  MHz.   This  graph  illustrates  the  fact  that  the  free-space
impedance condition  (E/H =  377 ) does not occur near  the  tower  in most cases.
Both the electric  and  magnetic field must be  considered for guidance purposes
whether one is measuring or calculating fields.

     Figure  75 is a  plot  of  electric  field  strength for  several  electrical
heights holding  frequency  and power  constant  at  1  MHz  and 50 kW.   No simple
trend is apparent for field strength as a function of electrical heights.

     Table 53  is a sample of the distances  away from AM transmitters necessary
to avoid exceeding various  alternative  guidance levels.   These values are from
the four dimensional array  described  in the text  (see page  62)  which accounts
for  both  electric  and magnetic  fields.   The   fields   from  a  1  MHz,  0.2
wavelength electrical  height  AM  station  will  drop  below the  field  strengths
shown in the  row headings  at  the distances  shown  in the  table  depending upon
the  station   power  (column  headings).   For  example,  fields  from   a  10  kW
stations will  drop  below 100  V/m (E < 100  V/m and (377   x H)  <  100 V/m)  at
14 meters from the tower.  Although  the distances  in Table 53 are specifically
for a 1 MHz, 0.2 wavelength  electrical  height  tower, Table  54 can  be used for
any  frequency  and  electrical  height.   The   distances   in this  table  were
obtained by  searching  the  array  for the  highest  values  occurring at  a given
power and field  strength.   They are  the  greatest  distances  necessary to fall
below the  specified  guidance  levels  for  any frequency  and  electrical height.
More simply,  Table 54 shows  the  worst case distances  necessary to  comply with
the specified  alternative  guidance  levels.   In most  cases, actual  distances
will  be somewhat  less than  those shown in the table.
                                      151

-------
                      WRVE  IMPEDflNCE  CE/H)
  1000
  800 •
~ 600
u
c
TJ
C
«-»  400 •
B
id
   200 -
                                                         8.18 Ltabd*
                                                         B.ZS Lubd*
                                                     MH*  8.SB Liabdt
                                                     MH>  8.EB Lubd*
                                125   158   175
                               Distance (Meters)
280  225   250   275  388
           Figure  74.  Wave  impedance (E/H) for several different
                       electrical heights at 1 MHz.
                                  152

-------
      NEC  flM  Model  for  50  kW,  1  MHz  Facilities
  100
   80
E
>
   60
L.
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U
•  40
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a
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   20
                                              B. IB Ltabd*
                                              B.25 L««bd»
                                              B.SB L««bd»
                                              8.6B L««bd»
               25
50        75        100
    Distance  (Meters)
                                                       125
150
          Figure 75.  Electric field strength for several different
                   electric  heights at 1 MHz and 50 kW.
                                 153

-------
TABLE 53.  DISTANCES (IN METERS) AT WHICH FIELDS FROM A 1 MHz
      0.2 ELECTRICAL HEIGHT AM  STATION WILL FALL BELOW
            EIGHTEEN ALTERNATIVE GUIDANCE LEVELS
Electric
Field Strength
V/m
10.00
31.62
44.67
70.79
86.60
100.00
141.25
173.18
200.00
223.87
244.91
264.55
281.84
300.00
316.23
446.68
707.95
1000.00
50.00
222
74
54
38
30
26
22
18
14
14
14
14
10
10
10
10
6
6
25.00
158
54
42
26
22
22
14
14
10
10
10
10
10
10
10
6
6
6
Transmitter Power
10.00 5.00 2.50
102
38
26
18
14
14
10
10
10
6
6
6
6
6
6
6
<2
<2
74
26
22
14
14
10
10
6
6
6
6
6
6
6
6
6
<2
<2
54
22
14
10
10
10
6
6
6
6
6
6
6
6
6
<2
<2
<2
(kW)
1.00
38
14
10
6
6
6
6
6
6
<2
<2
<2
<2
<2
<2
<2
<2
<2
0.50
26
10
10
6
6
6
6
<2
<2
<2
<2
<2
<2
<2
<2
<2
<2
<2
0.25
22
10
6
6
6
6
<2
<2
<2
<2
<2
<2
<2
<2
<2
<2
<2
<2
0.10
14
6
6
<2
<2
<2
<2
<2
<2
<2
<2
<2
<2
<2
<2
<2
<2
<2
                             154

