HIGH POWER RADIOFREQUENCY AND MICROWAVE RADIATION SOURCES: A STUDY
OF RELATIVE ENVIRONMENTAL SIGNIFICANCE
Herbert N. Hankin, Richard .A. Tell, T. Whit Athey, and David E. Janes, Jr.
U.S. Environmental Protection Agency
9100 Brookville Road
Silver Spring, Maryland 20910
Reprinted by: U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Radiation Programs
From 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), February 1976
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HIGH POWER RAIUOFRI-QUENCY AND MICHOWAVE RADIATION SOURCES: A STUDY
OF RELATIVE ENVIRONMENTAL SIGNIFICANCE
Norhcrt N. Hankin, Richard A. Toll. T. Whit Athoy, mul l>avld li. .Innos, Jr.
U.S. Hnvl roitmont ill 1'rotoctloit Ajir»cy
9100 hrookvll le Komi
Silver Spring, Murylnnd 20010
Abstract
Studies of environmental radiofrcqucncy and microwave radiation, high pov.cr radiation
sources, and source contributions to existing environmental nonionizing radiation levels
have contributed to an understanding of the relative environmental significanc? of the
major high power source categories. Analyses and measurements have produced information
relating to radiation characteristics, potential hazard evaluations, and environmental
radiation levels associated with high power source categories, which include satellite
communication earth terminals, radars (military and civilian), and broadcast transmitters
(UHF-TV, VIIF-TV, and FM). These results when considered along with other factors such as
number of sources in each category, relative numbers of persons possibly exposed, and
general system operating characteristics and procedures, lead to the conclusion that
broadcast transmitters constitute the most environmentally significant source category.
Introduction
This paper presents a view of the nonionizing radiation en.-ironment in terms of
categories of high power sources of nonionizing radiation, their contributions to
environmental levels, and the relative significance of the source categories considered.
This is only one of a number of results which hnve come from a comprehensive measurement
and analysis program whose objectives arc to determine if a notul exists for
population exposure standards and to define those standards if.they uro neoded.
A.program concerning the application of nonionizing rndiation exposure standards to
the environment requires an understanding of environmental nonionizing rndiation (NIR)
in terms of the intensities of existing electromagnetic fields as a function of fre-
quency-, identification of the 'sources and source categories which are important in pro-
ducing these fields, and a determination of the quantitative relationships between
sources and their contributions to environmental nonionizing radiation levels.
•The nonionizing radiation environment consists of electromagnetic radiation which
exists over a wide range of frequency (0-300 GHz). At any point in the environment the
environmental power density at any frequency is dependent upon the sources in the general
area and the geometry which exists relative to the source and that point in the environ-
ment. A high power source can be defined as one which is capable of producing a signif-
icant power density, arbitrarily chosen to be a minimum of 0.01 mW/cm2, at a distance
fronr-tbe"antenna where free access^would ordinarily-be possible, with inadvertent
exposure of individuals, unless restrictions were intentionally imposed. With this
definition, it is possible to differentiate between high power sources and potentially
hazardous sources which are those capable of producing power densities above this thres-
hold, and of the order of 10 mW/cm2, but only at distances close to a system antenna
where inadvertent exposure of an individual is highly unlikely.
The categories of high power systems studied, i.e., satellite communication earth
terminals, search and tracking radars, and broadcast transmitters (UHF-TV, VHF-TV, and
FM), arc those which include systems capable of producing significant power densities
in the environment.
High power source studies were .undertaken to obtain information about the maximum
power density capabilities of the sources studied, the variation of on-axis power density
-as 3 function of 'distance =ffdnf"t'hc source1,"-arid to aTlow the development of models which
could predict radiation characteristics useful in determining the significance of
individual sources and source categories relative to the NIR environment. In addition,
.the radiation characteristics obtained are important in performing hazard evaluations of
specific systems and facilities.
.A realistic evaluation of a source of environmental RF .or microwave radiation, in
terms of whether or not it may create a hazardous or undesirable exposure situation,
requires source and biological effects information. In the absence of sufficient
biological effects information, a potential hazard evaluation can be used to identify
and evaluate the sources which may be capable of producing radiation exposure levels
defined to be significant relative to some selected exposure criterion. This evaluation
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Is based upon en leu 1 n t I on of the •; i j;n if lean I elm rue t er i s t ics of a source tinder very basic
conditions and neglects the factors which aild realism to the situation. I'.Namples of suclt
criteria on which a potential hiiznrd evaluation is possible lire the maximum on-slte power
density which may exist nt-a specified distance, or the distance to which a specified
.power'dens 11y could exist.
