ORP tAD 78-4
,;une 1978
vvEPA
Near-Field Radiation
Properties of Simple Linear
Antennas with Applications
to Radiofrequency Hazards
and Broadcasting
.»".? 520/
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Office of Radiation Programs Technical Publications
Nonionizing Radiation
Publications of the Office of Radiation Programs are available
from the National Technical Information Service (NTIS), Springfield,
VA 22161. Current prices should be obtained directly from NTIS
using the indicated NTIS Order number. Single copies of some of
the publications listed below may also be available without
charge from the Office of Radiation Programs (AW-461), 401 M St.,
SW Washington, DC 20460.
EPA ORP/SID 72-3
EPA/ORP 73-2
EPA-520/2-73-001
EPA-520/1-74-005
EPA-520/2-74-008
ORP/EAD 75-1
ORP/EAD-76-1
ORP/EAD-76-2
EPA-520/2-76-008
ORP/EAD-77-2
Reference Data for Radiofrequency Emission
Hazard Analysis (NTIS Order No. PB 220 471)
Environmental Exposure to Nonionizing
Radiation, (Available NTIS only, Order
No. PB 220 851)
Nonionizing Measurement Capabilities: State
and Federal Agencies (Available NTIS only,
Order No. PB 226 778/AS)
RF Pulse Spectral Measurements in the
Vicinity of Several ATC Radars (NTIS Order
No. PB 235 733)
An Evaluation of Satellite Communication
Systems as Sources of Environmental Micro-
wave Radiation (NTIS Order No. PB 257 138/AS)
An Analysis of Broadcast Radiation Levels
in Hawaii (NTIS Order No. PB 261 316/AS)
Radiation Characteristics of Traffic Radar
Systems (NTIS Order No. PB 257 077/AS)
A Measurement of RF Field Intensities in
the Immediate Vicinity of an FM Broadcast
Station Antenna (NTIS Order No. PB 257 698/AS)
An Examination of Electric Fields Under EHV
Overhead Power Transmission Lines (NTIS
Order No. PB 270 613/AS)
An Investigation of Broadcast Radiation
Intensities at Mt. Wilson, California
(NTIS Order No. PB 275 040/AS)
ORP/EAD-77-3
An Analysis of Radar Exposure in the San
Francisco Area (NTIS Order No. PB 273 188/AS)
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NEAR-FIELD RADIATION PROPERTIES OF SIMPLE LINEAR ANTENNAS
WITH APPLICATIONS TO RADIOFREQUENCY HAZARDS AND BROADCASTING
Richard A. Tell
June 1978
U.S. Environmental Protection Agency
Office of Radiation Programs
Electromagnetic Radiation Analysis Branch
P.O. Box 15027
Las Vegas, Nevada 89114
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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 recommendation for their use.
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PREFACE
The Office of Radiation Programs of the U.S. Environmental
Protection Agency carries out a National program designed to
evaluate population exposure to ionizing and nonionizing radiation,
and to promote development of controls necessary to protect the
public health and safety. This report examines some of the
radiation properties of transmitting antennas, giving special
consideration to the area very near to such antennas which is of
most interest when evaluating potential radiofrequency hazards.
Readers of this report are encouraged to inform the Office of
Radiation Programs of any omissions or errors. Comments or
requests for further information are also invited.
FloyoL. Galpin, Director
Environmental Analysis Division
Office of Radiation Programs
iii
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ABSTRACT
Analytical expressions for the electromagnetic field have
been used to compute electric and magnetic field strengths in the
near-vicinity of dipole and monopole transmitting antennas. In
particular the fields about a A/2 dipole are mapped to illustrate
the magnitude and wide variation of field intensity which occurs
near the feed point, along the axis of the dipole, and about the
tip of the radiating arms. These results are analyzed in terms
of evaluating potential radiofrequency hazards which may exist
extremely near dipole like broadcast antennas and the aspect of
close proximity exposure of maintenance personnel is discussed.
The field expressions are used to determine the extent of near-
field gain reduction which occurs at distances close to the
antenna and this is compared with measured field intensity data
taken for a half-wave dipole using a short, nonperturbing field
probe. The results for a single dipole are used to model a
vertically stacked array of dipole radiating elements used to
simulate a typical FM broadcast transmitting antenna. Expected
field intensities are then determined for positions close to such
radiating structures to assess the potential of biologically
significant fields existing about present day FM broadcast
installations. It is found that near-field gain reduction can be
significant for typically encountered building exposures wherein
main beam illumination is possible.
iv
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TABLE OF CONTENTS
Page
ABSTRACT iv
LIST OF FIGURES vi
INTRODUCTION 1
FIELDS OF A DIPOLE 6
FIELDS OF A MONOPOLE 16
NEAR FIELD GAIN REDUCTION OF DIPOLE ARRAYS 18
VERTICAL RADIATION PATTERNS 30
SUMMARY 35
REFERENCES 36
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LIST OF FIGURES
Page
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
ACCUMULATIVE FRACTION OF POPULATION WITH
EXPOSURE <_ THE INDICATED LOG OF THE POWER
DENSITY
ELECTROMAGNETIC FIELD INTENSITY ASSOCIATED
WITH VARIOUS LEVELS OF THERMAL LOADING ON
SELECTIVELY ABSORBING TISSUES AND THE
WHOLE BODY
GEOMETRY FOR COMPUTATION OF FIELDS OF A
DIPOLE
ELECTRIC FIELD STRENGTH VS. DISTANCE FOR
DIPOLES OF DIFFERENT LENGTHS
MAGNETIC FIELD STRENGTH VS. DISTANCE FOR
DIPOLES OF DIFFERENT LENGTHS
WAVE IMPEDANCE VS. DISTANCE FOR DIPOLES OF
DIFFERENT LENGTHS
SPATIAL ELECTRIC FIELD PATTERN IN THE
VICINITY OF A HALF-WAVE DIPOLE ANTENNA AT
FIVE DIFFERENT RADIAL DISTANCES, F=100 MHz
ELECTRIC AND MAGNETIC FIELD STRENGTH IN THE
VICINITY OF MONOPOLES OF COMMON HEIGHT IN THE
AM STANDARD BROADCAST SERVICE
PHOTOGRAPH OF EXPERIMENTAL NEAR-FIELD GAIN
