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
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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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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
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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.
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
*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
!-•
0)
4-1
U.
4J
O
t-t
0)
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
e e
bel out
WfiVE
FIX
9
72
€4
79
€6
iei
26
95
7
144
11
134
e
7
e
guidance level with:
RUTENNR ONE 1/2
WRVE FIX
1
i
3
3
3
6
e
e
i
2
e
e
e
o
e
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
e
e
0
e
e
e
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
e
3
27
22
31
35
64
11
38
3
52
e
26
e
1
e
i
i
i
e
e
8
e
e
e
8
e
e
e
e
e
UNFIXRELE
5
3
5
6
e
i
e
e
e
e
e
e
e
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
8
8
0
0
8
8
e
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
-------
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
-------
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
-------
©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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
— 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
-------
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
-------
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
-------
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
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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
L.
U
QL
U.
O
I
U)
u
(/)
cr
i
a.
-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
U)
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.
*•
U
• 40
u
a
*>
o
»-
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
-------
10
* or
M
II
41
n
•- 11
&m
•-M M-M >»-4l 4»-SI
BITMI
CT71
77?
F73
u-u M-ri r^-n
tt CM
rn
H)
F
\
ii
ii
i
i
4
t
^
^
^
7*
P
1
^X
i
1 W-MW-IH >!• *->• '*-" X"1* »-4* 4*-M u~u
20 uH/cn~2
s or mmtM
ii
14
II
1
«
4
I
I
i
^
P
Pi
1 I
1 |
^ y/
V %
v> i
§
1
I
> or rrmiaa
„
ii
i
*
t
•
I
: |
OltTKCC (1C
III
1 ^
II
(•-71 7|-M •
TOBI
, i
.
y/mv/
// YSA //
'/, VA ft
»-MH-l« >!•
50 uH/cm-2
Y/
y-
'//,
'/>f
//
I
I
i
y/
M-tl 4*-M «-»• W-71 7»-M
tirmtx
•-II It-M »->• »•-«•
-------
75 uH/cm-2
1HH
> or EMUONC
14
II
<•
.
4
1
\
%
I
1
l-ll I*-M t»-M M-4
%
'/y
XX
7/
14
It
14)
1
1
4
1
i
7//
1
'/,
k,
1 4C-M M-M (•-?• ?»-M M-MM-IM »IM *-!• !»-»• M-M M-4« 4B-M M-t* U-71 W-M M-MM-IM >1M
llflM
cn
00
> or ttiniow
14
II
II
1
1
4
Y7*
1
^
\
^
•-II I»-M M-M M-4
300
ta (reran) unmtx (nn»i
uW/c»"2 300 uH/cn^S
« v cimiow
14
II
I*
•
1
4
1
' ?\
V-
^
7~r.
''',
\
Lrn
1 «•->• »il (*-?• r»-M M-MM-IM >IM »•!• !•-» M-M JtHI 4*-M M-M ••-'• 7«-M M-MM-IM >IM
OUIM
HO. IIC1UKI iliUWtt (ItnOtl
Figure 77. Percentages of SFMG exceeding alternative guidance levels to specified distances.
-------
« Or STBTIOMt
II
I
uN/cm-2
500 uW/cm*2
a or
4*-5i sa-M M-TI
outrun orient
OtSTIMCC (ICTOKI
en
10
ii
» Or fTHTlOM
I
608 uH/c«-2
IV I* ta-tl t»-M M-41
4IVW 34VU lt-71
DICTIMX
708 uH/co-2
* or trmie
ii
I-II ll-tl M-M M-41 4t-M M-M M-7t
OlSIIWCt IMCTOKI
Figure 78. Percentages of SFMG exceeding alternative guidance levels to specified distances
-------
800 uH/cm-2
900 uH/cm-2
» «r craria
it
it
•-II M-M M-M M-«« 4»-S» M-M W-r« M-M M-MM-IM MM
itcrma urtasi
•-II Ifr-M M-M M-W 4«-M
Bitnwoc CICTOWI
iroe
2080 uH/cp-2
i«
it
ii
it
w
•-II II-ZI M-M M-4» 4»-M M-M M-r«
•I>THNCC (irturti
•-II
M-M M-41 4»-M M-M M-7*
MfTIWX IKTOil
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
•ItTIMZ
»•!• !•-(• (»-M M-«« 4C-M MHM
Dtsima
Figure 80. Percentages of SFMG exceeding alternative guidance levels to specified distances,
-------
»-!• I»-M M-M
I uH/cn-2
IB uH/cm-2
t «r •lira
tm """
]
M
M
K
•
I
_. .* ••_•• M M — — *•_«• ««_«« u-n m-«a •••••• li
•
1
1
>!•
0>
ro
20
50
v nia
It
11
w
*
•
1
\
\
I
s
•
\
1
•^
*-!• I»-M M-M M-«« «-M MH« ••-» >M» M-MM-IM M
nnwa iiniMi
tin*
w
M
It
W
4
t
•
1
1
1
]
s
I
I
\
»-!• IHt M-M M-4« <»-5» MM §•-»• ?*-M M-MM-IM MM
MCTIWX (ICTCMI
Figure Bl. Percentages of MFMG exceeding alternative guidance levels to specified distances.