The evaluation of the relative environmental significance ol these sourer categories
is based not only on their potential for producing hazardous levels of nun Ion i z I nj; nidio
tion, but includes factors such as number of sources In each category, relative numbers
of persons possibly exposed to significant levels of power density, general system
operating procedures, and NIR hazard awareness by operating personnel. The generally
directional nature of the radiation distribution pattern of most high power source
antennas may significantly mitigate the exposure problem; i .c. , the power densities which
cxi.st at free access locations may be substantially less than on-axis or main beam power
densities depending upon antenna height above ground, main beam orientation, locations in
the environment to which persons may most closely approach the system, and the operation-
al procedures employed. Enhancement of environmental levels of power density can occur
if radiation is reflected from surfaces upon which it may be incident and interfere con-
structively with other radiation.
Generally, with the exception of a few unique systems, the anticipated power densities
at distances of closest approach for individuals not occupationally involved are less
mn the occupational exposure threshold of 10 mW/cm2 specified by current U.S. standards
for prolonged exposure times. However, environmentally significant power density
levels, could exist in the environment at great distances from many of the sources
considered. .
Satellite Communication Earth Terminals
Satellite communication earth terminals have the functional requirement to communicate
with earth orbiting satellites. Satellite communication (SATCOM) system antennas require
very directional antennas which radiate power into col lima tod beams with very little
divergence. The antenna diameter and maximum transmitter power arc characteristics of
particular interes-t from an environmental aspect since it is the combination of high
.transmitter power and large antenna diameter that is responsible for producing a region'
of significant power density which may extend over very large distances.
The antennas of all powerful satellite communication system earth terminals have
paraboloidal surfaces, are circular in cross-section, and have.Cassegrain geometries.
In the Gassegrain antenna, Fig. 1, power is introduced to the antenna from the primary
radiating source (power feed) located at the vertex of the paraboloidal reflector. The
radiation is incident on a small hyperboloidal subreflector located between .the vertex
and the focus of the antenna. Radiation from the power feed is reflected from the sub-
reflector, illuminates the main reflector as if it had originated at the focus, and is
then collimated.
^.Th.e .empirical modelt2) used to calculate the radiation characteristics of SATCOM
earth" terminals applies to antennas (reflectors) that are circular cross-section para-
boloids. It expresses the on-axis power density, the maximum which exists for any given
distance from the antenna, as a function of distance from the antenna in terms of basic
characteristics; i.e., the reflector diameter, radiation frequency, aperture efficiency,
and the maximum power which can be introduced into the antenna system for subsequent
radiation into space. The on-axis radiation field characteristics for circular cross-
section paraboloidal antennas based upon this model can be described using Eqs. 1-4. v2>3)
•' ' ' ' " . !
wnf = —y— wnf = maximum (on-axis) near-field (1) .
110 power density (W/cm2) • . j
n = aperture efficiency, }
e typically (KS
-------
W,,f
W,f • lar field power density (on
.-ixis) tit distance R
(4)
FEED
PARABOLOID
REFLECTOR
-^PARABOLOID
•** KOCAL POINT
HYPERBOLOID
SUBREFLECTOR
A -characteristic necessary to the
ca 1 cilia t ion of on-nxis power density as
a function of distance from im nntcnnn
-is the extent of the near field, Ri
(I:q. 2); i.e., the distance over which
the on-axis power density is .1 maximum
before it begins to decrease with
distance. The- magnitude of the power
density oscillates, as a function of
distance from the antenna, in the near-
field region, however the maximum
value of on-axis power density (liq. 1)
is constant. The hcam of radiation is
collimatcd so that most of the power- in .
the near- field is contained in a region
having approximately the diameter of the
reflector. The power density, W/f, in ...
the far-field, where R*2Ri, decreases -*«iE^
inversely as the square of the di-stance from the antenna. In the intermediate field
(RjsRs2Ri), a transition region between the near- and the far-fields, the power density,
wif» (En. 3) • decreases inversely with distance.
Comparisons between measured on-axis power density levels and predicted values for the
systems measured are sufficiently good to provide confidence in the model used.