REDUCTION MEASUREMENT SET UP
CALCULATED AND MEASURED RELATIVE ELECTRIC
FIELD STRENGTH NEAR AX/2 DIPOLE AT 144 MHz
RMS ELECTRIC FIELD STRENGTH VS. DISTANCE ON
THE AXIS OF THE CENTER OF RADIATION FOR A
12 BAY DIPOLE ARRAY WITH IX BAY SPACING AND
UNIFORM, IN PHASE POWER DIVISION
4
6
8
9
12
14
17
20
21
23
VI
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LIST OF FIGURES (Continued)
Figure 12. NEAR-FIELD GAIN REDUCTION FOR A 2 BAY
DIPOLE ANTENNA ARRAY OPERATING AT 100 MHz
WITH IX BAY SPACING AND UNIFORM, IN PHASE
POWER DIVISION TO EACH BAY
Figure 13. NEAR-FIELD GAIN REDUCTION FOR A 3 BAY
DIPOLE ANTENNA ARRAY OPERATING AT 100 MHz
WITH IX BAY SPACING AND UNIFORM/ IN PHASE
POWER DIVISION TO EACH BAY
Figure 14. NEAR-FIELD GAIN REDUCTION FOR A 4 BAY
DIPOLE ANTENNA ARRAY OPERATING AT 100 MHz
WITH IX BAY SPACING AND UNIFORM, IN PHASE
POWER DIVISION TO EACH BAY
Figure 15. NEAR-FIELD GAIN REDUCTION FOR A 5 BAY
DIPOLE ANTENNA ARRAY OPERATING AT 100 MHz
WITH IX BAY SPACING AND UNIFORM, IN PHASE
POWER DIVISION TO EACH BAY
Figure 16. NEAR-FIELD GAIN REDUCTION FOR A 6 BAY
DIPOLE ANTENNA ARRAY OPERATING AT 100 MHz
WITH IX BAY SPACING AND UNIFORM, IN PHASE
POWER DIVISION TO EACH BAY
Figure 17. NEAR-FIELD GAIN REDUCTION FOR AN 8 BAY
DIPOLE ANTENNA ARRAY OPERATING AT 100 MHz
WITH IX BAY SPACING AND UNIFORM, IN PHASE
POWER DIVISION TO EACH BAY
Figure 18. NEAR-FIELD GAIN REDUCTION FOR A 10 BAY
DIPOLE ANTENNA ARRAY OPERATING AT 100 MHz
WITH IX BAY SPACING AND UNIFORM, IN PHASE
POWER DIVISION TO EACH BAY
Figure 19. NEAR-FIELD GAIN REDUCTION FOR A 12 BAY
DIPOLE ANTENNA ARRAY OPERATING AT 100 MHz
WITH IX BAY SPACING AND UNIFORM, IN PHASE
POWER DIVISION TO EACH BAY
23
24
24
25
25
26
26
27
vii
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LIST OF FIGURES (Continued)
Figure 20. NEAR-FIELD GAIN REDUCTION FOR A 16 BAY
DIPOLE ANTENNA ARRAY OPERATING AT 100 MHz
WITH IX BAY SPACING AND UNIFORM, IN PHASE
POWER DIVISION TO EACH BAY 27
Figure 21. CALCULATED POLAR VERTICAL RADIATION PATTERNS
FOR DIPOLE ANTENNA ARRAYS'WITH 2, 3, AND
6 VERTICALLY STACKED BAYS SPACED IX APART
AND FED IN PHASE WITH UNIFORM POWER DIVISION 32
Figure 22. CALCULATED VERTICAL PATTERN FOR A 6 BAY
DIPOLE ANTENNA ARRAY WITH IX BAY SPACING
AND UNIFORM, IN PHASE POWER DIVISION TO EACH
BAY 33
Figure 23. CALCULATED VERTICAL RADIATION PATTERN FOR A
6 BAY DIPOLE ANTENNA ARRAY WITH 0.5X BAY
SPACING AND UNIFORM, IN PHASE POWER DIVISION
TO EACH BAY 33
Vlli
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INTRODUCTION
The Environmental Protection Agency (EPA) currently has
under consideration the development of environmental criteria
relative to exposure of the general population to radiofrequency
(RF) and microwave (MW) radiation. This nonionizing radiation
program within EPA has been investigating the problem since 1972.
A description of some of the preliminary efforts by EPA at that
time have been described by Mills et al., (1971) and Tell (1973).
A continuing effort has been to accurately determine the
range of present day RF and MW exposure of the general population
in that this environmental data is necessary prior to evaluating
the potential biological significance of typically encountered
exposures. Such an evaluation is, in turn, requisite to estab-
lishing environmental safety criteria.
Numerous investigations of RF and MW field intensities have
resulted from the EPA program and have included specific examina-
tion of radars (Tell and Nelson, 1974a; 1974b; Tell et al.,
1976a; Hankin, 1976; Tell, 1977a), satellite communication earth
terminals (Hankin et al., 1974; Hankin, 1974), microwave ovens
(Tell, 1978), high frequency emitters (Tell et al., 1974), and
broadcast stations (Tell, 1975; Tell, 1974; Tell and Nelson,
1974c; Tell and O'Brien, 1977; Tell, 1978b).
These studies reveal that the most likely contribution to
ambient environmental levels of RF and MW radiation is due to
transmitters in the broadcast service comprising the various AM,
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FM, and television stations. This contention had been previously
discussed (Tell, 1972) and has served as impetus for subsequent
examination of broadcast fields from the viewpoint that broadcast
stations, generally speaking, are our most significant sources of
exposure (Tell and Janes, 1975).
An extensive field measurements program since 1975 using a
specially developed mobile RF measurement system (Tell, et al. ,
1976b) has provided extensive environmental data on exposure of
the general population to broadcast fields (Janes, et al., 1977a;
1977b; Athey, et al., 1978). These analyses, relying on the
environmental measurement data and using computer techniques to
manipulate an automated population data base, reveal that the
typical median VHF and UHF exposure level of the population in
the cities studied by EPA so far is about 0.1 pJ/m3 electric
field energy density or the far field equivalent of 0.007 yW/cm2
power density. Figure 1 illustrates the results on population
exposure wherein the accumulative fraction of the population in
the 10 cities studied exposed to various levels of RF is plotted.
It is determined for example that less than 1 percent of the
population is exposed to intensities greater than the far field
equivalent of 1 yW/cm2.
In addition to the information developed on exposure of the
population as a whole, data relating to very high RF broadcast
fields which can be experienced occupationally have been obtained
(Tell, 1976). Another interesting aspect of relatively high
intensity broadcast fields is the illumination of tall buildings
adjacent to certain VHF and UHF broadcast facilities wherein
mainbeam illumination of the building and its occupants is possible
(Tell, 1978b). The identification of these hot spots is a continuing
effort within EPA and promises to provide added detail on the
extent of relatively high RF and MW exposure of various segments
of the population.