-------
75 uW/cm-Z
180 uH/cn~2
< or tins
14
II
M
1
1
4
1
•
1
1
1
1
ii
J
•-It I»-M M»-M *-*• <•-«• M>-M M-71 »-« M-M M-Itt »
« or titrt
14
II
4
1
•
I
8
#
|
^z
w
^j
I
I
'/',
1
1
S
1
•-It II-M W-M »-«•
4»-M W-M «•-/•
ntnwz
888
388
« or
14
It
II
1
4
1
•
P
1
I
•VII ID-tl N-M
Htm
ft-71
iictmi
« tr itia
14
II
II
I
1
4
t
•
I
^
1
1
!
^
!
ii
•-It It-ft I*-M M-41 4»-M M-M M-FI W-M W-MM-IH IIM
-M
•irnwcc
Figure 82. Percentages of MFMG exceeding alternative guidance levels to specified distances.
-------
400 uH/cm"2
500
« or tilts
l-ll II-N f*-M M-W
« or cttti
VTA
i*
14
If
II
1
t
•
1
^
y/
v/
VS
/'
\
I
\
4I~M »-il M-M rut M-MM-MH MM «-!• !•*• «•-»• *++• 41-SI M-W M-» »»«• W-MH-W >!•
•nmcc (icitMi oiiKMj. CIK.IIMI
6B0 uH/cn-2
ii
14
II
II
1
1
4
1
•
1
I
•-II ll-fl II-M M-«l
«»-M JI-M *•-»
nciani
?•-«• M-MM-IM >IW
700
« or nm
It
/.•
l-ll II-M t»-M >•-«• 41-51 M-M M-n W-M W-MW-IM >ll
Figure 83. Percentages of MFMG exceeding alternative guidance levels to specified distances.
-------
888
980 uH/cm-3
i or tiiu
II
14
It
•
1
4
t
•
l
i
1
i
II
14
It
1
«
f
•
I
m
\
%
\
1
®
•Ml lf-n M-M M-W 4*-M M-M M-/« W-M M-MM-IM MM
*-!• !•-(• M-M
M-41 4»-M M-M
ticnmi
en
on
I888 uH/CB-2
2088
* v MTU
It
14
It
M
•
1
4
1
•
s
•-It !•-«• t»-M M-M
4»-M M-M M-M
CICICMI
It
ntn
3
»•!• !•-<• W-M M-41 «*-M
Figure 84. Percentages of MFMG exceeding alternative guidance levels to specified distances,
-------
CTl
cn
n
it
ii
t v tins
I
seaa
laeee
*-!• W-M !4>-i
» «r Mid
•-II l»-n M-M t*-« ««-»• «^W M-M
.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
* or CTOTIMC
•
\
I
11 1 1 !••. IS/A m
J
N
IOC »»->• I»-M M-M M-« 4*-U M-M
nsriMCC
M-71 7»-M M-M M-1M >1M
DISTflNCE FROM TOWER TO FURTHEST FENCE - MFMG
I OF ITHTIOW
73
•
\
1
'X/ rm ^7/! rm VTA Y/A k/X
4S
19
MOW >•-!• It-M t»-M »•-«• 4»-M $•-*• tt-7t 71-M M-M M-IM >IM
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
« or rmnoe
n
Y/
%
%
yy
\
i
1
1
1
\
I
1
is
IOC >*-!• !•-(• »-M M-41 4*-U 9>-M ••-?• ?•-»• M-M •*-!• >!•
BIITIMCE (ICTCRS)
DISTRNCE FROM TOHER TO PROPERTY BOUNDRY - MFMG
* or cmuow
I
\
\
i
I
^ £3 ^
1
I
I
19
If
IOC >»-!• I»-n M-*l *-*• <«-M S*-M W-71 ?•-•• U-M M-1M >1H
umwcc cicTaa>
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.
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