Anticipated Environmental Levels of Power Density for Selected Systems
Calculations have been made for. the expected characteristics of several existing
satellite communication systems operating at maximum transmitter power. .In each case,
the distances from the antenna at which power densities of 10 mW/cm2, 1 .-nW/cm2, 100
cm?, .atid-10 uW/cni2 arc expei' ted have been... determined.: The results arc given in Table
I .-('* ) -
Fig. 1 Cassegraln Antenna
__ _
The* fa'c'to'rs which must l>c considered in realistically evaluating a system relative., to
the potent ial for exposure of persons at power density levels above a selected thrcsho.ld,
include, the transmitter power used in its regular operations (generally much less than
the maximum possible), the procedures employed in system operation, height an°yc._&<£pl.l.l.iiU-
tcrraih character jstics.,, antenna si-dcJobe— ra-di-at ion cha;r:rctcrt~sTic's"7' t'he mTiiimum cTcva-
tion.'ariglc of the antenna h^Io-w which the -system cannot operate, location and he"ight'"6f'
structures, and population characteristics within the area of interest. A system may
produce' significant power densities, and not constitute a hazard if there is no poss,i,bil-
ity fo.rexposij re _of_ ^persons. --._. 7r —j^r^r^—-^-^^''^-'"" ^TVrZTT* lZH^va-1^ --^t»-
. 'Table. 1 Anticipated Characteristics
System
LET -
AN/TSC-54
AN/MSC-46
AN/MSC-60
AN/FSC-9 '""
Intelsat
Antenna
Diameter
(ft)
15
18(eff)
40
60
60
97
A*
(cm)
3; 7
3.7
3.7
3.7i
3.7
4.8
PT
(kW)
2.5
8
10
8
20
5 -
Rl
(m)
9.98x10
1.44xl02
7.10xl02
1.60x103
K60xl03
3.22x103
of Selected
"nf (max)
.(mW/cm2)
30.4
50.8
„ 8.5,6 :.;?
,- 3.0.4 . .
7.61
.728
Satellite Communication
Systems
Distance (m) from Antenna
for Power Densities of:
10 mW/cm2
2.46xl02
4.58xl02
-: , -:-:-- , •- ."
^V C-> -'•-
-
. . _
1 mW/cm2
7.79xl02
1.45xl03
2, 94x1 03
, ^3,54x1 a3-.
6.23xl03
•
100 uW/cm2
2.46xl03
4.58xl03
9.29xl03
.1.25xl04
1.97xl04
1.23xl04
10 uW/cm2
7.79xl03
1.45xl04
2.9x104
.3.94X104--
6.23xl04
3.88xl04
Goldstone ' i * » *
Venus 85 12.6 450 9.43xl02 97.3 4.16xl03 1.32xl04 4.16xl04 1.32xl05
Goldstone
Mars 210 12.6 450' 5.76xl03 16.8 9.68xl03 3.34xl04 1.06xl05 3.34xl05
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The systems analyzed ronr.itiitc n potential hazard In thnt the maximum on-axls power
densities cnp.iblc of beinp. produced range from 0.73 to 97.3 mW/cm^ and the near-field
distances range from 10^ to 6x10?' m. These systems arc able to produce on-axis power
densities, H.mW/cm', at distances ranging from 7.8xl02 to 3.3xl04 m. High power SATCOM
systems arc relatively unique, few in number, and have.extremely well collimatcd beams.
The very large and-powerful sources such as the Goldstone Venus and Mars systems have
extensive general population exclusion areas which includes the air space above them.
The operational procedures employed were developed with safety as a consideration. In
many cases, elevation angles less than minimum; i.e., 7.5 degrees above the horizon, are
not possible during normal operation. An effort is made to avoid irradiation of persons
and structures by the main beam of radiation because of mission objectives and safety
considerations. It is of interest to see that for these systems having antenna diameters
which vary from 15 feet to 210 feet, the near-field extent, R\, increases from approxi-
mately 100 meters to almost 6xl03 meters. With the exception of the Intelsat system, the
on-axis near-field power densities, Wnr, are of the same order of magnitude or exceed 10
mW/cm'. Examination of the distances from the antenna at which various on-axis power
densities occur, shows the increase in the spatial extent over which significant power
densities exist, as the antenna diameter and near-field extent increase. Even for a
small diameter, low power system such as the LET, 10 pW/cm'.can exist as far as 7.8xl03m
from the antenna.
