EPA-5207 2-74-008
AN EVALUATION OF SELECTED SATELLITE
COMMUNICATION SYSTEMS AS SOURCES OF
ENVIRONMENTAL MICROWAVE RADIATION
1
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An Evaluation of Selected Satellite Communication Systems
as Sources of Environmental Microwave Radiation
*
By
Norbert N. Hankin
December 1974
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Radiation Programs
Field Operations Division
Electromagnetic Radiation Analysis Branch
9100 Brookville Road
Silver Spring, Maryland 20910
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FOREWORD
The Office of Radiation Programs carries out a national program designed
to evaluate the exposure of man to ionizing and nonionizing radiation, and to
promote the development of controls necessary to protect the public health
and safety and assure environmental quality.
Office of Radiation Programs technical reports allow comprehensive and
rapid publishing of the results of intramural and contract projects. The
reports are distributed to State and local radiological health offices,
Office of Radiation Programs technical and advisory committees, universities,
laboratories, schools, the press, and other interested groups and individuals
These reports are also included in the collections of the Library of Congress
and the National Technical Information Service.
I encourage readers of these reports to inform the Office of Radiation
Programs of any omissions or errors. Your additional comments or requests
for further information are also solicited.
W. D. Rowe, Ph.D.
Deputy Assistant Administrator
for Radiation Programs
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TABLE OF CONTENTS
Section 1 - Introduction
Background 1
Measurement Objectives 1
Section 2 - Satellite Communication System Earth Terminals
General Description 2
Source Identification 3
Antenna Characteristics . . 6
Use of a Model in the Calculation of Satellite
Communication Earth Terminal Characteristics 7
Section 3 Satellite Communication System Measurements
System Description 14
AN/TSC-54 22
LET 24
AN/MSC-60 24
Satellite Communication Systems Measurements 24
Instrumentation Description ..... 32
Measurement Results 37
Section 4 - Discussion and Evaluation
Measurement Considerations 39
Evaluation of Model Applicability 42
Anticipated Environmental Levels of Power Density for
Simulated Satellite Communication Systems 45
Anticipated Environmental Levels of Power Density for
Selected Systems 46
Potential Hazard Evaluation 48
ECAC as a Resource in the Identification and Evaluation
of Potentially Hazardous Sources 53
Section 5 - Conclusions and Summary 54
Acknowledgments 56
References 58
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LIST OF FIGURES
Page
1. Cassegrain Antenna ........... .... 8
2. Radiation Zones for Paraboloidal Reflectors ........... 9
3. Antenna Gain vs. Wavelength .............. 15
4. Near Field Extent vs. Wavelength (Diameters: 1 to 20 feet) ... 16
5. Near Field Extent vs. Wavelength (Diameters: 30 to 120 feet) . . 17
6. On-Axis Near Field Power Density vs. Antenna Diameter ...... 18
7. Far Field Power Density/On-Axis Near Field Power Density vs.
Distance from Antenna/Near Field Extent 19
8. Intermediate Field Power Density/On-Axis Near Field Power
Density vs. Distance from Antenna/Near Field Extent . . 20
9A and 9B. AN/TSC-54 Antenna Array ......... 23
10A. LET Geodesic Housing ......... ... 25
10B. Rear View of LET Antenna in Housing ............... 25
11A and 11B. AN/MSC-60 Satellite Communication Earth Terminal .... 26
11C and 11D. Close-up Views of AN/MSC-60 Antenna 27
12. Other Satellite Communication System Earth Terminals at
Ft. Detrick 28
13. Geometry for On-Axis Power Density Measurements 31
14. Staircase Location for LET Near Field Measurement 33
15. Measurement of AN/TSC-54 Near Field Power Density 33
16. Measurement of AN/MSC-60 Near Field Power Density 34
17. Possible Method of Identification of Satellite Communication
Systems ..... 51
VI
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LIST OF TABLES
Page
1. Ranking of Sources by EIRP 5
2. Paraboloidal Antenna Characteristics 21
3. Summary of Pertinent Characteristics for Selected
Satellite Communication Systems 29
4. Measurement Geometry and Instrumentation ..... 31
5. NARDA Isotropic Probe Characteristics 36
6. Results of On-axis Power Density Measurements . . 38
7. Comparison of Measured and Calculated Values of
On-Axis Power Density 45
8. Anticipated Characteristics of Simulated Satellite
Communication Systems 47
9. Anticipated Characteristics of Selected Satellite
Communication Systems 49
10. Hazards Evaluation Summary 57
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ABSTRACT
Selected satellite communication (SATCOM) systems are evaluated
analytically and, for some of these systems, through measurement of the
microwave radiation power densities generated by them. The evaluation
is directed toward assessing the radiation exposure hazards which exist
for specific systems and generally for SATCOM systems as a class of high
power nonionizing radiation source. The paper includes determinations
of anticipated maximum power density levels as functions of distance from
the source, a description of the analytical method used, and the results
of measurements of the power densities produced by certain SATCOM systems,
Included also is a discussion of potential hazard analysis and its uses
in identifying systems which may constitute environmental hazards.
VI 1 1
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AN EVALUATION OF SELECTED SATELLITE COMMUNICATION SYSTEMS
AS SOURCES OF ENVIRONMENTAL MICROWAVE RADIATION
Section 1 - Introduction
Background
The U.S. Environmental Protection Agency supports a field operations
group to obtain data on the levels of existing environmental radiation, to
determine any change in the radiological quality of the environment, to
identify sources of radiation and their contributions to environmental
levels, to provide data for estimating population exposure to ionizing and
nonionizing radiation, and to determine if environmental radiation levels
are within established guidelines and standards. This report evaluates
some selected satellite communication systems, a category of microwave
emitting source with the potential for significant environmental exposure.
This evaluation and others concerning specific source types and the general
ambient environment will be used together with the results of biological
effects studies to determine the need to establish guidelines for environ-
mental exposure to nonionizing radiation.
Measurement Objectives
Satellite communication systems, evaluated on the basis of effective
isotropic radiated power, are the most powerful continuous wave (CW) sources
of environmental microwave radiation. The determination, by measurement, of
the environmental radiation levels generated by certain of these sources has
several different objectives, the primary ones being:
(1) to determine actual environmental radiation levels (power density)
as a function of distance from the system antenna,
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(2) to determine the applicability of a model, using known system
characteristics in predicting power density as a function of distance from
the source, by comparing measured and predicted levels, and
(3) to evaluate the potential of a source to produce hazardous environ-
mental radiation levels.
Other important objectives may be realized through use of the results
of the measurements performed, i.e., (1) identification of other factors
which may be involved in affecting the sources contributions to environmental
levels, e.g., the procedures used in system operation, system power losses
which occur before power is introduced into the antenna system, and the
effect of reflections from structures on power density levels in the environ-
ment; (2) determination of the most significant criteria which can be used
to identify sources having the potential for creating significant radiation
levels, and to rank these sources relative to each other; and (3) evaluation
of the contents of the source inventory used to identify sources which may
be involved in an environmental situation under investigation, or have the
potential for significant contribution to environmental radiation levels.
Section 2 - Satellite Communication System Earth Terminals
General Description
Satellite communication system earth terminals, as a source type, have the
greatest potential for creating hazardous environmental situations in that
significant power densities may exist at greater distances from the antenna
than would be possible for other types of radiating systems. They have the
potential for irradiating a particular region of the environment for long
periods of time while tracking satellites in various earth orbits out to
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geostationary (synchronous) orbits at a height of 22,300 miles above earth.
