RF radiation hazards to space station personne

Inman, R. A.

English

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CO-_
NASA TECHNICAL
MEMORANDUM
NASA tM X-64523
RF RADIATION HAZARDS TO
SPACE STATION PERSONNEL
By R. A. Inman
A strionics Laboratory
May 20, 1970
NASA
$	 (ACCESSI NU BEK-
o	 , .J(PAGES)
u (NASA CR OR TMX OR AD NUMBER)
(:) fI L' r
w	 (THRU)
(CODE)
(CATEGO Y)
George C. Marshall Space Flight Center
Manhall Space Flight Center, Alabama
MSFC • Farm 3190 (September 1968)
TECHNICAL REPORT STANDARD TITLE PAGE
I,	 REP('i%T	 NO. 2.	 GUVLRNMENT ACCLS51ON NO. 3.	 RECIPIENT'S CATALOG NO.
TM X-64523
5	 REPORT DATEq	 TITLE AND SUBTITLE
May 20, 1970
RF Radiation Hazrrds to Space Station Personnel 6.	 PERFORMING ORGANIZATION CUDE
7 .	AUTHORS) 8. PERFORMING ORGANIZATION REPOR f n
It. A. Inman
9.	 PERFORMING ORGANIZATION NAME AND ADDRESS 	 10.	 WORK UNIT NO.
George C. Marshall Space Flight Center
Marshall `pace Flight Center, Alabama	 35812	 11.	 CONTRACT OR GRANT NO.
19 .	TYPE OF REPOR'. 	 Q PERIOD COVERED
12.	 SPONSORING AGENCY NAME AND ADDRESS
Technical Memorandum)
14.	 SPONSORING AGENCY CODE
15,
	
SUPPLEMENTARY NOTES
Prepared by Astrionics Laboratory, Science and Engineering Directorate
16,	 ABSTRACT
Although there is controversy concerning possible effects of exposure to microwave
radiation, some harmful effects Have been proven, such as formation of cataracts in the eyes.
The proven effects alone justify precautions to insure that personnel are not exposed to harm-
ful levels of RF radiation.
Calculation of typical values of RF power density in the vicinity of space station antennas
are included in this document.
	 More definite calculations can be made when the system con-	 I
figurations have been more completely defined.
	