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FRftCTION OF POPULRTION EXPOSED flS fl
FUNCTION OF POWER DENSITY
.99
.95
.9
.8
.7
.6
.5
A
,3
.2
.1
.05
.01 .
CITIES:
BOSTON
ATLANTA
HIAAI
PHILADELPHIA
NCN VORK
CHICAGO
MASHINOTON
LAS VEftflS
SAN OIEOO
PMTUWO
-5 -4 -3-2-1 01 2
LOG S; S » POWER DENSITY IN UW/CM/CM
Figure 1. Accumulative fraction of population with
exposure <_ the indicated log of the power
density.
Fundamental advances, within especially the last three
years, in the study of RF and MW dosimetry as it pertains to the
human body, have shown that man is not transparent to radiowaves,
even at low frequencies. In fact, it is now known that the adult
exhibits a whole body resonance phenomena in the range 70-80 MHz
wherein the body absorbs significantly more power from incident
RF fields than had previously been envisioned. Tell has reviewed
this dosimetric development (Tell, 1978c) and Figure 2 illustrates
the frequency response of human absorption. It is interesting to
note that at resonance the adult body may absorb energy at a rate
equivalent to 2.4 times the basal metabolic rate of the resting
body when exposed in free space to a field intensity equivalent
to the currently accepted RF occupational exposure standard,
i.e., 10 mW/cm2 (OSHA, 1971; ANSI, 1974). Due to rather selective
absorption rates of different tissues, the complex inhomogeniety
of the internally distributed absorbed power, and physiological
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FREE SPACE, PLANE WAVE EQUIVALENT POWER DENSITY (mW/cm2)
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-------
mechanisms which control heat dispation throughout the body,
there is considerable attention presently being given to the
reexamination of the OSHA safety standard even for occupational
applications (NIEHS, 1977).
Because of the particular resonance frequency found for man
and the possible local intensification of energy deposition in
specific tissues of between 10 and 30 times the average rate for
the body as a whole (Gandhi et al., 1977; Guy et al., 1976) it is
of special interest to examine the possibility of high intensity
exposure to FM broadcast radiation. This paper presents analytical
results obtained on the near-field radiation properties of simple
linear antennas in the form of monopoles and dipoles. In particular,
the fields about a half wave, tuned dipole are mapped to illustrate
the magnitude and variation of field intensity when in its immediate
environment. Results are also provided for the local fields
around monopoles.
These findings are discussed from the practical aspect of
defining hazardous areas near AM and FM radiating structures.
Computed values of near field gain for a tuned dipole are presented
with additional field measurement data to verify the analytical
results. The phenomenon of near field gain reduction is then
examined from the viewpoint of using the results to predict the
exposure fields in buildings, or any area, immediately adjacent
to VHP and UHF broadcast antennas. Where possible, field measure-
ment data is used to supplement the analytical results for array
near field gain reduction.
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FIELDS OF A DIPOLE
Figure 3 dipicts the geometry used to calculate the electric
and magnetic fields near a dipole. Field equations describing
the electromagnetic field near such a radiator have been taken
from Jordan and Balmain (1968) and are given here.
Ez=-j30I (£j(^_ + e j BR -2cos3H £
R
E =j3Im(Z-H. e
y R
H =j30I (e~j3R
R
Z+H e
-2Zcos3H e
~j3R -
m
2cos He
~jBr
(1)
(2)
(3)
Figure 3. Geometry for computation of fields of
dipole.
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E and E are the parallel and radial components of the electric
field originating from the dipole and H is the magnetic field.
n is the intrinsic impedance of free space or 120irn. Im is the
maximum loop current along the radiator which is assumed to be
sinusoidal in its distribution. A computer program was written
which takes as input the frequency, the diameter of the radiating
arms of the dipole, their length, and the RF power delivered to
the antenna at its feed point. A relation between the RMS
transmitter output power and the current on the antenna was
obtained by finding the feed point impedance of the dipole using
a method developed by Schelkunoff (1943). Because the RMS input
power is used in the procedure all fields are determined to be
RMS fields. For the purposes of this analysis it has been
assumed that all of the transmitter power is effectively delivered
to the antenna. In the case of short, non resonant antennas,
considerable difficulty in obtaining the required impedance
matching and in actuality being able to deliver all available
transmitter power into the antenna is encountered.
It is instructive to explore the nature of the fields very
near to a dipole antenna and in particular look at the variation
in electric field as a function of distance from the antenna.
Using the analytical approach described above, the electric and
magnetic fields of a family of dipole antennas, each with different
lengths, was computed and are presented in Figures 4 and 5. In
this case an RMS input power of 1 watt at 100 MHz was used and
the radius of the wire radiator was 10 m. These results are
similar to those obtained by Mills (1966) except that in this
instance the data are presented in absolute units of distance and
for a specific transmitter frequency and power rather than in a
normalized system of power and distance units. It has been the
i
experience of the author that, though some generality is lost in
using these results, the use of absolute units provides a keener,
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VARIATION OF ELECTRIC FIELD STRENGTH WITH DISTANCE
FOR DIPOLES OF DIFFERENT LENGTHS
P=1 WATT
r=10~10meter
-2
LOG DISTANCE (METERS)
Figure 4. Electric Field Strength vs. distance for dipoles of
different lengths
8
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VARIATION OF MAGNETIC FIELD STRENGTH WITH DISTANCE
FOR DIPOLES OF DIFFERENT LENGTHS
P=1 WATT
r=10-10meter
-4
-3
-2-10 1
LOG DISTANCE (METERS)
Figure 5. Magnetic Field Strength vs. distance for dipoles of
different lengths
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conceptual insight to the spatial distribution of fields that
would be expected near antennas with which we have common day
experience.
From Figure 4 we see that for distances beyond A/2iT (in the
case of 100 MHz radiation A/2ir=0. 48m) the electric field varies
as 1/r or -6dB/octave (-20dB/decade) which corresponds to the
classical far field. For sufficiently short dipoles at distances
less than \/2-n the field varies as 1/r3 or -18dB/octave (-60dB/decade)
and this comprises the so called near field or reactive zone of
the antenna. The slope of the electric field curves go through a
transition region in the neighborhood of A/2ir wherein the slope
varies between -20 and -60 dB per decade or between 1/r3 and 1/r.