The antenna diameter and maximum transmitter power are characteristics
of particular interest from an environmental aspect. The need to transmit
power over large distances determines the transmitter power to be used with
an antenna whose diameter has been determined usually on the basis of
reception requirements. Generally, as systems are required to provide high
data transmission rates at increasing distances, the earth terminal trans-
mitter power and antenna diameter increase. It is the combination of high
transmitter power and antenna diameter that is responsible for producing a
region of significant power density which may extend over very large
distances.
Source Identification
Characterization of the environment, either analytically or through
measurement, requires identification of the radiating sources which con-
tribute to that environment. These sources must be identified in order to
determine how many there are, where they are, and how they may affect the
environment. In addition it is desirable to rank the sources to determine
their potential relative environmental importance. Sources were identified
for EPA through the use of a computerized inventory of sources operated by
the Electromagnetic Compatibility Analysis Center (ECAC) located in Annapolis,
Maryland.
The ECAC data base contains an inventory of transmitting sources and
their characteristics, and includes all U.S. equipment, both military and
civilian, common carrier microwave equipment, and all FCC licensed equipment
except for amateur and citizens bands. The sorting criterion, used in the
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search of the inventory, was effective isotropic radiated power (EIRP).*
This system characteristic is the product of the antenna gain and the
transmitter power. The sources of interest include both non-pulsed (CW)
systems and pulsed (radar) systems. Computer listings of unclassified
sources were derived for both source categories (]_). The CW sources were
ranked in the order of decreasing EIRP. The listing includes all CW sources
whose EIRP is greater than or equal to 1 megawatt (106 watts). Other
source characteristics resident in the ECAC data files were included in the
computer printout in order to provide additional information on the character-
istics of these systems. Twenty of the most powerful CW systems identified
are listed in table 1 (2). They are all satellite communication earth
terminals. On the basis of these results, selected systems in the category
of satellite communication earth terminals were studied, analytically and
through measurement.
Since the date of the inventory sort yielding the results of table 1
(July 1972) other powerful sources have been constructed and are now in
operation. A search of the current ECAC source inventory would result in a
rearrangement of the source ranking presented in table 1, and include several
sources which did not previously exist. One of the new sources which would
*
Effective isotropic radiated power is defined as the hypothetical total
power which a non-isotropic source of EM radiation would be required to
radiate isotropically (assuming it to be a point source of radiation) so
that the power radiated per unit solid angle would be the same as that
actually radiated. EIRP is obtained by multiplying the power radiated by
a source by its antenna gain characteristic, gain being a measure of the
antenna's directivity, or concentration of radiation, and ideally equiva-
lent to the ratio of 4ir to the solid angle subtended at the source by its
collimated beam.
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Table 1
Ranking of Sources by EIRP
Frequency Average
Rank Location (MHz) Use EIRP (GW)
Satellite
1 Westford, MA 7748 Communication 31.6
2 Lakehurst, NJ 8004 " 20.0
3 Roberts, CA 7985 " 20.0
4 Rosman, NC 5925 " 11.3
5 Paumalu, HI 5925 " 7.9
6 Jamesburg, CA 5925 " 7.9
7 Etam, WV 5925 " 7.9
8 Brewster, WA 5925 " 7.9
9 Andover, ME 5925 " 7.9
10 Bartlett, AK 5925 " 7.9
11 Archer City, TX 217 " 6.4
12 Mojave Desert, CA 5985 " 6.4
13 Pt. Loma, CA 7997 " 5.0
14 Helemano, HI 7990 " 5.0
15 Ft. Monmouth, NJ 7990 " 5.0
16 Brandywine, MD 7986 " 5.0
17 Camp Parks, CA 7990 " 5.0
18 Wildwood, AK 7986 " 5.0
19 Floyd Test Annex, NY 7986 " 5.0
20 Elgin, IL 8004 " 5.0
144 CW unclassified sources have average EIRP's of 1 MW or greater.
79 nonpulsed, classified emitters, none had an EIRP greater than
GW.
Of
5.0 GW.
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be included was the subject of a measurement and evaluation discussed in
this paper. In addition, several other sources have become operable, and the
transmission characteristics for the Mars and Venus communication systems at
Goldstone, California are presented in another section of this paper.
The initial selection of effective isotropic radiated power as the
criterion used in identifying and ranking sources resulted because EIRP is
a characteristic commonly used in describing the power radiating capability
of an antenna and transmitter system. The system characteristics used in
calculating EIRP; i.e., antenna gain and transmitter power, are entered
directly into the ECAC data inventory.
These systems, identified on the basis of selection by EIRP, have a
functional requirement to communicate with earth orbiting satellites and
possess a capability to produce significant radiation levels at great
distances. This illustrates, at least qualitatively, a degree of applica-
bility of EIRP in identifying categories of sources potentially capable of
producing the most hazardous exposure situations; i.e., existence of
relatively high power density at considerable distances from the source.
Therefore, if it is desired to perform measurements relating directly
to high power CW sources with the greatest potential for creating hazardous
environmental situations, satellite communication systems provided the logical
first choice for evaluation.
Antenna Characteristics
The satellite communication systems which are studied analytically and
by measurement, and most of the systems listed in table 1, have paraboloidal
antennas with a Cassegrain design. In the Cassegrain geometry, power is
introduced to the antenna from the primary radiating source (power feed)
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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 (figure 1). Radiation from the power feed is
reflected from the subreflector, illuminates the main reflector as if it
had originated at the focus, and is then collimated.
The Cassegrain antenna has advantages relative to a system which has
the power input at the focus; i.e., the very low-noise microwave receiving
preamplifier and the transmitter power amplifier are located immediately
behind the antenna resulting in lower transmission line loss and noise
which may interfere with the reception of transmissions from satellites
which operate at much lower transmitter powers than the earth situated
systems. In addition, spillover radiation from the power source is
directed toward space resulting in a lower antenna noise temperature.
The systems which have been studied all transmit and receive circularly
polarized microwave radiation. The polarization generally is right-hand
for transmission and left-hand for reception.
Use of a Model in the Calculation of Satellite Communication Earth Terminal
Characteristics
An empirical model was selected which allows the characteristics of
satellite communication (SATCOM) earth terminals to be calculated for use
in evaluations of hazards (_3). This model applies to antennas (reflectors)
that are circular cross section paraboloids, a characteristic of almost all
large SATCOM systems, and calculates the on-axis power density at any
distance from the antenna as a function of antenna diameter, radiation wave-
length, and transmitter power.
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PARABOLOID
FOCAL POINT
HYPERBOLOID
SUBREFLECTOR
PARABOLOID
REFLECTOR
H
30
>
z
en
m
D
03
m
CO
Geometric Relationship Between Antenna Feed, Subreflector, and
Main Reflector In a Conventional Cassegraln Design
Figure 1 Cassegrain Antenna
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The on-axis radiation field characteristics for circular cross-section
paraboloidal antennas can be described using figure 2.* The maximum value
of power density at any given distance from the antenna exists on the
antenna axis. In the near field of an antenna the magnitude of the power
density oscillates with distance, however, the maximum value of the on-axis
power density, Wnf, is constant over the extent of the near field, and the
beam is collimated so that most of the power is contained in a region having
approximately the diameter of the reflector.
Wnf = 4nP/A
5.66A
-RI -
Near Field
= Wnf(R/R.)
Wff = 2 Wnf(R/R,)
-2
Rl =
Intermediate Field
Far Field
Figure 2 Radiation Zones for Paraboloidal Reflectors
*The model used in defining the extent of the various regions and the
magnitudes of the on-axis power density which exist in these regions were
developed by the U.S. Army Environmental Hygiene Agency (3j.