Radar Systems
Radar systems in the categories of acquisition and tracking radars have been studied.
(3) Included were military systems, a;r traffic control (ATC) radars, and weather
radars. The results are used to specify, for entire system categories where possible,
the ranges of on-axis power de sity levels produced and the distances from the system
for which significant environmental levels exist.
The variation of system characteristics is greatest in the categories of acquisition
and tracking radars, resulting in a wide range of on-axis near-field power densities and
effective near-field distances. The systems studied represent the majority of powerful
.radars used for civilian application and include examples of military systems considered
to be among the most hazardous under worst-case operating conditions relative to power
"denTs liry~ reveals and The distance •over-rwh->ch--t;-ho.s.e.-.ifvels .ip.ay,. occur.
--The .character is tic of primary interest, in radar system radiation measurements is
..time-averaged power density, not peak- power-density. The radiation is pulsed, and, for
mo.st systems, the pulse width and--rcpctit-ion rate are'such. that the average transmitter
power and power density created at any point are roughly two to four orders of magnitude.
less than.the peak value. In addition,..many radar system antennas rotate, further
;feducing" the. time-averaged»power~densi-t-y-'whichrmight-;Occurr at. a particular location.
r :',._. . " '"" '."' "-'' •'-•'• ^ • '•" -:-~ •: : - -.= i-s - - : •
.An evaluation of peak radiation-characteristics, is appropriate when considering
• interference effects, as exist for the operation;of certain electronic systems in a
•pulsed microwave radiat4on field, and any biological effects caused by pulsed fields.
All of the radars studied have antennasjwith paraboloidal surfaces. Those with
—ci-rcular-cross-sections can be-analyticall^trea^ed by tke_ model used in the analysis of
SATCOM system antenna radiation characteristics. The acquisition and tracking radar
category includes radars which have circular, rectangular, and ellipsoidal cross-
• sections. Noncircular cross-section antennas collimate radiation in a manner such that
..the radiation beam is better collimated in the plane which contains the antenna axis
having the larger dimension, arid has greater divergence in the plane containing the
..antenna axis of lesser length, the antenna axes being mutually orthorgonal.
,. _ The time-averaged system characteristics used in evaluation of radar hazards are
^*T~ T7- . . - --_.
"The model, used^to determine -ort^axisj^'timc-averagcd power, dcnsi ties at a distance
beyond the near-field of the antenna, has been modified for paraboloidal antennas that
have other than circular cross-sections. The effective near-field distance, Rjeff is
expressed as:
... " .,,-. Rleff = 0.318 A/X (5)
Antenna rotation further reduces the time-averaged power density which would be
produced by a stationary (non-rotating) radar antenna. The power density produced
at any point, by a system with a rotating antenna is:
-------
W • Ws-f 16)
where Ws is the t i mo -nvt-r.-ifird power .density proiliirccl hy the antenna If stationary and f
is the rotntion.il reduction factor which applies at the point of interest.
Anticipated characteristics of several types of radar systems are presented in Table 2
and Table 3'3) for acquisition and tracking systems, respectively. Shown are the dis-
tances from the antenna at which power densities of 10 mW/cm2, 1 mW/cm2, 0.1 mW/cm2, and
0.01 mW/cm2 are expected if the antennas were stationary.
Table 2 Acquisition Radar Sys t
System
Antenna Dimensions* (ft)
Frequency (GHz)
Average Transmitter Power (W)
Near-Field Extent (m)
Near-Field Power Density (mW/cm2)
Distance (m) to
10 mW/cm2
1 mW/tm2
0.1 mW/cm2
0.01 mW/cm2
Rotational Reduction Factor
(far-ficldj
FPN-
cm
"40
1 = T 1 i - Q 1 1
^v •' » ''h I
azim. I
9.0
180
22
12.8
28.1
111
351
l.llxlO3
3.8xlO"2
Characteristics
v=10, Lh-2.5
elev.
17
13.9
24.2
8«).h
283
8 18
1.33-
2.8xl03
31.2
15.5
48.3
174
54!)
1.74x1 0s
Lh * 42
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lllir-TV Transmitters
IIIIF t olcv i:; ion I r;in:;ini •'•'.< ion liar,
the environment to xvhich millions
tion and radar systems .ire sources
collimatcd beams with relatively 1
source, the 'UHF-TV antenna is an a
horizontal plane, hut having some
disseminate information and in the
numbers of people may reside. Ulll:
CW sources which have high cri'cctivc
areas.