-------
TABLE 54.  DISTANCES (IN METERS) AT WHICH FIELDS FROM AM STATIONS
   WILL FALL BELOW EIGHTEEN ALTERNATIVE GUIDANCE LEVELS.  THIS
       TABLE APPLIES TO ANY FREQUENCY OR ELECTRICAL HEIGHT
Electric
Field Strength
V/m
10.00
31.62
44.67
70.79
86.60
100.00
141.25
173.18
200.00
223.87
244.91
264.55
281.84
300.00
316.23
446.68
707.95
1000.00
50.00
270
90
70
50
42
38
30
30
26
26
22
22
22
22
22
18
14
10
25.00
174
70
54
38
34
30
26
22
22
22
18
18
18
18
18
14
10
10
Transmitter Power (kW)
10.00 5.00 2.50 1.00
114
50
38
30
26
26
22
18
18
14
14
14
14
14
14
10
6
6
90
38
30
26
22
22
18
14
14
14
10
10
10
10
10
6
6
6
70
30
26
22
18
18
14
10
10
10
10
10
10
10
10
6
6
6
50
26
22
14
14
14
10
10
10
6
6
6
6
6
6
6
6
<2
0.50
38
22
18
14
10
10
10
6
6
6
6
6
6
6
6
6
<2
<2
0.25
30
18
14
10
10
10
6
6
6
6
6
6
6
6
6
<2
<2
<2
0.10
26
14
10
6
6
6
6
6
6
6
6
<2
<2
<2
<2
<2
<2
<2
                               155

-------
                                  Appendix  E
     The output from the FM model is more specific than the  summarized  results
presented in Tables 6  through  43.   As indicated in  Figure  13,  the model  also
calculates  the  farthest   distance  from  the  station  at  which  each   of  18
alternative power density  levels is exceeded.   This information is useful  in
determining property or fencing  requirements  in order to  comply with  a  given
guidance  level.   Figures   76  through  85   are  histograms   illustrating  the
percentages of stations exceeding  each  guidance  level  at  various distance
intervals.  The  results for  single  ground  mounted  stations  are presented  in
Figures 76  through  80.  Figures 81  through  85  show  the  results for multiple
ground mounted stations.
                                      156

-------
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Figure 77.  Percentages  of  SFMG exceeding alternative guidance levels  to  specified distances.

-------
               « Or STBTIOMt
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              Figure  78.   Percentages  of  SFMG  exceeding alternative guidance levels  to  specified distances

-------
                     800 uH/cm-2
                                                                                900 uH/cm-2
   » «r craria
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Figure 79.  Percentages of SFMG exceeding alternative guidance  levels to specified distances.

-------
                 5000 uH/c«"2
                                                                             10000 uH/cm-2
    77*
                                                            it
    «-!• l»-*t M-M
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Figure 80.  Percentages of SFMG exceeding  alternative guidance levels to specified distances,

-------
                »-!• I»-M M-M
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             Figure  Bl.   Percentages of MFMG exceeding  alternative guidance  levels  to specified  distances.

-------
                    75 uW/cm-Z
                                                           180 uH/cn~2
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Figure 82.  Percentages  of MFMG exceeding alternative  guidance  levels to  specified  distances.

-------
                    400 uH/cm"2
                                                                                   500
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Figure 83.  Percentages  of MFMG exceeding  alternative guidance  levels to  specified distances.

-------
                                  888
                                                                                                 980 uH/cm-3
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                 Figure 84.   Percentages of MFMG exceeding  alternative  guidance levels to  specified distances,

-------
CTl
cn
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                                    .KID..
              Figure 85.   Percentages of MFMG exceeding  alternative  guidance  levels to  specified  distances.

-------
                                  Appendix F
                          Preliminary Survey Results
     Early in 1984, a survey was conducted to  obtain  more  detailed information
about  FM  broadcast  facilities.   Since  the  proposed  Guidance   level  at  FM
                                                                       2
frequencies   is   not   anticipated  to   be   lower   than   100   uW/cm ,   the
questionnaire was sent only  to  those stations which  the model  predicted  could
exceed this  value.   Thus the survey results  apply most  directly to  guidance
level  6  (100  uW/cm )   but  also  provide  a  source  from  which  information
concerning higher guidance alternative levels can be extracted.

     Station-by-station  analyses ire planned which will  use the  modeling  and
survey  results  to determine  whether each  station has  sufficient fencing  or
property  to  exclude  areas  in  which it  is  predicted  to  exceed the  various
guidance  levels  (above  guidance level 6).   This more detailed  application  of
the  modeling  results  will reduce  the  impact predicted for  the FM service  by
introducing the less expensive  "fix" of  fencing or posting  the necessary area
around the station.

     Approximately  52  per  cent  of  the  1,118  questionnaries  mailed  were
returned.   Preliminary  analyses  of  the  results  have  been  performed  which
provide a statistical view of certain aspects  of FM facilities  and nearby land
usage.  A copy of the questionnaire is shown below.