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The value of the maximum on-axis near field power density, Wnf, is
given by eq.(1):
ii - 4np m
Wnf - -£- U;
where
P = total power radiated by the power feed
A = cross sectional area of antenna
n = antenna efficiency
The efficiency of the antenna (4) is the ratio of the power radiated into
the main beam to the total power fed into the antenna system, and is the
product of two factors:
1) the fraction of the total feed power incident on the reflector
2) the efficiency of the reflector in concentrating the available
energy into the peak of the main beam.
The efficiencies of circular paraboloidal antennas in concentrating the
available energy into the peak of the main beam typically range from 0.50
to 0.75 depending upon the method used in irradiating the hyperboloidal
subreflector.
The maximum on-axis near field power density for a circular paraboloidal
antenna expressed in terms of antenna diameter, D, using eq.(l) and A = ^2
is
W f = 16nP (2)
wnf ^02 {*•>
An important characteristic of high power sources, in addition to the
maximum power density which can be generated, is the extent of the near
field; i.e., the distance from the antenna over which the power density can
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be a maximum before it begins to decrease with distance. This parameter
and the maximum power density in the near field determine the value of the
on-axis power density at any distance from the antenna. The extent of the
near field, R-j , is given as eq. (3).
(3)
(6)
" 5.66X
where X is the wavelength of the transmitted radiation and is expressed in
the same units as the antenna diameter.
The far field of the antenna is the region in which the beam diverges
and the power density in the far field decreases inversely as the square
of the distance from the antenna. The far field on-axis power density,
Wff, can be expressed relative to Wnf by
W / D -2
where the distance from the antenna, R, must be at least as great as 2R-] .
The intermediate field region is a transition region between the near
and far fields in which the power density decreases inversely with distance.
The transition from one region to another is continuous, and this is taken
into account in the model; the on-axis power density, in each region, being
equal at the "transition boundary." The on-axis power density in the
intermediate field, W.., can be expressed as
where R, the distance from the antenna, lies in the interval between the end
of the near field region and the beginning of the far field region.
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The on-axis radiation field characteristics which have been described
were determined empirically (_3) and yield the maximum power density, as a
function of distance, of which the system may be capable. This constitutes
an assessment of the potential of the source for creation of a hazardous
exposure situation.
A frequent occurrence in evaluating the potential hazard situation
created by a specific source, is the lack of information regarding antenna
diameter and efficiency. However, if another characteristic, antenna gain,
is available, and the antenna is known to be a circular paraboloid, the
antenna diameter may be approximated assuming r\ to be 0.50. The antenna
gain is a measure of the directivity (collimation) of a reflector as com-
pared to an isotropic radiator and may be expressed (4) as
where Ae is the effective area of the antenna.
For the specific case of a circular paraboloidal reflector the
expression for aperture gain becomes
/ \2
j-\ __ | 7TLJ I
\ A /
\ /
The gain is generally expressed in dB, and is then defined
G - 10 log
(8)
The effective isotropic radiated power (EIRP) , previously described as the
basis for the identification of sources in the order of decreasing potential
hazard from the ECAC inventory, is commonly used also in describing satellite
communication earth terminals. The value is related to the antenna gain and
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radiated power capability of the system by
EIRP = G-P (9)
where P = maximum power capability of the system, including losses of power
which occur before power is fed into the antenna, and the value used for G
is the absolute gain.
A simple computer program has been written which calculates the various
pertinent characteristics for circular paraboloidal antennas and plots these
characteristics on a cathode ray tube (CRT). The display may represent
characteristics for one or several antenna diameters. A printout of the
numerical values of the characteristics may also be obtained on a CRT dis-
play or in a teletype printout.
The calculations performed include: (1) gain (dB) as a function of
wavelength for various antenna diameters and for any specified antenna
efficiency (using eq. 7), (2) the extent of the near field region as a
function of wavelength for various antenna diameters (eq. 3 ), (3) the
maximum (on-axis) near field power density as a function of antenna diameter
for 1 kW of transmitter power for any specified antenna efficiency (from eq.
2), (4) a dimensionless presentation of the ratio of the far field on-
axis power density to the near field on-axis power density as a function of
the ratio of the distance from the antenna to the extent of the near field
(eq. 4), and (5) a dimensionless plot of the on-axis intermediate field power
density to the on-axis near field power density as a function of the ratio
of the distance from the antenna to the extent of the near field (eq. 5).
The latter two curves allow rapid predictions of on-axis power density
during measurements at locations in the far and intermediate fields.
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Displays of these characteristics, generated by a Varian ADAPTS mini-
computer system, are presented in figures 3 to 8. An example of a listing
of characteristics displayed on the CRT is shown in table 2. Characteristics
can be displayed for paraboloidal systems over a range of diameters of 1 to
200 feet and a wavelength range of 1 to 60 cm.
These mathematical expressions and the resulting graphical displays
characterize the pertinent characteristics and generally describe the on-
axis power density as a function of distance in terms of the near field on-
axis maximum power density and the near field extent for antennas used in
satellite communication system earth terminals.
Section 3 - Satellite Communication System Measurements
System Description
Measurements were made of environmental power densities produced by
three high power, large diameter, satellite communication earth terminals
located at Fort Monmouth, New Jersey and Fort Detrick, Maryland. An
invitation extended by the U.S. Army Environmental Hygiene Agency to join
them in hazards evaluation studies at these locations provided the oppor-
tunity to study the radiation characteristics of the satellite communication
systems located there. These systems were operated in accordance with
directions furnished by the personnel performing the measurements in order
to evaluate them with respect to their potential hazards; the
systems were not operated under normal operational procedures.
The systems studied were the AN/TSC-54 and the Lincoln Experimental
Terminal, both located at Fort Monmouth, and the AN/MSC-60 located at Fort
Detrick. The characteristics for these systems are summarized in table 3.
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GRIN
190
78
38
28
10
e
16 19 20 25 38 35 40 45
WflVELENQTM
Figure 3 Antenna Gain vs. Wavelength
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lit i
let e
5 10 15 29 25 30 35 40 45 50 55 66
WflVELENGTH
-------
1ft 4
tet 3
18 15 28 25 3d 35 40
MflVEUENGTH
-------
let 4
POWER
DENSI TV
let 2
lit i
let e
iet-1
\
P = 1
kW
co
40
120
60 80 108
REFLECTOR DIflM (FT)
Figure 6 On-Axis Near Field Power Density vs. Antenna Diameter
140
160
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i
Figure 7 Far Field Power Density/On-Axis Near Field Power Density vs.
Distance from Antenna/Near Field Extent
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wa/wi
i
i
5
i
i. i i.
1. 3 i. 4
i, 6 i.
1>
1. 9 2
ro
O
Figure 8 Intermediate Field Power Density/On-Axis Near Field Power Density vs,
Distance from Antenna/Near Field Extent
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Table 2
Paraboloidal Antenna Characteristics
DlflM(FT)
6©
WflVELENGTH
1
2
3
4
5
6
7
8
9
18
13
26
29
38
35
48
45
38
55
68
QfllNCDB)
Ri(CM)
72. 1769
66. 1762
62. 6343
68. 1356
58. 1974
56. 6137
53. 2747
54. 1149
53. 8918
52. 1766
48. 6548
46. 1559
44. 2177
42. 6341
41. 2951
48. 1353
39. 1122
38 197
37. 3692
36. 6134
596992
2954i31
196967
147725
116180
96463. 8
84414. 7
73862. 7
65655. 8
59898 2
39393. 4
29545. 1
23636 1
19696. 7
16882 9
14772. 5
13131. 1
11818
18743. 6
9848. 36
WKHW/CMt2>
761372
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The AN/TSC-54 comprises items 28 through 33 in the ECAC identification
(1) of the most powerful CW sources, ranked on the basis of decreasing EIRP.