It is observed that in the region which is less than the dipole
half length the slope of the curve returns to -6dB/octave. An
interesting observation is that the steep sloped 1/r3 region of
the electric field is really only apparent for sufficiently short
dipoles, i.e., typically less than 0.08A in length (0.04A half
length). What this means is that one does not expect to see 1/r3
fields in the vicinity of the typically encountered, tuned
dipole. In fact the exactly half-wave dipole exhibits the
interesting behavior of a constant field strength in its immediate
vicinity.
Figure 5 shows the variation in magnetic field strength for
dipoles of differing lengths. In this case we see that the slope
of the magnetic field line again changes according to the distance
from the antenna. Beyond the A/2ir point the magnetic field
varies by -6dB/octave (-20dB/decade) or as 1/r. For distances
between A/2-rr and the dipole's half length, the field has a
-12dB/octave (-40dB/decade) slope or 1/r2 characteristic. For
shorter distances, less than the half length, the slope returns
to a -6dB/octave (-20dB/decade) value. For practical applications
of tuned dipoles we see that one expects the magnetic field to
continuously increase until the dipoles surface is reached.
10
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The ratio of the magnitude of the electric field strength to
the magnitude of the magnetic field strength is the magnitude of
the electromagnetic wave impedance. Figure 6 shows the the-
oretical variation of the magnitude of the wave impedance of the
wave radiating from dipoles of various lengths as a function of
distance from the antenna. The dipole is assumed to be in free
space having an intrinsic impedance of 377Q. In the far field
the wave impedance approaches the intrinsic impedance of the free
space medium. In the near field of short dipoles with half
lengths less than about 0. l/\ the magnitude of the wave impedance
exceeds the impedance of the surrounding medium and increases at
a rate of 6dB/octave until the distance reaches a value equal to
the dipole's half length. For shorter distances the wave impedance
remains constant. This high impedance is characterized as being
a high capacitive reactance. The wave impedance decreases as the
electrical length of the dipole is made longer. Mills (1966)
suggests that a logical boundary between the near and far fields
of a dipole is a distance equal to A/271. At this point the field
intensity curves, electric and magnetic, assume a slope of
-6dB/octave and the wave impedance is within about 3dB of the
free space value. A significant feature of short dipoles is
illustrated by these impedance curves. Very short dipole sensing
probes will exhibit a very high wave impedance in their immediate
environment and consequently are desireable for use in non-
perturbing measurements of field intensity, i.e., they will
minimize the load placed on the measured field thereby reducing
the error in such measurements. This is particularily important
when attempting to make field intensity measurements within the
near field of a source wherein the wave impedance of the field
is already high (Bowman, 1974; Wacker, 1970).
Field intensity measurements made near the radiating structures
of a linear, vertically stacked FM broadcast array have shown
that very intense fields exist particularily near the tips of the
dipole like radiators (Tell, 1976). This property of electric
11
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VARIATION OF WAVE IMPEDANCE WITH DISTANCE
FOR DIPOLES OF DIFFERENT LENGTHS
P=1 WATT
r=10-10meter
LOG DISTANCE (METERS)
Figure 6. Wave impedance vs. distance for dipoles of
different lengths
12
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field like convergence has been examined by using the above
discussed analytical formulation and some of the results are
given in Figure 7. This figure shows the spatial variation of the
magnitude of the electric field in the vicinity of a half-wave
dipole antenna driven with 1 kW of power at 100 MHz at five
different radial distances from the dipole's center. At a distance
of 5m, which is clearly in the far field as seen in Figure 4, the
classical far field pattern of a dipole is seen to consist of a
figure eight shape wherein the maximum field intensity is observed
at points broadside to the dipole, or at 0° in Figure 6, with
nulls in the field at points on the axis of the dipole. As the
distance from the dipole is decreased to 2m, the electric field
increases in amplitude and the pattern begins to broaden with
less well defined nulls. At a distance of 1.5m the field pattern
is observed to be almost circular. At distances closer than 1.5m
the pattern begins to elongate showing the intensification of the
local field near the dipole arm tips. At a distance of 0.76m,
when at an angle of 90°, the distance to the tip is only 1cm and
the electric field is extremely intense.
A circular shaded area in Figure 7 defines the spatial area
within which the electric field is less than the ANSI (1974)
recommended safe level for RF workers of 194 V/m. Thus, for
distances less than 1.5m, or about 5 feet, areas about a half
wave dipole radiating 1 kW at 100 MHz will exist in which the
ANSI safety standard for indefinitely long exposure is exceeded.
This analytical verification of the potential of high fields
close in to dipole radiators resonant in the FM band carries a
strong implication. Extreme care should be exercised by personnel
performing tower work in which they may come into close proximity
of VHF broadcast antennas using dipole or dipole-like radiating
structures. The local fields about each such structure in a
multiple bay broadcast antenna is a function of the power delivered
to each bay and stations using relatively low power with small
antenna systems may require just as much or more caution than
13
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Figure 7. Spatial electric field pattern in the vicinity of a
half-wave dipole antenna at five different radial
distances, F = 100 MHz
14
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higher power stations using larger antennas. The power per bay
is the critical factor in localized fields which may exceed the
ANSI safety standard.
Our observation of tower workers routinely working on towers
supporting energized broadcast antennas, often times supporting
several active antennas, suggests that this common practice
merits further attention to minimize or prevent hazardous exposure.
Field measurements near a typical FM array showed that localized
fields could exceed 18 times the accepted safety standard defined
for indefinitely long exposure (Tell, 1976). The ANSI and OSHA
standards permit higher field strengths for exposure durations
less than 6 minutes. But because of the potentially very intense
fields that tower workers might encounter, the permissible
duration of exposure is so small that it is questionable whether
individuals involved in such situations could maintain their
exposure within the allowable limits in view of their limited
mobility on a tower.
An area of particular caution involves the use of auxilliary
transmitting antennas during times when the main antenna is not
being energized. Because auxilliary antennas are often composed
of a single bay or substantially fewer bays than the main antenna,
the possibility of very high intensities exists in its immediate
area. Also, because auxilliary antennas commonly are mounted on
the same supporting tower as the main antenna, the possibility of
working in the associated high field is increased.
The qualitative implications of this analysis for FM
broadcast arrays apply as well to similar considerations for
television antennas and the same degree of caution to insure
minimizing exposure is required.