     Question 2  was  included to determine  the number of  potentially  impacted
stations  which  are  remote from other human  activities.   Such  stations  may  be
required  only  to  post  warning  signs  in  order  to  comply  with   a  given
alternative guidance level.  It is unknown at  this time whether or not posting
will be considered a sufficient compliance measure although  it  does seem to be
a  reasonable approach  for  mountaintop  and  other remote  station  locations.
Table  55  shows   the   breakdown  of   responses  to  this  question   as   of
March 28, 1984.    Respondents  were  permitted  to  check   one   or  two  of  the
descriptions  in  question  2  so the  total  responses  to  this   question  exceed
100 percent.  A prioritizing scheme  will  be applied  to these responses  when a
more detailed analysis is performed.   The table headings are described below:
                                      167

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                                                   OMB Clearance No. 2060-0045
                               EPA QUESTIONNAIRE
                                                           Please do  not
                                                           remove this  label
1.  Name and telephone number of person responding  to  survey:
2.  Check  one  or  two of  the  following  statements which  best  describe  the
    location of your transmitter:
        |~|     Downtown or Urban
                Residential or Suburban
                Industrial - Comercial
                Rural
                Remote from other human activities
3.  What  is  the shortest  distance,  d, between  the base  of your  transmitter
    tower  and the  fence  surrounding  the tower?   Write  "0"  if  there  is  no
    fence.  Approximate  this value if site plan is not readily available:
Example:
              Fence
                   Tower
                     lo'
                     A
d = 20'
                                    168

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4.
What is the  shortest  distance,  r,  between your transmitting tower and  the
boundary  of   the   property  owned  or   leased   for  operation  of  your
transmitting  facility?   Approximate  this distance  if  site  plan  is  not
readily available:
      r =
Example:
                Property Boundary
                Tower
            •/oo'	X
                   So'
            	1
                                                              r = 80'
5.  Is the property boundary fenced?
        H     Yes
        Cl     No

6.  Check the box(es) which best describe your antenna facility:
        l_|     Only broadcast antenna on tower
        M     Co-located with other broadcast antennas  on  same  tower
        l~l     On tower located near other transmitting  facilities  (antenna  farm)

7.  Do you anticipate an antenna replacement within:
        l~l     0-3 years
        l~|     3-5 years
        |~|     5-10 years
        l~l     Not anticipated
                                   169

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Answer the  following  questions only  if  your antenna  is  located on  top  of a
building.
8.  Is the rooftop accessible to the public?  (observation  deck,  swimming
    pool, etc.):

         PI     Yes

         PI     No
9.  Are  there  any  nearby  buildings  of  comparable  or  greater  height?   (We
    define "nearby" as within one city block.):

        II!     Yes

        PI     No
10. What  is  the   street  address  of  the  building  on  which  your  tower  is
    located?  Include city,  state, and zip  code:

    Building: 	


    Street Address: 	


    City, State, Zip Code: 	
Please return  this  questionnaire  to the U.S. Environmental  Protection  Agency,
Attn:  Paul  Gailey,  Office  of Radiation Programs,  P.O.  Box 18416, Las  Vegas,
NY  89114.
                                    170

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     SFMG - Single FM stations on ground-mounted towers
     SFMB - Single FM stations on building-mounted towers
     MFMG - Multiple FM  stations  at the  same  site on  ground-mounted  tower(s)
            (questionnaire was sent to only one station at the site)
     MFMB - Multiple FM stations at the  same  site on  building mounted tower(s)
            (questionnaire was sent to only one station at the site)

              TABLE 55.  PRELIMINARY RESULTS FOR SURVEY QUESTION 2
Location of
SFMG
SFMB
MFMG
MFMB
Transmitter   Number  Percent   Number  Percent   Number  Percent   Number  Percent
Downtown or     37      8.2       44     53.7
  Urban
Residential    108     23.9       39     47.6
  or Suburban
Industrial -    45     10.0        4      4.9
  Commercial
                                  4

                                  9

                                  4
                       9.8

                      22.0

                       9.8
                7

                1

                2
    70

    10

    20
Rural
Remote
214
142
47.3
31.4
11
8
13.4
9.8
15
18
36.6
43.9
1
2
10
20

     Question  3 was  designed  to  reveal  the  number  of  stations  which  are
already  fenced  to  sufficient  distance  to  prevent  exceeding  the  various
alternative guidance levels.   The responses  were compiled into  histograms for
SFMG and MFMG for an overview  of  existing  fences  (Figure 86).   An extention of
this  analysis  could include  a comparison of  each  survey  response with  the
modeling results for that  station to determine the number of  stations  already
possessing sufficient fencing.  It  should  be  noted that some stations may have
one fence close to the tower and  another fence  at  some distance or surrounding
the property boundary.   Question  5  is intended to help  reveal  this  condition.
Responses to question 3 probably  refer to  the closest  fence,  so in cases where
the answer to  question  5  was yes,  the  response to question  4 was used  as the
fencing distance.  Results shown  in  Figure 86 can be  compared  to the modeling
                                      171