The LET studied at Ft. Monmouth, an earth terminal used to communicate with
an experimental communication satellite developed by the Lincoln Laboratories,
was not identified in the ECAC source listings.
The system studied at Ft. Detrick, Maryland, the AN/MSC-60, had
recently been constructed, was in the system calibration stage, and was not
yet considered operational. Its EIRP, 5.0xl09 W, makes it one of the most
powerful in the U.S. with respect to EIRP (refer to table 1).
A system located at Ft. Monmouth, an AN/MSC-46, is listed in table 1
among the systems having the highest EIRP values. In fact, the systems
numbered 13 through 20 are all examples of an AN/MSC-46 satellite communica-
tion system. It was one of the systems to be studied, but unfortunately was
not operational at that time.
AN/TSC-54
The AN/TSC-54 satellite communication terminal is a part of the Defense
Satellite Communication System. The system is transportable by air or
vehicle, and provides the capability for tracking a near synchronous orbit
communications satellite and for transmitting voice and teletypewriter
communications through satellites to other communication facilities. It
transmits in the frequency range from 7.9 to 8.4 GHz, and receives satellite
transmission in the 7.25-7.75 GHz range.
The antenna consists of an array of four 10-foot diameter parabolic
reflectors, each having a special high efficiency power feed in a Cassegrain
geometry. Figures 9a and 9b are photographs of the antenna. The overall
reflective surface area is approximately equal to that of a single 18-foot
diameter reflector.
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9A
....... .
9B
Figures 9A and 9B AN/TSC-54 Antenna Array
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LET
The Lincoln Experimental Terminal has a 15-foot diameter parabolic
reflector. The transmitter has a maximum power output of 2.5 kW and trans-
mits over a frequency range of 7.9 to 8.4 GHz. The antenna is sheltered in
a weather-proof geodesic housing (figures lOa and lOb).
AN/MSC-60
The AN/MSC-60 satellite communication system is considered a heavy
transportable earth terminal. The antenna has a 60 foot diameter and uses a
Cassegrain geometry. The system, used for communication with satellites
in synchronous orbit (at a height of 22,300 miles), has three
transmitters, one high power (8 kW maximum) and two low power (3 kW maximum),
using only one at a time. The maximum EIRP for the system (taking into
account a 3 dB power loss occurring in the waveguide and swivel joints) is
5.0x10 W at a transmitting frequency of 7.9 GHz. The system is shown in
figures lla and lib. Close-up photographs of the antenna are presented in
figures lie and lid.
As a matter of interest, two other extremely powerful, 60-foot diameter
systems are under construction at Ft. Detrick. These systems are located
within 1000 ft. of the AN/MSC-60. The photograph showing these systems,
figure 12, was taken from the AN/MSC-60 site.
Satellite Communication Systems Measurements
The power density, using a known transmitter power, was measured for each
of the systems on the reflector axis at a location in the near field and
also, where possible, at a point on-axis as far from the antenna, as
practical. Due to the height of the antenna axis above ground, the limita-
tions on the minimum elevation angle to which each of the antennas could be
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Figure 10A LET Geodesic Housing
Figure 10B LET Antenna (Rear View) In Housing
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11A
11B
Figures 11A and 11B AN/MSC-60 Satellite Communication Earth Terminal
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Figures 11C and 11D Close-up Views of the AN/MSC-60 Antenna
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28
Figure 12 Satellite Communication Earth Terminals
under Construction at Fort Detrick, Md.
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Table 3
Summary of Pertinent Characteristics for
Selected Satellite Communication Systems
Antenna
Type
Diameter (ft)
Aperture Efficiency
Gain (dB)
1/2 Power Beamwidth (deg)
Polarization of Trans-
mitted radiation
Height from center of
antenna to ground (ft)
LET
Cassegrain
15
.50
48
.58
RHC
Frequency (GHz)
System EIRP (109 W)
AN/TSC-54
Cloverleaf Array Using
4 Modified Cassegrain
Reflectors
18 (eff)
.75
52
.5
RHC
Transmitter power output (kW)
(Maximum) 2.5
7.9-8.4
.161
8.0
7.9-8.4
1.27
.63*
AN/MSC-60
Cassegrain
60
.50
61
RHC
63
8.0
7.9-8.4
10.0
5.0*
*Includes 3 dB power loss in waveguide and swivel joints.
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30
oriented, and the means available to elevate personnel and instrumentation
to the system axis, the number of locations at which measurements can be
made is severely restricted. For the measurements of the LET and AN/TSC-54
systems at Ft. Monmouth, the only practical way for personnel to reach the
height at which the maximum power density could be measured was to use
buildings to which access was possible. A "cherry picker" was available for
the measurements of the AN/MSC-60 system at Ft. Detrick, however, the number
of locations at which it could be situated was limited because of structures
and streets in the area, and the availability of suitable ground which could
provide a stable base.
In all measurements, the antenna is oriented so as to illuminate the
measuring instrumentation, and the maximum power density is found by moving
the instrumentation until the maximum reading is obtained.
The geometry for the measurements is generally illustrated in figure
13. The information specifying the distances, elevation angle, and detector
used for each measurement is given in table 4.
The on-axis power density generated by the LET system was measured at
two locations: one in the near field, on a metal staircase on the outside
of a building with a brick exterior; the second measurement in the inter-
mediate field at the second story window of a wooden building. The loca-
tion for the near field power density measurement is shown in figure 14.
The measurement of the near field maximum power density generated by
the AN/TSC-54 system was the only on-axis measurement attempted. The
system's operating personnel limit the minimum elevation angle to 7.5° to
avoid any possibility of creating a potentially hazardous exposure situation,
-------
31
Antenna
Detector
Figure 13 Geometry for On-Axis Power Density Measurements
Table 4
Measurement Geometry and Instrumentation
System
LET
LET
AN/TSC-54
AN/MSC-60
AN/MSC-60
R (ft)
^200
•^600
50
60
360
9 (deg)
•\4
0.3
7.5
0
0
hi (ft)
15
15
16
63
63
h2 (ft)
^28
•v-18
^22
63
63
Instrumentation
Narda 8323 probe
HP 432A power meter,
AEL APN101A antenna
Narda 8321 probe
Narda 8321 probe
Narda 8321 probe
-------
32
and for this reason the measurement could be performed only on the roof of
the wooden building which housed the transmitters and electronic systems of
the AN/TSC-54. Figure 15 illustrates the actual measurement in progress.
The on-axis power density measurement of the microwave radiation
transmitted by the 60-foot diameter reflector of the AN/MSC-60 was possible
only with the use of a "cherry picker" which elevated the measurement
personnel and instrumentation to the height of the center of the antenna for
an elevation angle of 0°. The fiberglass basket could accommodate two
persons and their hand-held instruments. The antenna was oriented so as to
direct its radiation at the basket, and then the "cherry picker" boom was
moved and the basket oriented in order to intercept the on-axis beam.
The extent of the near field for this antenna, 1.6x10-3 m, made it
impossible to measure power density beyond the near field. Measurements
were made at two different locations with respect to the antenna, but both
were in the near field. The antenna and measuring personnel are shown in
figure 16.
Instrumentation Description
The instrument primarily used in performing these measurements was the
NARDA 8300 which employs the model 8321 broadband isotropic probe for
measurements of power density <20 mH/cm2 and the 8323 broadband isotropic
probe for measurements of power density >20 mW/cm2. The Hewlett Packard
power meter, model 432A, with calibrated thermistor mount and calibrated
antenna, sensitive to the frequency emitted by each satellite communication
system, was used in one of the measurements (see table 4).