15
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FIELDS OF A MONOPOLE
A similar analytical method may be used to investigate the
electric and magnetic fields of monopole antennas. This is of
some practical significance with respect to evaluating potentially
hazardous fields near monopole radiators as used in the AM
standard broadcast service. Figure 8 shows the variation in the
magnitudes of the electric and magnetic field strengths for
monopoles of heights 0.1 and 0.5X. These two heights define the
typically encountered extremes of tower heights found in the AM
broadcast service. The calculations assume 50 kW of RMS power are
delivered to the monopole since this corresponds to the maximum
authorized by the FCC for AM broadcast stations. Furthermore it
is assumed that the monopole is driven against a perfectly
conducting ground plane; i.e. , it is perfectly lossless. Thus
these computed field values are conservative in that the finite
conductivity of real earth will reduce the actual fields from
the computed values. Smith (1969) has examined the effects of
finite ground conductivity on monopole fields over a plane earth
model. This assumption provides a degree of conservativeness in
the predicted field amplitudes. A dashed line in Figure 8 is
drawn at the field intensity value that corresponds to both the
maximum electric field strength and the maximum magnetic field
strength permitted for long time exposure in the OSHA and ANSI
personnel safety standards.
It can be seen that for distances less than 10 meters, the
field amplitudes will likely exceed the safe levels recommended
for frequencies above 10 MHz, this being the lower frequency
limit for the present OSHA and ANSI standards. The Air Force
(1975) has established a much higher safety limit for Air Force
personnel exposure below 10 MHz in view of the significantly
16
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decreased body absorption at these lower frequencies (refer to
Figure 2). This higher Air Force standard allows exposure field
intensities equivalent to a far field, plane wave of 50 mW/cm2
power density or a squared field strength value of 188,000 V2/m2.
The strongest implication of these computed results is that field
strengths very near and on the surface of such towers can reach
extremely high values and may represent a hazardous condition for
workers climbing them while energized with RF.
H(.U)
POWER RADIATED-Hk W
CALCULATED ON THE SURFACE OF THE
fiROUND PLANE
I
i
K
fc
--IS"
JUCTRWMA6NETIC_FI£LD_FORMR FIELD
POWER DENSITY OF 1S»w/Mi2
3
n
ii«
IB*
it'
1|2
103
1I«
1I«
DISTANCE FROM MONOPOLE (m)
Figure 8. Electric and magnetic field strength in the
vicinity of monopoles of common height in the
AM standard broadcast service
17
-------
NEAR FIELD GAIN REDUCTION OF DIPOLE ARRAYS
Recent field measurement data collected by the author in
tall buildings situated adjacent to various high power broadcast
installations, in particular FM stations/, has revealed that the
observed field intensity could not, in all cases, be accounted
for by the far field specified effective radiated power (ERP) of
the station (Tell, 1978c). An investigation of near field gain
reduction for linear broadcast arrays showed that when in relatively
close proximity to extended multiple bay arrays, significant
reductions in the measured field intensity would be expected.
To examine the nature of this array near field gain reduction
phenomena, computations and subsequent field strength measurements
were performed first on a single tuned dipole to confirm the
predicted extent of gain reduction. A half-wave dipole was
constructed for operation in the 144 MHz (2 meter) amateur radio
band using 7/8 inch outside diameter rigid copper tubing. A
sleeve balun (KLM model 144-148-62-N) was used to couple a 50fi
coaxial transmission line to the dipole. The dipole was supported
at one wavelength above ground for the measurements. A swept
VSWR measurement technique was used to trim the dipole to resonance
at 144.0 MHz with a final VSWR of 1.02 being obtained. This
dipole was then driven with a Collins KWM-2A amateur transceiver
in conjunction with a Collins 62S-1 VHF transverter to deliver 60
watts of continuous wave power as determined with a Bird model 43
thruline watt meter. The watt meter has a specified occuracy of
0.2 dB at an indicated 60 watts. Test transmissions on 144.020
MHz were made using the author's amateur radio license.
A field sensing probe was constructed for the field'measure-
ments. This probe consisted of a short (0.0624X) dipole of 1.6 mm
18
-------
diameter stainless steel tubing (Omega Engineering) with a
Hewlett-Packard Schotky barrier diode (model 5082-2800) attached
at the center gap using a silver conductive epoxy. At the feed
point very high resistance parallel conductors (20^k /ft) (Polypenco
TFE) were attached and brought through a plastic handle to a
connector which mated with the input to a high impedance digital
voltmeter (DVM). The voltmeter was used to measure the dc voltage
produced as a consequence of the diode rectification of the
induced voltage on the short dipole arms when immersed in an RF
field. This assembly provided a relatively high impedance probe
which minimized undesireable disturbance of the fields which were
to be measured near the half-wave transmitting dipole. Free and
Stuckey (1969) have discussed the implementation of non perturbing
criteria in probe design. Figure 9 is a photograph of the
transmitter set up and the high impedance sensing probe.
The transmitting equipment was set up in a vacant parking
lot and a fiberglass measuring tape was stretched from the half-
wave transmitting dipole center gap to a distant fence some 15m
away. The dipole was activated and voltage measurements were
made using the small sensing probe at small intervals of distance
along the axis of the transmitting dipole. Figure 10 shows the
measured data along with the calculated values of relative field
for the 2m dipole. Calculated field strength values are for the
electric field component parallel to the transmitting dipole. All
DVM voltage measurements were normalized to the voltage obtained
at 1 meter from the transmitting antenna and then the normalized
value obtained for 1 meter was shifted to coincide with the
theoretically determined relative field strength at 1 meter. In
the range of 20 to 170 cm there is good agreement between the
theoretical and experimental values of relative field intensity
clearly verifying the reduction in antenna gain at close distances.
Next, using the same rigorous analytical expressions for the
electric field of a dipole, computations were performed to determine
the resultant electric field magnitude for a vertical array of
19
-------
/\
144 MHz half-wave transmitting dipole, small field
sensing probe (pen points to probe), and DVM
HF amateur radio transmitter, two meter transverter
and dummy load
Figure 9. Experimental near-field gain measurement equipment
20
-------
CALCULATED AND MEASURED RELATIVE ELECTRIC
FIELD STRENGTH NEAR A HALF-WAVE DIPOLE
100C AT 144 MHZ
E
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1000
DISTANCE FROM DIPOLE
Figure 10.
Calculated and measured relative electric field
strength near a A/2 dipole at 144 MHz
21
-------
horizontally polarized half-wave dipoles at any point on the axis
of the center of radiation (CR) of the array. Since the principle
interest in this computational exercise was to examine the near
field gain aspect of FM broadcast arrays, a dipole spacing of one
wave length was used, this being the almost universal spacing
found in FM antennas. It was assumed that the dipoles were
excited at 100 MHz. The calculational procedure used the principle
of superposition of fields or vector addition to determine the
resultant field strength at any point from 1 to 1000 meters on
the axis of the CR. A principle consideration is the relative
phase of each signal being radiated from each dipolar radiator in
the array. At positions sufficiently close to the array, significant
phase cancellation occurs tending to reduce the apparent gain of
the array compared to the gain found in the far field. Wave
reflections from the ground will tend to create an oscillatory
behavior in the measured field intensity from a multi-element
array which will depend on phasing of the direct and reflected
waves and consequently on the height of the antenna above ground.