However, the values calculated demonstrate
that the currently accepted safe limits of RF radiation can be exceeded easily in the near field
of high gain antennas on the space station.
	 Personnel working in the field of these antennas
should be protected from overexposure to this radiation.
17.	 KEY WORDS 18.	 DISTRIBUTION STATEMENT
microwave radiation STAR Announcement
RF radiation hazards
space station
19.	 SECURITY CLASSIF, (of this report) 20.	 SECURITY CLASSIF, (of this page) 21,	 NO. OF PAGES 22,	 PRICE
Unclassified Unclassified 13 $3.00 
M5FC - Form 3292 ( Ma Y 1969)
eRECEDiNG PAGE BLANK NOT FILMED.
TABLE OF CONTENTS
INTRODUCTION ...................................
RF EXPOSURE STANDARDS ....... . .... . . . . . . . . . . . . . . . .
POWER DENSITY CALCULATIONS . . ... . .... . . . . . . . . . . . . . .
SHIELDING FROM RF RADIATION ...... . ... . . . . . . . . . . . . . .
CONCLUSIONS ....................................
REFERENCES ......................................
Page
i
1
3
4
5
7
LIST OF TABLES
Title
Examples of Allowable Exposure Times
Examples of Low Gain Antenna Radiation
Levels at 20W Power . . . . . .. . . . . ...... .. . . . . . . . .
Examples of High Gain Parabolic Antenna
Radiation Levels at 20W Power . . . ...... . . . . . . . . . . .
Table
i.
2.
3.
Page
2
5
6
iii
TECHNICAL MEMORANDUM X-64523
RF RADIATION HAZARDS
TO SPACE STATION PERSONNEL
INTRODUCTION
There is currently much controversy and uncertainty concerning the
possible effects on the human body of exposure to electromagnetic energy in
the microwave frequency ranges. The maximum safe level of radiation for
all-day exposure is currently accepted in the United States as 10 mNk'/cm 2.
However, there is some pressure for the adoption of a more stringent limit
on exposure levels [ fl..
The purpose of this document is not to argue what exposure level
should be allowed or what the possible biological effects might be. The pur-
pose is to examine the possible levels of RF radiation to which personnel on
the space station and similar spacecraft might be exposed. Because of the
lack of a firm design, definite calculations cannot be made at this time.
However, some typical situations that might be encountered are examined
for the possibility of radiation hazards. The radiation level to be allowed
on the space station should be a matter of much further study.
RF EXPOSURE STANDARDS
In the United States, the currently accepted permissible level of RF
radiation is 10 mW/cm 2 for continuous exposure. Higher power densities
are permitted for situations in which the time of exposure is closely con-
trolled. For power densities from 10 to 100 mW/cm 2 , the allowable time of
exposure is given by the formula [ 2]
T 6000/VW 2	 ( 1)
TABLE 1. EXAMPLES OF
ALLOWABLE EXPOSURE TIDIES
[power Density
(mW/cm 2 )
Exposure Time
(min/hr)
10 6U
15 26.7
20 15
25 9.6
30 6.7
. 10 3.7
50 2.4
where T is the allowable exposure time in minutes per hour and \V is the
power density expressed in nAV/cm 2. Examples of allowable exposure times
are given in Table 1.
The Soviet Union enforces a
much more stringent maximum of
U. U1 mW/cm 2 for all-day exposure.
For exposures of 15 to 20 minutes per
day, a power density of 1. 0 m\k /cm2
is allowed 13J.
These widely differing standards
have been one source of the controversy
that ha g arisen. Another area of con-
troversy is that of the biological
effects caused by exposure to RF
radiation. Some effects, however,
have been conclusively proven. The
excitation of atoms by microwave radiation results in the production of heat.
The depth of penetration, and therefore the depth at which heating occurs, is
dependent on the frequency of the radiation. At frequencies below 1000 MHz,
the energy penetrates into the body, and vital organs may be heated. Above
3000 MHz, the heating is near the skin, but heat may be conducted to the
deeper tissues. In the frequency band between 1000 and 3000 MHz, a com-
bination of penetration and the skin effect may occur.
There have been some alleged cases of death resulting from exposure
to very high microwave power densities. Formation of cataracts in the eyes
of both experimental animals and man has been demonstrated after exposure
for a few minutes. Some people hear a buzz when exposed to microwave
radiation; what is heard is prob phly the pulse repetition frequency. A sense
of warmth may be experienced by those exposed to microwave radiation,
especially in the frequency range from 8 to 26 GHz. Stomach distress and
nausea have occurred in cases of exposure to radar at power densities as
low as 5 to 10 n,W/cm 2. This effect usually is associated with the frequency
range from 8 to 12 GHz. There are other, more questionable effects that may
be produced by exposure to high levels of microwave radiation. Those
mentioned here, however; are sufficient to demonstrate the need for protecting
personnel from such exposure.
2
( 4)
POWER DENSITY CALCULATIONS
The power density at a distance R meters from a lossless isotropic
antenna is given by the formula
P. D. = Prad/4nR2
	
W/nit
	 (2)
in which Prad is the power in watts radiated from the antenna. if the
antenna is not isotropic but has a directivity D , the expression for maximum
power density becomes
P. D. max PradD/41rR2	 'w/m2
	 ( 3)
The maximum power density would exist only in the directi.-n of maximum
radiation, which is the peak of the antenna pattern. For the general case of
an antenna having both directivity and losses, the expression becomes
P. D. max = PradDrl/47r1Z2	 W/m2
where -q is the efficiency of the antenna. Since the gain of the antenna is
given by
G=Dry	 (J)
the expression for maximum power density becomes
P. D. max = PradG/4rrR2	 W/m2
	 ( 6)
3
The far field of an antenna begins, by definition, at a distance It
where
R = 2d 2/X	 m2
	