-------
33
Figure 14 Staircase Location for LET Near Field Measurement
Figure 15 Measurement of AN/TSC-54 Near Field Power Density
-------
34
Figure 16 Measurement of AN/MSC-60 Near Field Power Density
-------
35
The NARDA broadband isotropic monitor responds to the electric field
component of radiation, in the frequency range of 300 MHz to 18 GHz, with
equal sensitivity over all polarizations and direction of propagation, and
can measure the power density accurately in the near and far field. This
isotropic response characteristic is derived from the probe design employing
thin film resistive thermocouple dipoles arrayed in three mutually
perpendicular planes.
The HP-432A power meter uses a log periodic antenna as the radiation
sensor. The antenna must be oriented to maximize the signal generated by
the detected radiation. The maximum signal measured must be multiplied by
a factor of 2 in order to correct for the measurement of power density for
the circularly polarized radiation radiated by the satellite communication
systems measured; or measurements of power density must be made in two
orthogonal directions and then added.
The characteristics of an instrument using the HP-432A power meter
depend to a great degree on the antenna used to detect RF or ywave
radiation. The meter displays the power in the RF or ywave field which was
detected by an antenna having a specified cross-section and efficiency.
The 432A has 7 ranges of sensitivity from 10 uW to 10 mW.
The characteristics of the NARDA 8300 with the 8321 and 8323 isotropic
probes are presented in table 5 (5).
-------
36
Table 5
NARDA Isotropic Probe Characteristics
Frequency range (GHz)
Power reading ranges
(full scale)
Frequency sensitivity (dB)
1 to 12 GHz
0.85 to 16 GHz
0.30 to 18 GHz
Isotropic responses
Accuracy
Overload threshold
CW (mW/cm2}
Peak (W/cm2)
Power source
Weight
8321
0.3 to 18
2 mW/cm2 &
20 mW/cm2
8323
0.3 to 18
10 mW/cm2 J
100 mW/cm2
+0.5 +0.5
+0.5 to -1 +0.5 to -1
+0.5 to -3 +0.5 to -3
+0.5 dB from energy incident in any
direction (excluding handle)
+3% of full scale
100 300
20 60
battery battery
4 pounds including carrying case
A problem which existed in the NARDA probes, especially the more
sensitive 8321, was the effect of static charge buildup on the 4-inch dia-
meter sphere of foamed polystyrene which contains the probe elements. The
amount of static charge present causes deflection of the meter indicator
above the zero level, and may be responsible for inaccuracies in low-level
power density readings, observed in field measurements, of ^+0.8 dB at 2
o
mW/cm . The amount of static charge varies continuously and cannot be
accurately compensated for by the zero control. It appears possible to
have up to ^+2 dB maximum error in measured power density for the most
sensitive scale. The manufacturer has recently eliminated the static
charge buildup problem with a design change.
-------
37
Measurement Results
The results of the measurements performed to determine the on-axis
power density levels generated by the three satellite communication system
earth terminals studied are presented in table 6. The values of near field,
on-axis, power density predicted by the model are given for a transmitter
power of 1 kW. In addition, the predicted on-axis power density levels at
the point of measurement are given for the actual transmitter power used.
The physical geometry and instrumentation involved are as presented in
figure 13 and table 4.
The maximum on-axis near field, power density for each system was
calculated using eq 2, the antenna diameter, transmitter power used, and
antenna efficiencies of 0.50 for the LET and AN/MSC-60 systems and 0.75 for
the AN/TSC-54. The only measurement of on-axis power density made beyond
the near field extent of an antenna was that for the LET at a distance of
l.SxlO2 m to l.SxlO2 m. Equation 5 was used in calculating the expected
power density. In addition, the near field extent was calculated for all
systems, using eq 3, because of its importance in the calculation of power
density variation with distance from the antenna at distances beyond the
near field.
-------
Table 6
Results of On-axis Power Density Measurements
Diameter (ft)
Near Field Distance (m)
Predicted Maximum Near Field On-
Axis Power Density (mW/cm2) for
1 kW Transmitter Power
Transmitter Power (kW)
R, Distance from Antenna (m)
to Point of Measurement
o
Measured Power Density (mW/cm )
Predicted Power Density (mW/cm2)
at R
LET
15
9.3X101
12.2
2
1.5xl02
to
l.SxlO2
12
15.1
to
12.6
2
e.ixio1
50*
24.4
AN/TSC-54
18
(eff)
1.4xl02
12.7
1
1 .5X101
7
12.7
6.4**
AN/MSC-60
60
1.6xl03
.76
.38b
.500
l.SxlO1
^.3
.38
^.19**
6.7
l.SxlO1
^2.2
5.1
2.5**
6.0
1 .OxlO2
to
1.1x10^
^3
4.6
2.3**
GO
oo
*Measurement performed on metal staircase (see figure 14).
**Includes a 3 dB loss of power into waveguide and swivel joints not originally known but
discovered after an inquiry was made to operations personnel about the discrepancy
between the measured values and those predicted.
-------
39
Section 4 - Discussion and Evaluation
Measurement Considerations
The measurement procedure used in performing an on-site measurement of
potentially hazardous environmental levels of power density requires that
the system must be operated in a manner consistent with good safety
practices which consider the system characteristics, so that the transmitter
power, antenna orientation, and system activation be under the control of
the persons performing the measurement.
The determination of the procedure used in measuring the power densities
generated by any given satellite communications system involves: a calcula-
tion of the environmental power density levels generated by the system based
on reflector and transmitter characteristics, knowledge of the distances
from the centerline of the reflector to ground for the minimum elevation
angle of the system at possible measurement locations, location of structures
in the vicinity of the system, a need to avoid the exposure of persons other
than personnel involved in measurements, provisions for the means to elevate
personnel and equipment to the proper height above ground for the measure-
ment, and the need for a reliable communication link between measurement
personnel and the system operators.
The calculation of maximum near field power density (on-axis) per kW
transmitter power and near field extent specifies the transmitter powers
(other than maximum) to be used during a measurement of system characteristics
and the distances at which measurements may be considered. The selection of
measurement locations also involves consideration of the minimum antenna
elevation angle possible, the need to avoid exposure to other persons, the
location of structures and the associated effects due to reflection of
-------
40
radiation, terrain characteristics which affect the height above ground at
which it may be necessary to locate measurement personnel, and the avail-
ability of a means to reach that height.
As antenna diameters increase, and therefore the near field extent also
increases, it becomes more impractical to measure power density beyond the
near field for elevation angles significantly greater than zero degrees.
The initial analysis used in the selection of systems to be investigated,
and the preliminary determination of instrumentation needs and measurement
procedure was based upon the data residing in the ECAC information inventory
and the use of an idealized model. However, there are factors which could
modify this analysis and impose other requirements which must be satisfied.
After an initial evaluation has been made, complete and accurate character-
istics of the identified systems should be obtained. Recent system modifi-
cations, i.e., transmitter power capability, may not have been included in
the data obtained from ECAC.
The data resident in the ECAC information inventory includes the site
location, system frequency, transmitter power, antenna or system gain,
antenna diameter, system nomenclature, antenna height above ground, a
qualitative description of the terrain, and the name of the operating agency.
However, there is much information which could be of use, does not exist in
the data file, and must be obtained from the persons operating the source
prior to a measurement. This information concerns power losses which occur
between the transmitter and the antenna, the coupling efficiency between the
antenna feed and reflector, the minimum antenna elevation angle, and the
normal operating power of the transmitter. The maximum power capability of
a transmitter and its usual operational power may be much different, however,
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41
the maximum transmitter power should be used in evaluating a system as a
potential hazard prior to its measurement.