In-come :i common and w i ill-spread ra'diatinn component of
of persons arc exposed. Whereas satellite cominun lea-
.with very directional antennas radiating power into
ittle divergence compared to an isotropical 1'y radiating
Imost isotropic radiator, omnidirectional in the
collimation in the vertical plane. It is designed to
process irradiate very large areas in which great
-TV transmitters comprise a significant ninnher of the
radiated power, and arc commonly located in urhan
The transmission frequencies range from 432 to 728 Mil?., equivalent to a range of
radiation wavelengths of (>!). <1 cm to 41.2 cm, respectively. The maximum effective
isotropic radi.ltcd power (I-IRP) allowed by the Federal Column nh: at ions Commission for a
single UHF-TV transmitter is 5.0 MW There were 306 UHF-TV stations in operation in the
United States as of December 1971.'5' There are 52 transmitters having an E1RP greater
than 2 megawatts and approximately 100 transmitters with an LIRP greater than I megawatt.
I.t has become a common practice in certain very large urban areas to locate two or
more powerful UHF-TV antennas at the same location, and in some instances atop the
tallest buil'dings. The possibility that other buildings, of the same height may be,
located within close proximity to the antennas results in the possibility for exposure"
of ma^y persons to microwave radiation at relatively high .power density levels.
The field strength and power density for the general case of UHF-TV transmission
(assuming no propagation losses) are described by the following equations:' •*
Ra(30 Pj)1/2
(7)
..2
whore L = field strength (volt/meter), R^, - relative field strength and is a function of
the depression angle, a, relative to horizontal, Pj = total KIRP (W) for a transmitter
for both visual and aural signals, and r = distance from antenna (meters). The geometry
used in the model is shown in Figure 2. The distance to the antenna is. given in terms of
the^relative vertical height, h, and horizontal distance, d. R^, , is determined from the
vertical radiation pattern of the antenna. A typical radiation pattern, that of WDCA
(Bethesda, MD), is shown in Figure 3.
.."The. resul ts of power density for several
-relative antenna/exposure point heights as a
function of horizontal distance from the
'antenna for a transmitter/antenna EIRP of 1
megawatt are shown in Figure 4.^
The results can be applied to the case of
a transmitter or transmitter complex, in
order to estimate environmental power density
levels and population exposed to power den-
sities above any selected threshold. The
1.0
.8
06
C
4]
W
T3
4) *1
,|j---
W
i-l
4>
<* .2
Depression Angle (dcg)
Exposure Point
Fig. 2. Geometry Used in Calculation of Field
Intensities for UHF-TV Transmission.
-2
0
10
Fig. 3. Vertical Radiation Pattern,
WDCA
-------
power density characteristics for
different transmitters will differ
dependinjj upon their iiulividu/il
antenna radiation patterns.
The results, assuming for
simplicity that the radiation
pattern (Fig. 3) is typical of all
IJIIF-TV transmitters, have been
applied to a worst-case prediction
of population exposed to power
densities equal to or greater than
1 uW/cn)2 and 4 pW/cm^ from selected
transmitter complexes in four urban
areas. In this worst-case determina-
tion, the field strength was doubled
to allow for reflections. The
results of this estimate .ire presented
in Table 4.17'
In urban areas, where population
distribution may vary greatly over
relatively small distances, differ-
ences between predicted power
densities and those which actually
exist, especially in the rep ion of
the second power density maximum
(Fig. 4) may be responsible for
significant differences in the loca-
tion and extent of the region in which
exceeded, and subsequently responsible
exposurcd population and the number o1.'
CM
U
19
10°
10-
10
g
a.
-2
c
03
.O
10
r3
,-4
.10
10"5
h=]000_
.01
0.1 1.0
Distance (miles)
10
Fig. 4. UHF-TV Radiation Characteristics, 1 MW
a selected power density threshold may be
for large differences between the estimated
persons actually exposed.