The calculated fields provided here do not incorporate the addi-
tional complication of ground effects.
Figure 11 is a plot of the calculated magnitude of the
electric field strength on the CR axis for a 12 bay dipole array
with 1 kW of power delivered to the array. Uniform power division
among the bays is assumed and the bay spacing is one wave-length.
The straight line shown in Figure 9 represents the expected field
strength if the far field gain of the array applied to the near
field region as well. Thus it is clear that significant gain
reduction will occur for this model arrciy at sufficiently close
distances. Results are presented in Figures 12-20 which give the
computed compression of array gain as a function of distance for
2, 3, 4, 5, 6, 8, 10, 12, and 16 bay arrays respectively. In
these figures the gain reduction is given directly in decibels
below the far field value. These results are similar in appearance
22
-------
RMS ELECTRIC FIELD STRENGTH M - 12
1000
t100
Ul
O
P
CO
oc
10-
1
0.1
F=100MHz
P=1kW
III -I
10 100
DISTANCE (METERS)
1000
Figure 11.
RMS electric field strength vs. distance on
the axis of the center of radiation for a
12 bay dipole array with IX bay spacing an
uniform, in phase power division
-10
to
ui
ec
a.
o
oc
-40
F=100 MHz
i i Hi
-t-
*
t I I I I HI
10 100
DISTANCE FROM ARRAY (METERS)
1000
Figure 12.
Near-field gain reduction for a 2 bay
dipole antenna array operating at 100 MHz
with 1A bay spacing and uniform, in phase
power division to each bay
23
-------
0 _
Z -10
o
g
O -20
O
Z
-30.
-40
F=100 MHz
I I I I I i III
I I I I I I ill
10 100
DISTANCE FROM ARRAY (METERS)
1000
Figure 13.
Near-field gain reduction for a 3 bay
dipole antenna array operating at 100 MHz
with IX bay spacing and uniform, in phase
power division to each bay
Z -10
o
111
K
a.
§-20
O
Z
1-3.
-40
s
F=100MHz
V
-II-
10 100
DISTANCE FROM ARRAY (METERS)
1000
Figure 14. Near-field gain reduction for a 4 bay
dipole antenna array operating at 100 MHz
with IX bay spacing and uniform, in phase
power division to each bay
24
-------
0
z -10
o
35
CO
ui
ec
Q.
O
Z
-30 J
oc
-40
FMOOMHz
i I I I i in
I I I I I MM
10 100
DISTANCE FROM ARRAY (METERS)
1000
Figure 15. Near-field gain reduction for a 5 bay
dipole antenna array operating at 100 MHz
with IX bay spacing and uniform, in phase
power division to each bay
z -10
o
55
CO
iij
E
-40
F=100MHz
*
> I I I M11
10 100
DISTANCE FROM ARRAY (METERS)
1000
Figure 16. Near-field gain reduction for a 6 bay
dipole antenna array operating at 100 MHz
with IX bay spacing and uniform, in phase
power division to each bay
25
-------
Z -10
o
35
(0
uj
£
O -20
U
Z
oc
-30
-40
FMOOMHz
10 100
DISTANCE FROM ARRAY (METERS)
1000
Figure 17. Near-field gain reduction for an 8 bay
dipole antenna array operating at 100 MHz
with IX bay spacing and uniform, in phase
power division to each bay
ffl
§-io
(0
CO
Ul
oc
111
O -20
U
Z
3
i -30
/
^ /^\/ F=100MHz
^^-*>» / \ / v / N^
^/ V v
v I
I
10 100
DISTANCE FROM ARRAY (METERS)
1000
Figure 18.
Near-field gain reduction for a 10 bay
dipole antenna array operating at 100 MHz
with IX bay spacing and uniform, in phase
power division to each bay
26
-------
CO
ui
E
a
g-20
o
E -30
DC
-40
F=100MHz
*-
i i i MM i i i i ino
10 100 1000
DISTANCE FROM ARRAY (METERS)
Figure 19. Near-field gain reduction for a 12 bay
dipole antenna array operating at 100 MHz
with IX bay spacing and uniform, in phase
power division to each bay
CD
o
-10
ui
E
a
O
O
-20
O
i
E
-30
-40
F=100 MHz
-t-
10 100
DISTANCE FROM ARRAY (METERS)
1000
Figure 20. Near-field gain reduction for a 16 bay
dipole antenna array operating at 100 MHz
with 1A bay spacing and uniform, in phase
power division to each bay
27
-------
to those obtained for parabolic microwave dish antennas by Cain
et al. , (1973) in which significant gain nulls are observed at
close distances.
From Figures 12-20 the IdB reduction point has been used to
determine an approximate expression, in analytical form, that
gives the distance at which the gain is reduced by IdB. This
expression is: D(m)--4.7+1.7N+1.2N2 where D is the distance in
meters to IdB gain compression and N is the number of bays in the
antenna.
A careful measurement of the field intensity near a 12 bay
FM broadcast array in Houston, Texas (Tell, 1978c) revealed that
the measured electric field strength was significantly below what
would be expected on the basis of the far field gain of the array
and known transmitter power. Modification of the ERP by reference
to Figure 19 resulted in a correspondence within IdB between
measured and calculated values of the field.
The near field gain reduction phenomenon observed for
stacked dipole arrays is due, in practical circumstances, i.e.,
distances greater than about 3 meters, to the phase properties of
the individual fields from each radiating bay rather than any
near field radiation characteristic of the fundamental radiating
structure itself. The practical significance of these findings
is that the field intensities in buildings located close by to
high power VHF and UHF broadcast stations will not necessarily
experience the high fields predicted from simple theory. A
detailed examination of the transmitting antenna is required in
order to accurately estimate potential exposure. Secondly,
because of the oscillatory nature of the amplitude of the fields
near such arrays, significantly higher fields may be found in
areas farther away than in closer by locations. Thus the field
measured inside a large room near a window facing an extended
array antenna, may be lower than that measured at a position
28
-------
substantially farther from the window. Other confounding factors,
such as standing wave patterns within such structures, often also
tend to distort the observed results.