The source must be operated with particular considerations being given
to safety so that the personnel performing measurements are not exposed to
excessive levels of radiation and that exposure of persons not involved in
the measurement be avoided. Thus an understanding of the measurement pro-
cedure and purpose by the satellite communication system operators is
extremely important, as is a reliable system for communication between the
measurement personnel and the system operators. This direct communication
link and the source operator's knowledge of the measurement procedure
facilitates the measurement and results in knowing all of the pertinent
source operating characteristics at the time of each measurement.
The instrumentation used to measure the power density from satellite
communication systems under less than ideal conditions (from a roof-top or
"cherry picker" basket) must not only have the necessary sensitivity over
the desired frequency range, but must be portable, lightweight, physically
small, extremely reliable, battery powered, thermally stable, and easily
read. The effect of various weather conditions on the operation of an
instrument must be considered in determining its suitability during the
measurement.
An important requirement is that the measured power density be directly
indicated by the instrument so that possible complications affecting the
measurement may be immediately obvious instead of being detected at a later
time. This requirement also serves to protect both the measurement
personnel and instrumentation from possible exposure to excessive or
hazardous levels which could be accidentally produced.
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42
Evaluation of Model Applicability
The model used in predicting characteristics and on-axis power density
levels for satellite communication systems earth terminals having circular
paraboloidal reflectors cannot be entirely validated by the measurements
reported because relatively few measurements were made, and of these, none
were made in the far field. However, the results obtained, table 6, lend
support to the use of the model for predicting on-axis power density.
If the on-site measurements agree with the anticipated values derived
through use of the model within the expected measurement accuracy of the
instrumentation used, the model can be used for preliminary determinations
of maximum on-axis power density at any distance from the antenna for
satellite communication systems. If all sources of instrument error would
add to produce the maximum possible error, deviations between measured and
actual power densities of up to ^40%, are possible. A difference of 30%
between measured and power densities predicted by the model would be
acceptable.
There are certain complicating factors not taken into account which
will cause the predictions based on the model to deviate from the actual
on-site measurements. These complications include the effect on power
density of interfering structures, and the power losses which occur before
power is introduced to the antenna system. In addition, there is a possible
degradation in antenna gain due to geometrical considerations, but this
latter factor is infrequently encountered since most systems have undergone
rigorous testing prior to operation. However, it is conceivable that
deviations from the idealized antenna geometry can occur, especially for
systems which are assembled in the field, i.e., the smaller transportable
systems.
-------
43
The effect of interfering structures and the discrepancy between
system transmitter power and the power fed to the antenna were demonstrated
in the satellite communication system measurements. The effect of inter-
fering structures on measured power density was seen in one of the LET
system measurements. The near field measurement of power density yielded
o
a maximum value of 50 mW/cnr as compared to a maximum theoretical value of
o
24.4 mW/cm^ for the transmitter power used. The measurement was made on
the metal staircase shown in figure 15 and undoubtedly the high value of
power density was due to reflected radiation interfering constructively with
incident radiation. While this measurement has no value as far as confirm-
ing the validity of the model, it is a fine example of the potential effect
of structures having good reflective characteristics.
The predicted system characteristics, including near field power
density, for the AN/MSC-60 system had been determined on the basis of the
antenna diameter and an assumed aperture efficiency of .50. During the
measurements, it was noticed that the predicted values of on-axis, maximum
near field power density were a factor of ^2 greater than the measured
values of power densities significantly greater than zero; i.e., 2.2 and 3
The discrepancy was pointed out by the system operators as being due
to a 3 dB power loss in the waveguide and swivel joints of the system. This
power loss is typical of large systems where the antenna is massive and the
distance between the transmitter and antenna power feed is relatively large.
The near field measurement of power density produced by the AN/TSC-54
system yielded a maximum value of ^7 mW/cm^ as compared to an anticipated
-------
44
value of 6.4 mW/cm2, based on the assumption that the power losses incurred
prior to the antenna power feed is 3 dB and confirmed by persons having
experience with this system.
System documentation and ECAC data do not necessarily provide
information describing the power loss which occurs in waveguide and swivel
joints, but may only specify maximum transmitter power and antenna gain from
which a theoretical maximum EIRP may be calculated. The maximum on-axis
power density should be calculated using known values of antenna power or
including the effect of power losses between transmitter and antenna.
The results of the satellite communication systems measurements lend
support to the use of the model for calculation of on-axis maximum power
density in the near and intermediate field for the initial hazard evaluation
of these systems. Unfortunately, because of the limited number of systems
measured and the difficulty in placing personnel and instrumentation at the
antenna axis during the on-site hazards surveys performed, only three near
field measurements and one intermediate field measurements were possible.
The extensive near field region associated with the AN/MSC-60 system
and the minimum elevation angle of 7.5° possible for the AN/TSC-54 system
made it impractical to make measurements beyond the near field for these
antennas. The definition of near field extent cannot be substantiated
because the model based calculation of near field extent is much greater
than the actual separation distance involved in the measurement. The valid
LET system measurement was made at approximately the transition region
between the intermediate and far fields as defined by the model.
The comparisons between the measured values of on-axis power density
and the calculated values of maximum on-axis power density, based upon the
-------
45
model are given in table 7. The AN/MSC-60 measurement made with a
transmitter power of .5 kW (table 6) must be disregarded because of the
inherent inaccuracy of the isotropic probe at the low end of its most
sensitive scale. Static charge buildup effects can add significantly to
small indicated readings. Measurements of low power densities whose value
is within an order of magnitude of inherent inaccuracies in instrument
performance, show large percent errors for relatively small deviations.
Table 7
Comparison of Measured and Calculated Values of On-Axis Power Density
LET AN/MSC-60 AN/TSC-54
Measured Power
Density (mW/cm2) 12 %2.2 ^3 7
Calculated Power
Density (mW/cm2) 15.1 to 12.6 2.5 2.3 6.4
% difference 26 to 5 14 23 9
The agreement shown supports the use of the USAEHA model for initial
hazards evaluations of satellite communication systems in the near and
intermediate fields. More measurements are needed to validate the model
in these regions and in the far field, but in order to illustrate the ideas
considered in the remainder of this report, the model will be used.
Anticipated Environmental Levels of Power Density for Simulated Satellite
Communication Systems
The mathematical model previously described has been used to determine
expected characteristics of various diameter circular paraboloidal reflectors
assuming 5 kW transmitter power. This combination of antenna and transmitter
-------
46
simulates satellite communication system earth terminals. The characteristics
are determined using realistic aperture efficiencies, transmitted wavelengths,
and system power losses which occur between the transmitter and antenna feed.
These anticipated characteristics are presented in table 8 for reflector
diameters which range from 15 feet to 100 feet. The gain, maximum on-axis
near field power density, EIRP, and distances at which on-axis power density
would reach selected levels have been calculated for two typical aperture
efficiencies for antenna diameters where they may apply.
While these systems are simulated, they represent approximations to some
satellite communication systems which are in use so far as antenna diameters
are concerned. Transmitter power capability for actual systems may be
different than the 5 kW selected for the illustration being as low as 1 kW
for some tasks, however, this value may be typical for the power output of
transmitters during usual system operation.
The contents of table 8 generally identify potentially hazardous systems
directly from their maximum near field power densities and the distance
intervals (with respect to antenna location) over which power densities
exceed a selected threshold. The values in the table show magnitudes of
expected maximum near field power density, for a given transmitter power and
wavelength, and that the power density levels close to the antenna (in its
near field) are more hazardous for smaller diameter antennas even though the
EIRP increases with antenna diameter. However, the distances over which
levels exceed a selected threshold increase with diameter, but generally not
at the rate shown by EIRP.