Table 4
Station
WPHL
WDCA/WETA
WSNS/WFLD
WSBK/WKBG
Approximate Population Exposure from
Selected UHF-TV Transmitters
Total
EIRP
(mW)
4.9
5.7
6.0
5.5
Location
Philadelphia
Washington
Chicago
Boston
Population Exposed
(Thousands) to Power
Densities of
1 uK/cm* 4 uW/cmz
1300 3
800 20
46 20
10 1
Discussion.and-Conclusions
Satellite communication earth terminals, tracking radars, and UHF-TV transmitters are
categories of high power sources of nonionizing radiation capable of producing main beam
time-averaged power densities considered hazardous; i.e., >10 mW/cm^, at distances of
the order of 100 meters from the system antenna. In general, all of these source
categories contain systems which can generate environmentally significant, main beam
power densities, defined in this paper to be >10 pW/cm^ at distances on the order of 100
m from a source. However, significant differences between the radiation characteristics
of these sources occur because of differences in beam collimatiqn, antenna location
relative to population centers, antenna height above ground,.and system operational
procedures. VIIF-TV. and KM broadcast transmitters have not been specifically discussed
in this" paper since environmental data and analyses arc extremely recent and not yet
avaiLabie 'for presentation. However, similarities in' environmental radiation
characteristics of broadcast transmitters allow the discussion.of UHF-TV transmitters
to be generally applicable to the category of high power broadcast sources and indicate
that VHF-TV and FM transmitters are environmentally significant sources.
SATCOM and tracking radar antennas produce very well collimated radiation beams and
off-axis radiation levels are greatly reduced relative to main beam radiation levels.
The ratio of off-axis power density to on-axis power density at any far-field location
is generally less than 0.01 at angles greater than S degrees relative to the antenna
axis for systems having small diameter (MO ft) antennas. This ratio decreases as off-
axis angle increases, and the angle at which a given ratio may exist decreases as
antenna diameter increases and wavelength decreases. Radiation produced by a UHF-TV
transmitter is collimated vertically, but for an exposure point out of the main beam
at depression angles >7 degrees (refer to Fig. 2), the ratio of field intensity at that
-------
point to main beam field intensity nt that same distance from the antenna Is rclntivcly
constant and approximately equal to 0.1 (refer to Fig. 3),
A determination of the relative environmental significance of these source cii-tcgories
should include the differences in beam collimation characteristics, the fact that UIU'-TV,
VHF-TV, and FM pntcnna height above ground is. much great or than that of trucking rndar
and SATCOM antennas, and that generally radar and SATCOM systems a IT lociitod whcro nccoss
Is restricted and population sparse when compared to the location of broadcast sources.
Tracking radar and SATCOM systems arc considered to hnvo a greater potential for the
production of hazardous environmental power densities closer to the ground than IMIP-TV
sources. However, the number of persons that could be exposed to these levels and to
•environmentally significant levels would be expected to be very small because of (1)
system location in sparsely populated ureas, (2) operation generally in accordance with
procedures to minimize the possibility for exposure of persons, and (3) the well
collimatcd radiation beam.
UHF-TV and broadcast sources generally should be considered to have the greatest
environmental significance. It has been shown that they have the capability to produce
significant power densities outside of the main beam and irradiate large numbers of
people. The number of transmitters and persons involved in situations where exposure
levels should be reduced will ultimately depend upon verification of these predictions
with measurements and the results of biological effects research in determining the
threshold levels for exposure upon which standards would be based.
- References
1. Department of Labor, "Occupational Safety and Health Stand-.rds, National Consensus
Standards and Established Federal Standards," Federal Register, Vol. 36, No. 105, 10522-
10523, 1971.
2. U.S. Army Environmental Hygiene Agency, "Laser and Microwave Hazr.rds Course Manual,"
l:dgewood Arsenal, MD.
3. Hankin, N.N., tell, R.A., At hey, T.W., and Janes, D.E., "Environmental Nonionizing
Radiation from High Power Sources," U.S. Environmental Protection Agency, .Silver Spring,
MD (to be published).-.;-^ --:. ._-.•,;•-.• - ... . -
.ar ^«-»- .'•"-'-' ". _
4. Hankin, N.N., "An Evaluation of Selected Satellite Communication Systems as Sources
of Environmental Microwave Radiation," F.PA-520/2-74-008, U.S. Environmental Protection
Agency, Silver Spring, MD, 1974.
5. "Broadcasting 497-2^Ttrarbook.,:" Broadcasting Publications, Inc., Washington, DC, 1971.
6. Tell, R.A. and Nelson, J.C., "Calculated Field Intensities Near a High Power IIHF
Broadcast Installation," Radiation Data and Reports, July 1974.
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