(7)
In this expression, d is the largest linear dimension of the antenna 14J and
A is the wavelength of the radiation. Beyond this range, almost all of the
power exists in a mode which is propagating outward from the antenna, so
that the power density varies as i/1t 2 . At ranges in the near field, however,
not all of the power exists in this outward propagating mode. Some of the
power goes into other modes, such as the radial modes, so that tl:e outward
propagating or axial mode no longer varies as 1/R2.
Formulas and curves of axial power density for parabolic antennas
are given on pages 186 and 187 of Reference 5. The axial power densi y at a
distance 2d 2/X meters from the parabolic antenna is given there as
P. D. max = 158. 4Pk`'V/d 2	mw/cm2	 ,	 (8)
where d is the antenna diameter in feet and PkNL, is the radiated power in
kilowatts. The curve on page 187 of Reference 5 shows the axial power
density variation in the near field. It can be seen that the axial power density
in the near field can be as much as 41 times as great as the axial power
density existing at the 2d 2 /A distance.
Using these curves and the equations that have been given, some
possible radiation levels on the space station are shown in Tables 2 and 3.
For low gain antennas, equation (6) has been used to calculate power density.
For the high gain parabolic antennas, the equations and curves given in
Reference 5 have been used.
SHIELDING FROM RF RADIATION
Various materials may be used to shield personnel from RF radiation.
A thickness ^--)f 1 or 2 mils of a metal that is a good conductor will provide
4
mss: +.•...•r.•-^•^	 —	 --^.	 ----^.	 ^,
FABLE 2. EXAMPLES OF LOW GAIN ANTENNA
RADIATION LEVELS AT 20W POWE It
Distance from
Antenna
l ml
Maximum Power
Density
l mN%'/cm21
Case I
0 dB antenna gain (G = 1) 1 0. 159
5 0.0064
10 0.0016
50 0.00006
Case II
6 dB antenna gain (G = 4) 1 0.637
5 0.025
10 0.0064
50 0.00025
very nearly total protection from direct radiation. In the frequency range
from 1 to 12 GHz, 60 by 60 mesh screen will allow only about 1 percent of
the incident radiation to pass. Window screening (32 by 32 mesh) will pass
less than 2 percent in the same frequency range; 6. ^.5 mm mesh will pass
less than 2 percent at 1 GHz, but at 12 GHz it allows 10 percent transmission.
Glass will allow 40 to 65 percent transmission depending on frequency.
When shielding from microwave energy, one should consider the
possibility of reflected or refracted waves. The path of least attenuation
should always be considered.
CONCLUSIONS
It can be seen that a possible RF radiation hazard exists for space
station personnel exposed -:,j the main beam of high gain antennas. The
greatest hazard exists in the near field region of the high gain antennas;
where power densities may easily exceed 10 mW/cm 2. The power densities
calculated here are for power in the outwardly propagating mode only. In
the near field of an antenna, personnel would be exposed also to the power
existing in the near field modes in addition to that calculated here. Care
should be taken to keep unshielded personnel out of the main beam and the
near field of the high gain antennas.
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REFERENCES
i.	 Hardeman, Lyman J.: Microwave Oven Leakage: Federal Regulations
Soon. Microwaves, February 1970, pp. 17-24.
2. Control of Hazards to Health from Microwave Radiation. Technical
Bulletin No. Med 270, Air Force Manual No. 161-70, Departments
of the Army and the Air Force, Washington, D. C. , December 6, 1965.
3. Soviet Health Standards on Microwave Exposure. Microwaves, April
1969, p. 26.
4. Jasik, Henry: Antenna Engineering Handbook. McGraw-Hill Book
Company, New York, 1961, pp. 2-13 through 2-15.
5. Hansen, It. C.: The Microwave Engineer's Handbook and Buyer's
Guide. Horizon House, Dedham, Mass. , 1966, pp. 186-187.
7
APPROVAL 
	 TAI x -64523
RF RADIATION HAZARDS
TO SPACE STATION PERSONNEL
By It. A. Inman
The information in this report has been reviewed for security classifi-
cation. Review of any information concerning Department of Defense or
Atomic Energy Commission programs has been made by the NISFC Security
Classification Officer. m. is report, in its entirety, has been determined to
be unclassified.
This document has also been reviewed and approved for technical
accuracy.
PAUL M. 5'WINDALL
Chief, Ant. , . na Section
C ') II	 r _
C. T. PA LUDAN
Acting Chief, Communications and Tracking Branch
0. T. DUGGA N
Chief, Instrumentation and Communication Division
!:^ . ^ t^s W\Q,
F. B. MOORE
Director, Astrionics Laboratory
MSFC—RSA, Ala
8
:^


RF radiation hazards to space station personnel

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