Anticipated Environmental Levels of Power Density for Selected Systems
Calculations have been made, for the expected characteristics of
-------
Table 8
Anticipated Characteristics of Simulated Satellite Communication Systems
Antenna
Diameter
(ft)
15
20
30
40
60
90
100
i
.5
.75
.5
.75
.5
.75
.5
.75
.5
.5
.5
Gain*
(dB)
48.1
49.8
50.6
52.4
54.1
55.9
56.6
58.4
60.1
63.7
64.6
Maximum Near Field Near
Power Density Field
(mW/cm2)** 5 kW Distance*
Transmitter Power (m)
60.9
91.4
34.3
51.4
7.61
11.4
4.28
6.42
1.90
0.846
0.685
9.23x10
1.64xl02
3.69xl02
6.57xl02
1.48xl03
3.32xl03
4.10xl03
EIRP
(109 W)
PT-G P-G
1 **
0.32
0.48
0.57
0.87
1.28
1.94
2.28
3.44
5.11
11.7
14.4
_
_
0.64
0.97
1.14
1.72
2.56
5.86
7.2
Distance (m
10 mW/cm2
3.20x10
3.95xl02
4.25xl02
5.27xl02
4.21xl02
-
-
-
-
) from Antenna for Power Densities of:
1 mW/cm2 100 yW/cm2 10 uW/cm2
l.OlxlO3
1.25xl03
1.34xl03
1.67x103
1.44xl03
1.76xl03
1.92xl03
2.36xl03
2.82x103
-
-
3. 20x1 O3
3. 95x1 O3
4.25xl03
5. 27x1 O3
4.55xl03
5. 58x1 03
6. 08x1 03
7.45xl03
9.13xl03
1.37xl04
1.52xl04
l.OlxlO4
1 .25xl04
1.34xl04
1.67xl04
1 .44xl04
1.76xl04
1.92xl04
2.36xl04
2. 89x1 O4
4. 32x1 O4
4. 80x1 O4
*Assuming 4 cm ywave radiation transmission.
**3 dB loss between transmitter and power feed for reflectors with diameters >30 ft.
-------
48
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/cm^, 1 mW/crrr, 100 pW/cm^, and 10 yW/cirr are
expected have been determined. The results are given in table 9.
The points of interest are, that for these systems having antenna
diameters which vary from 15 feet to 210 feet, the near field, RI, increases
from approximately 100 meters to almost 6x10-3 meters. With the exception of
the Intelsat system, the on-axis near field power densities, Wnf, are
significant relative to 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.
The larger systems are included in the list of CW sources having the
highest values of EIRP with the exceptions of the AN/MSC-60 and the two
deep space communication systems. These more recent arrivals would not be
expected to appear in the ECAC listing.
Potential Hazard Evaluation
A potential hazard evaluation assesses the potential of a source of
radiation to produce a hazardous exposure level relative to a defined thres-
hold for some selected exposure criterion. Potentially hazardous sources may
be identified through an examination of the ECAC inventory. The inventoried
sources may be evaluated relative to one another and assigned a priority for
further examination.
An initial evaluation of the potential hazards of an individual
satellite communication system may be performed by determining its worst
-------
Table 9
Anticipated Characteristics of Selected Satellite Communication Systems
System
LET**
AN/TSC-54**
AN/MSC-46
AN/MSC-60**
AN/FSC-9
Intelsat
Goldstone
Venus***
Goldstone
Mars***
Antenna
Diameter
(ft)
15
18(eff)
40
60
60
97
85
210
(cm)
3.7
3.7
3.7
3.7
3.7
4.8
12.6
12.6
Gain
(dB)
48.8
52.1
57.3
60.8
60.8
62.7
53.8
61.9
PT
(kW)
2.5
8
10
8
20
5
450
450
EIRP
(109 W)
P*G
.189
.651
2.68
4.82
12.0
4.68
54.0
348
On)
9.98x10
1.44xl02
7-lOxlO2
1.60xl03
i.eoxio3
3.22xl03
9.43xl02
5. 76x1 O3
wnf (max)
(mW/cm2)
30.4
50.8
8.56
3.04
7.61
.728
97.3
16.8
Distance (m) from Antenna
for Power Densities of:
10 mW/cm2
2.46xl02
4. 58x1 O2
-
-
-
-
4.16xl03
9. 68x1 O3
1 mW/cm^
7.79xl02
1.45xl03
2. 94x1 O3
3. 94x1 O3
6. 23x1 O3
-
1.32xl04
3.34xl04
100 uW/crrr
2.46xl03
4. 58x1 O3
9. 29x1 O3
1.25xl04
1.97xl04
1.23xl04
4.16xl04
1.06xl05
10 yW/cm^
7.79xl03
1.45xl04
2.9xl04
3. 94x1 O4
6. 23x1 O4
3. 88x1 O4
1.32xl05
3. 34x1 O5
*Includes a 3 dB loss of power into waveguide and swivel joints.
**Systems measured.
***These are not satellite communication systems, but systems which are used to communicate with space vehicles on planetary
exploration missions.
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50
case (on-axis) power density capability and the distance from the system at
which the power density may equal a selected threshold value of interest.
Simplified models exist which can calculate reasonably well the significant
radiation characteristics of a source from 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.
If the potential hazard evaluation of satellite communication system
were carried further to include other factors, i.e., operational procedures
and site characteristics, power density produced at any location relative to
the source would be predicted with greater accuracy. However, these factors
are difficult to incorporate into calculations and they may be the factors
responsible for significant differences between measured and calculated
radiation field characteristics. In addition, biological effects information
is inadequate for quantitative biological effects hazard analysis. The
analysis depends on the power density at which effects occur and their
frequency dependence, the exposure time, and the characteristic and distribu-
tion of the exposed population.
A simple illustration of the technique of applying a selected threshold
value in a potential hazard evaluation of the inventoried satellite communi-
cation systems treated in this paper is presented in a graphic display (figure
17). Sources whose on-axis power density is greater than any imposed thres-
hold are identified and their near field distances are immediately available
for use in further analysis.
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51
OJ
e
o
1
to
c
0)
o
1-4
0)
3
o
10
cfl
0)
2
10'
-1
10
• TSC-54
LET
GOLDSTONE
VENUS
GOLDSTONE
MARS
•MSC-46
'FSC-9
MSC-60
• INTELSAT
I
10
102 10*
Near Field Distance (m)
Figure 17 Possible Method of Identification of Satellite Communication Systems
-------
52
Once this initial identification has been made, the sources can be
comparatively ranked using chosen criteria. One method of comparing sources
is based upon the distance from the source at which a selected on-axis power
density exists. The results shown in table 9 can be used. Another method
can be based upon the on-axis power density which exists at a selected
distance from the source. It must be remembered that these evaluations do
not take frequency, power density, and exposure time into account; they are
based upon a model used for calculational purposes and assume a threshold
value for power density.
Procedures which can be used to perform these evaluations for systems
using modified circular paraboloidal reflectors and other types of antennas
are available, although more complex, so that the evaluation techniques can
be applied more generally.
The factors which must be considered in a realistic evaluation of the
potential environmental hazard of any particular system include the trans-
mitter power used in its regular operation (generally much less than the
maximum possible), the procedures employed in system operation, the distance
between the center of the antenna and ground, terrain characteristics, the
minimum elevation angle of the antenna below which the system cannot operate,
and the distribution of population within the area for which power density
equals or exceeds the selected threshold level.
The operational transmitter power may not equal the transmitter power
capability, resulting in power densities produced during operation which
are less than the maximum possible. The power densities vary directly as
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transmitter power, so that if Pj (used)/Pj (max) =0.1, the power densities
produced will be one-tenth of that calculated for use of maximum transmitter
power.