It should be reemphasized that the computed gain reductions
provided in the figures do not take into account the effects of
grounds of varying conductivity which will lead to ground reflections
and thereby influence the actual field on the axis of the array,
usually in an oscillatory manner. These reflected waves will in
turn be dependent upon the actual height of the transmitting
antenna above ground and will affect the exact near-field gain
exhibited by the array. The results, though they do not incorporate
ground effects, are provided to illustrate the nature of expected
gain reductions but their precision in terms of actual installations
will be subject to localized environmental conditions in the
vicinity of the transmitting antenna which may introduce signifi-
cant reflections.
The extent to which these results may be quantitatively
applicable to various VHP and UHF TV antennas has not been
determined but the qualitative findings of significant gain
reduction and the oscillatory nature of the electric field near
such arrays are directly pertinent to similar investigations of
building exposure. The use of larger numbers of radiating
structures, the structures basic similarity to a dipole, and the
radiating structure vertical spacing determine the exact form of
the associated gain reduction curves for TV antennas.
29
-------
VERTICAL RADIATION PATTERNS
For several years EPA has observed, in the course of environ-
mental field measurements, that FM broadcast arrays often exhibit
extremely strong lobes of radiation straight downward below the
antenna. To understand how such radiation can come about, it is
only necessary to know the vertical radiation pattern of the
basic radiator structure used in the antenna array. If the
array, for example is composed of a series of dipoles, horizontally
polarized and spaced one wave length apart, the array will
exhibit pronounced downward and upward radiation lobes due simply
to the isotropic pattern of a dipole in the vertical plane and
the adding in phase of signals from all of the bays. To what
extent commonly used FM array bay structures may be approximated
by a dipole pattern in the vertical plane is not clear. Though
such radiation properties are not efficient from a broadcast
objective, it appears that some, if not a significant portion of
FM broadcast antennas, exhibit this property of emitting a strong
"grating" lobe. Others have discussed various techniques for
reducing the presence of grating lobes in FM transmitting antennas
(Silliman, 1975) and grating lobes have played a significant roll
in hazard evaluations of some broadcast installations (Silliman,
1975b; Tell, 1975). A problem in more accurately identifying how
common our observations might be is that manufacturers of FM
broadcast antennas virtually never make vertical radiation
pattern measurements beyond depression angles greater than 30
degrees for practical reasons. Considerations of distant propaga-
tion for broadcast purposes commonly entail radiation pattern
data only to a few degrees below the horizontal axis of the CR.
Observations of grating lobe effects have not occurred for
TV antennas and it must be concluded that the present common use
30
-------
of one wave length bay spacing and relatively simple radiating
structures, which in their own right exhibit relatively significant
downward radiation, continue to produce this interesting effect
only in some FM antennas.
Calculations have been carried out to determine the relative
electric field magnitude as a function of the vertical angle
about a dipole antenna array to illustrate the expected grating
lobe phenomenon. Figure 21 provides the results for 2, 3, and 6
bay dipole arrays having one wavelength spacing and all bays
being fed in electrical phase. Zero degrees represents the
direction of the axis of the CR while -90 degrees represents a
direction vertically below the transmitting antenna. A basic
dipole radiator can be seen to account for significant downward
radiation, the angular width of the lobe in fact being greater
than the main lobe out to the horizon.
Another method of viewing these results can be used wherein
the relative field strength is plotted in a rectangular format
rather than a polar format. For a 6 bay dipole array Figures 22
and 23 have been prepared in this rectangular format and illustrate
the difference obtained in the vertical pattern under identical
conditions except for the bay spacing. Figure 22 is for a one
wave length spacing while Figure 23 is for 0.5 lambda spacing. In
this case, the even number of total bays, 6, and the half wave
length spacing produce a null directly below the antenna, i.e.,
at 90 degrees. The principle reason for using one wave length
element spacing in FM broadcast antennas is the enhancement in far
field main lobe gain which can be achieved over 0.5 lambda spacing
of close to 3dB. In practice this means that twice as many elements
are required to obtain the same gain and consequently the cost of
production is essentially doubled.
31
-------
32
-------
RELflTIVE FIELD PfJTTERN WITH N
PHME DCMCES SPflCIMO - 1
1.0
90 80 70 60 50 40 30 20 10 0 -10 -20 -30 -40 -50 -60 -70 -80 -90
ANGLE ABOVE AND BELOW AXIS OF CENTER OF
RADIATION (DEGREES)
Figure 22. Calculated vertical pattern for a 6 bay dipole
antenna array with IX bay spacing and uniform,
in phase power division to each bay
RELftTIVE FIELD PfiTTERN WITH « * 6
PHflSE - 0 DEGREES SPACING » . S LWIBOfl
Figure 23.
0 -10 -20 -30 -40 -50 -60 -70 -80 -90
ANGLE ABOVE AND BELOW AXIS OF CENTER OF
RADIATION (DEGREES)
Calculated vertical radiation pattern for a 6 bay
dipole antenna array with 0.5X bay spacing and
uniform, in phase power division to each bay
33
-------
If the actual vertical radiation pattern of the individual
radiating structure is known, then it is a simple matter to apply
this space factor to the analytical method to arrive at the
actual vertical pattern of the array.
The possibility of grating lobe irradiation of the area
directly beneath an FM transmitting antenna warrants considera-
tion when evaluating possible hazards near such installations.
An analysis of the FM broadcast service in 1975 (Tell and Janes,
1975) indicated that about 3 percent of all FM broadcast stations
might be expected to produce field intensities near the base of
their antenna supporting structures in excess of 1 mW/cm2 power
density. Recently, fields in excess of 10 mW/cin2 were actually
observed in areas beneath a low-to-the-ground FM station antenna
(Tell, 1977b).
34
-------
SUMMARY
This report has discussed the significant contribution of
broadcast stations to radiofrequency exposure of the general
population. An analysis of the radiation properties of dipole
and monopole antennas has been conducted which illustrates the
potentially high radiation fields near broadcast antennas,
concentrating on FM antenna arrays, which have been observed in
the course of field measurements. Several areas are examined in
particular.
The very high field intensities which exist near the active
radiators in an FM antenna are deserving of special caution
during tower work on energized towers to prevent unsafe personnel
exposures.
Secondly, the phenomenon of gain compression within the near
field of an antenna array is examined by modeling an FM antenna
as a series of horizontally polarized dipoles with uniform power
division and equal phasing. It is seen that significant reductions
in field strength near the array are expected and have been
observed via actual field measurements. The reductions in near
field gain are due to the vector summing of fields from the array
as opposed to the inherent near field properties of the basic
dipole radiator itself. Results are presented for commonly
encountered FM arrays which allow estimates of the field on the
axis of the CR of the transmitting antenna.