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         DISTHNCE FROM TOWER TO FURTHEST FENCE   -   SFMG
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         DISTflNCE  FROM TOWER TO FURTHEST  FENCE  -  MFMG

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   Figure  86.   Distribution of  distances from FM towers
                      to  furthest  fence.
                               172

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results  for 100  wW/cm2 shown  in Figures  77  (for  SFMG)  and  82  (for MFMG).
The modeling results  indicate  that  96.4  percent of the SFMG stations exceeding
          2
100  nW/cm  do  so only  at  distances  less than 50 meters.   The  survey  results,
however,  show  only 20.1  percent of  the stations  having  fences  to distances
greater   than  50  meters.   It  can  thus  be  roughly   estimated  that  about
                                                                2
20 percent  of  SFMG  stations  predicted  to   exceed  100  wW/cm    are  already
sufficiently  fenced   and  would not  actually  be  impacted  by  such  a  guidance
level.   Similarly, 90.2 percent of MFMG sites predicted to  exceed 100
do  so  to distances  of  70 meters  or  less.   The  survey  results  show  only
11 percent  of  MFMG   stations   to  have  fences   at  distances  greater  than
70 meters.  A  reduction in impact of  10  percent  or greater  might  be expected
for these stations.

     Figure  87 illustrates  the  responses  to  question 4  in  histogram form.
Although  the  question  3 responses  indicate  only  a modest,  yet  significant,
reduction in impact due  to  existing  fences, the question 4 results reveal  that
a  substantial   decrease  in  predicted  impact   may  occur  because   of  property
control.   In   cases  where  a  station owns  or controls  sufficient  property,
erection  of  a  fence  to exclude  areas exceeding  the  guidance  may be  a  less
expensive  "fix."   When  the  final  analyses  of  the  survey  responses  are
completed, the  results  will be sent  to  Lawrence  Livermore National Laboratory
for economic analysis.   As  mentioned previously,  96.4  percent of SFMG stations
                                 2
predicted  to  exceed  100  wW/cm   do  so  only  to  distances  of  less  than
50 meters.  The survey  results  indicate that  54.8  percent of  these  stations
own or  control  property  to  distances greater  than 50 meters  from  their tower.
Over  30 percent of MFMG sites  own  or control property to  distances  greater
than 70 meters from their tower.   Question  6 was  included  to identify multiple
sites  and  distinguish between  cases where stations  are  located  on the  same
tower and cases where towers are located close together.

     Actual  impact of  the "antenna fix" mitigation  strategy  depends  partly on
the time frame in which  a station  intends to replace their antenna for  reasons
other  than  Federal  Guidance.   The  responses to  question   7,  as  shown  in
Table 56, give an indication of this time frame.
                                      173

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        DISTRNCE FROM TOWER  TO PROPERTY BOUNDRY  -  SFMG
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        DISTRNCE FROM TOHER TO PROPERTY BOUNDRY -  MFMG

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      Figure 87.   Distribution of distances  from tower
         to  property boundary from survey  results.
                               174

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                          TABLE  56.   TIME  FRAME FOR ANTICIPATED ANTENNA REPLACEMENT
Time Until Anticipated
Antenna Replacement
   0-3  years
   3-5  years
   5-00 years
Not anticipated
   SFMG
Number Percent
  95
  20
  22
 304
20.0
 4.6
 4.9
69.5
SFMB
Number
09
9
6
48
Percent
23.2
00.0
7.3
58.5
MFMB
Number
00
3
0
26
Percent
26.8
7.3
2.4
63.4
MFMB
Number
6
0
0
3
Percent
60.0
00.0
0.0
30.0
TOTAL
Number
030
34
29
390
Percent
22.4
5.8
5.0
66.8

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     Questions 8, 9,  and  10 relate only  to building-mounted stations.  Of  76
responses to question 8, 75  indicated  that  the rooftops on which  their towers
are mounted are not accessible to  the  public.   Question 9 asks whether  or not
there are buildings of  comparable  or greater  height  within  one city  block  in
order to address the  problem of  beam interception by nearby buildings.  Of  76
responses,  30  stations  (39.5  percent)   Indicated  that  there  were  nearby
buildings of comparable or greater height.  Question  10  of the  survey provides
exact information about  the locations of building-mounted  stations so  that  a
more detailed  analysis  of  building-mounted stations can be  performed  in the
future.
                                      176

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