Many satellite communication systems cannot operate at elevation angles
less than a specified angle relative to horizontal. The knowledge of that
angle and other system and site characteristics; i.e., terrain features,
antenna height above ground, population distribution, location and height of
structures, and sidelobe radiation characteristics of the antenna, will
realistically determine the possibility for exposure of persons within a
radius for which the power density exceeds or equals the threshold level.
A system may produce significant power densities, and still not con-
stitute a hazard if there is no possibility for exposure.
It is apparent that measurements of existing radiation levels are
necessary to a realistic hazard evaluation. A potential hazard evaluation
may be most useful in identifying those systems to be studied by measure-
ment, to assist in assigning a priority to them for a more intensive
evaluation, and to analytically determine radiation characteristics for a
specified source. However, it is necessary to quantitatively measure the
environmental levels of power density in a realistic evaluation due to the
difficulty of incorporating into an analytical evaluation the factors which
may significantly affect the results.
ECAC as a Resource in the Identification and Evaluation of Potentially
Hazardous Sources
The value of potential hazard evaluations depends on the quality of
the information which resides in the computerized data inventory. The
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Electromagnetic Compatibility Analysis Center has the most extensive
inventory of sources containing most of the system characteristics
required, but the quality of the information is not always consistent. The
organizations operating the systems have the responsibility to provide
accurate information to ECAC but there are inaccuracies and inconsistencies.
The effective radiated power determined by ECAC and used in ranking
sources, usually the maximum EIRP for the systems considered, uses the
maximum transmitter power capability and, in many cases, the theoretical
system gain, not the actual system gain. EIRP may be reported as defined
in eq. (9), but usually as G-Pj, where Pj is the maximum output power
capability of the transmitter. EIRP reported in this manner may be a
factor of two greater than that which includes losses in the system.
The extent to which the ranking of systems would be affected by
inclusion of actual system gain characteristics needs to be determined, but
the effectiveness in identifying sources or categories of sources according
to their potential for creating potentially hazardous environmental radia-
tion levels may not be affected.
Section 5 - Conclusions and Summary
This study applies in general to the concept of potential hazard
evaluations involving sources of environmental microwave radiation, and
specifically to satellite communication earth terminals. The general con-
clusions reached are that:
1) In general, satellite communication systems, operated in accordance
with prescribed system operational procedures, should not constitute a
thermal effects hazard. The possibility of exposure of persons (not
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employed in system operation) should be extremely small because radiation
is directed generally upward. The avoidance of population exposure is
intended, as indicated by the minimum elevation angles specified. Certain
satellite communication systems, if operated improperly, can create thermally
hazardous situations due to the large on-axis power densities which can be
produced at great distances from the antennas.
2) Models exist which predict, under ideal conditions, maximum on-axis
near field power density, and appear to be applicable in defining the
intermediate and far field zones and on-axis power density as a function
of distance from the antenna.
3) The calculated values of on-axis near field power density, near
field extent, and on-axis power density as a function of distance from the
source can be applied to a method for potential hazard evaluation. A
selected power density threshold can be used as a basis for the identifica-
tion of potentially hazardous systems, assigning a relative ranking to them,
and in the initial evaluation of a system with regard to its potential as
a source of hazardous exposure levels.
4) Effective isotropic radiated power is not the most directly
applicable characteristic to be used in the potential hazard evaluation.
Small diameter antennas, having lower gain characteristics, can produce
greater on-axis near field power densities, for equal transmitter powers
and radiation wavelengths than larger diameter systems.
5) A more realistic hazards evaluation will include other criteria
defining the exposure problem in addition to power density; i.e., the
population exposed and the exposure duration.
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6) A true hazard evaluation will not be possible until effects are
identified, thresholds defined, and the relationships between the degree
of effect to radiation frequency, power density, and exposure as a
function of time are known.
7) Nonthermal effects hazard evaluations would give greater importance
to the off-axis power density characteristics of satellite communication
systems.
8) A potential hazard evaluation procedure can be used to identify
those systems to which priority can be assigned for a more realistic hazard
evaluation. Factors exist which significantly affect the results of an
analytical evaluation, and make measurement a necessary part of a realistic
hazard evaluation.
The requirements and uses for hazard evaluations are summarized in
table 10.
Acknowledgments
I wish to express my appreciation to Colonel James E. Anderson and
John Taylor for making possible my participation in the U.S. Army Environ-
mental Hygiene Agency radiation protection surveys at Fort Monmouth, New
Jersey and Fort Detrick, Maryland. I am particularly grateful to Marcus
Dieterle and Arthur Riggs for their cooperation and efforts in coordinating
the investigations at Fort Monmouth and Fort Detrick so that my interest in
satellite communication system earth terminals could be satisfied, and to
Charles Hicks, Captain Robert Fenlason, and Francis Rura for their coopera-
tion, assistance, and good humor in helping to make the experience useful
and thoroughly enjoyable.
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Table 10
Hazards Evaluation Summary
Potential Hazard Evaluation:
1. Used to identify potentially hazardous sources on the basis of a selected power density threshold
2. Systems can be ranked according to selected criteria
3. Uses existing simplified models to calculate:
near, intermediate, and far field regions
power density (on-axis) vs. distance from source
Realistic Hazard Evaluation requires:
1. Quantitative identification of biological effects; i.e., determining for each effect ^
a. dependence upon frequency, power density, exposure time
b. threshold values
2. Knowledge of the population exposed, including
a. unique characteristics
b. distribution
3. Measurement of significant radiation field characteristics for those sources identified; i.e.,
power density as a function of frequency and time.
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References
(]_) Electromagnetic Compatibility Analysis Center. Metropolitan Radiation
Hazards II. ECAC-PR-72-034, ECAC, Annapolis, Maryland, 1972.
(2J Tell, R.A. Environmental Nonionizing Radiation Exposure: A Preliminary
Analysis of the Problem and Continuing Work within EPA. Session Pro-
ceedings: Environmental Exposure to Nonionizing Radiation, U.S. Environ-
mental Protection Agency, Hashington, DC, 1973.
(3j U.S. Army Environmental Hygiene Agency. Laser and Microwave Hazards -
Course Manual. USAEHA, Aberdeen Proving Grounds, Maryland (current).
(4) Silver, S. Microwave Antenna Theory and Design. Dover Publications,
Inc., New York, 1965.
(5j Narda Microwave Corporation. Operation and Maintenance Manual for
Broadband Isotropic Radiation Monitor. NMC, Plainview, New York, 1972.
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-520/2-74-008
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
AN EVALUATION OF SELECTED SATELLITE COMMUNICATION
SYSTEMS AS SOURCES OF ENVIRONMENTAL MICROWAVE RADIATION
5. REPORT DATE
December 1974; Issuing date
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Norbert N. Hankin
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Electromagnetic Radiation Analysis Branch
9100 Brookville Road
Silver Spring, MD 20910
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
Field Operations Division
Office of Radiation Programs
Environmental Protection Agency
401 M Street, SW. Washington, DC
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
20460
15. SUPPLEMENTARY NOTES
16. ABSTRACT
Selected satellite communication (SATCOM) systems are evaluated analytically and,
for some of these systems, through measurement of the microwave radiation power
densities generated by them. The evaluation is directed toward assessing the radia-
tion exposure hazards which exist for specific systems and generally for SATCOM
systems as a class of high power nonionizing radiation source. The paper includes
determinations of anticipated maximum power density levels as functions of distance
from the source, a description of the analytical method used, and the results of
measurements of the power densities produced by certain SATCOM systems. Included also
is a discussion of potential hazard analysis and its uses in identifying systems which
may constitute environmental hazards.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS C. COSATI Field/Group
Environment; microwave; nonionizing
radiation; power densities; satellite
communication.
18. DISTRIBUTION STATEMENT
Release to public
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
20. SECURITY CLASS (This page)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
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