Finally, the presence of grating lobes in some FM broadcast
antennas suggests that the fields near the base of FM antenna
towers are another area of special concern since relatively
intense fields may exist, particularily in the case of antennas
mounted close to the ground or on roof tops.
35
-------
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Tell, R. A. (1975): "An Analysis of Broadcast Radiation Levels
in Hawaii," Technical Note, ORP/EAD 75-1, U.S. Environmental
Protection Agency, Washington, DC, August 1975 (NTIS Order No.
PB 261 316/AS).
Tell, R. A. (1976): "A Measurement of RF Field Intensities in
the Immediate Vicinity of an FM broadcast Station Antenna."
Technical Note, ORP/EAD-76-2, U.S. Environmental Protection
Agency, Silver Spring, MD, January (NTIS Order No. PB 257 698/AS)*,
Tell, R. A. (1977a): "An Analysis of Radar Exposure in the San
Francisco Area," EPA Technical Note ORP/EAD-77-3, 11 pages, March
1977.
Tell, R. A. (1977b): Letter report to Department of Occupational
Safety and Health, Nevada Industrial Commission. Environmental
Protection Agency Correspondence, November 2.
Tell, R. A. (1978a): "Field Strength Measurements of Microwave
Oven Leakage at 915 MHz." Presented at Symposium on Microwave
Mobile Communications, held September 29-October 1, 1976 at the
Institute for Telecommunication Sciences, Boulder, Colorado and
submitted for publication to IEEE Transactions on Electromagnetic
Compatibility (in press), May.
38
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Tell, R. A. (1978b): "Measurements of Radiofrequency Field
Intensities in Buildings with Close Proximity to Broadcast
Stations," Environmental Protection Agency Technical Note, April.
Tell, R. A. (1978c): "An Analysis of Radiofrequency and Microwave
Absorption Data with Consideration of Thermal Safety Standards."
Environmental Protection Agency Technical Report, March.
Tell, R. A., and D. E. Janes (1975): "Broadcast Radiation: A
Second Look." In Biological Effects of Electromagentic Waves,
ed. by C. C. Johnson and M. L. Shore, (Selected papers of the
USNC-URSI 1975 annual meeting, Boulder, Colorado, October (2
Volumes), USDHEW Publication (FDA) 77-8011.
Tell, R. A. and J. C. Nelson (1974a): "RF Pulse Spectral Measure-
ments in the Vicinity of Several Air Traffic Control Radars."
EPA Technical Report EPA-520/1-74-005, 45 pages, May.
Tell, R. A., and J. C. Nelson (1974b): "Microwave Hazard Measure-
ments Near Various Aircraft Radars." Radiation Data and Reports,
Vol. 15, No. 4, pp. 161-179, April.
Tell, R. A. and J. C. Nelson (1974c): "Calculated Field Intensities
Near a High Power UHF Broadcast Installation." Radiation Data
and Reports, 15:401-410, July 1974.
Tell, R. A. and P. J. O'Brien (1977) : "An Investigation of
Broadcast Radiation Intensities at Mt. Wilson, California." EPA
Technical Report ORP-EAD-77-2, 20 pages, April.
Tell, R. A., J. C. Nelson, and N. N. Hankin (1974): "HF Spectral
Activity in the Washingotn, DC Area." Radiation Data and
Reports, 15;549-558, September.
Tell, R. A., N. N. Hankin, and D. E. Janes (1976b): "Aircraft
Radar Measurements in the Near Field." In Operational Health
Physics, Proceedings of the Ninth Midyear Topical Symposium of
the Health Physics Society, compiled by P. L. Carson, W. R.
Hendee, and D. C. Hunt, pp. 239-246, Central Rocky Mountain
Chapter, Health Physics Society, Boulder, CO, February.
Tell, R. A., N. N. Hankin, J. C. Nelson, T. W. Athey, and D. E.
Janes (1976b): "An Automated Measurement System for Determining
Environmental Radiofrequency Field Intensities II." In Proceedings
of NBS symposium on Measurements for the Safe Use of Radiation.
March 1976, NBS publication NBS SP456, (ed. S. P. Fivozinsky),
pp. 203-213. Also presented at 1974 Meeting at USNC/URSI, October
14-17, 1974, Boulder, Colorado.
USAF (1975): US Air Force Regulation 161-42, November.
Wacker, P. F. (1970): "Quantifying Hazardous Microwave Fields:
Analysis." US Department of Commerce, National Bureau of Standards
Technical Note 391, April.
39
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
2.
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
NEAR-FIELD RADIATION PROPERTIES OF SIMPLE LINEAR
ANTENNAS WITH APPLICATIONS TO RADIOFREQUENCY HAZARDS
AND BROADCASTING
5. REPORT DATE
June 1978
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Richard A. Tell
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
U.S. Environmental Protection Agency
Office of Radiation Programs
Electromagnetic Radiation Analysis Branch
P.O. Box 15027
T.qg Vppag TJp-uaHa 8Q1 1 U
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
Same as above
13. TYPE OF REPORT AND PERIOD COVERED
Technical Note
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT
Analytical expressions for the electromagnetic field have been used to compute
electric and magnetic field strengths in the near-vicinity of dipole and monopole
transmitting antennas. In particular the fields about a A/2 dipole are mapped to
illustrate the magnitude and wide variation of field intensity which occurs near the
feed point, along the axis of the dipole, and about the tip of the radiating arms.
These results are analyzed in terms of evaluating potential radiofrequency hazards
which may exist extremely near dipole like broadcast antennas and the aspect of
close proximity exposure of maintenance personnel is discussed. The field expressions
are used to determine the extent of near-field gain compression which occurs at.
distances close to the antenna and this is compared with measured field intensity
data taken for a halfwave dipole using a short, nonperturbing field probe. The
results for a single dipole are used to model a vertically stacked array of dipole
radiating elements used to simulate a typical FM broadcast transmitting antenna.
Expected field intensities are then determined for positions close to such radiating
structures to assess the potential of biologically significant fields existing about
present day FM broadcast installations. It is found that near-field gain compression
can be significant for typically encountered building exposures wherein main beam
illumination is possible.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS C. COSATI Field/Group
8. DISTRIBUTION STATEMENT
Release to public
19. SECURITY CLASS (ThisReport}
Unclassified
21. NO. OF PAGES
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (Rev. 4-77) PREVIOUS EDITION is OBSOLETE
6 U.S. GPO:1979-684-155/2086
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