1962
MICROWAVE RADIATION AS A USAF ENVIRONMENTAL
HEALTH HAZARD
Captain Marvin E. Kennebeck
Regional Environmental Health Laboratory
Mcclellan AFB, California
1. Microwaves - What Are They?
Microwaves originate as electromagnetic impulses in a high frequency oscillator tube. They are carried, via a waveguide, to a transmitting antenna which radiates them outward at the speed of light.
Microwaves occur in a band of the electromagnetic spectrum just above ordinary radio waves, from about 300 to 300, 000 megacycles per second. The most common frequencies encountered in the AF lie between 390-1550 megacycles (L band) 1550-5200 inc (S band) and 5200-10, 900 (X band).
2, Effects of Microwaves on the Body.
When microwave radiation is transmitted in space, its energy is either reflected or absorbed by interferring objects. The total amount absorbed is dependent on the nature of the material and on the frequency and power density, or flux of the radiation,
The absorbed energy produces localized heating of tissue in varying degrees. Through animal experimentation, it has been shown that short wave lengths cause maximum heating effects at the skin surfaces, while longer wave lengths produce heating of the deeper tissues and internal organs.
Microwave radiation at frequencies above 3000 mc and above is usually reflected or absorbed by outer skin layers and will give immediate warning as a result of superficial heating. Radiation occurring below 3000 mc is more serious, as the heating will occur in the fatty and deeper tissues of the body without adequate warning. Chronic injury may result from repeated low level exposures, if the heat cannot be readily dissipated.
Personnel in the AF and in civilian industry who are occupationally exposed to microwave radiation have been closely followed during the past ten years. The results of physical examinations show no significant evidence of temporary or permanent body injury as a result of such exposure.
3. Exposure Limits.
The maximum permissmble limit for USAF occupational exposure has been established at 10 mw/cm , for all frequencies. No stipulation has
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been made as to duration of exposure, such as an eight-hour period, due to lack of data correlating time, frequency and power density. It is interesting to n~te in a recent Russian technical report that limits as low as 0. 01 mw/cm have been recommended (for frequencies above 300 mc per second).
4. Use of the Ramcor Densiometer.
The Rancor Densiometer (Models 1200 and 12.50) is a portable battery operated instrument that will detect and measure VHF, UHF and microwave energy.
The scale is ca~ibrated in decibels (db), plus2or minus in reference to zero, or 10 mw/cm . For conversion to mw/cm , a chart is provided, or a simple rule of thumb may be used; +3 db indicates a doubling or halving of power density. Limits are 1-20 mw/cm
The principle of operation is similar to that of the Wheatstone Bridge. A balance resistor varies the current through a thermistor, changing its resistance; when the thermistor resistance is equal to that of a reference resistor, no current flows through the meter.
When microwave energy is applied via the antenna, the thermister resistance will change, unbalancing the bridge; current flows through the meter indicating field strength.
In making measurement, the antenna is pointed toward the radiating source and oriented so that a maximum reading is obtained. Care must be used in locating the maximum reading as the antennae are very direction sensitive.
5. Survey Procedure.
Standardized survey procedures are not recommended due to varying conditions and types of units. However, the following information should be obtained for all units surveyed, whether ground or airborne radar.
a. Nomenclature, including type, peak and average power in watts, size and configuration of antenna, frequency(ies) used.
b. For search and other fixed ground units, maximum negative tilt of antenna, use of blankers, areas or buildings in direct path of beam with elevations, distances, and azimuths, and degree of occupancy in inhabited areas.
c. For mobile and airborne units, the average power levels used are so low that no hazard will exist under normal operating conditions. In
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maintenance and repair operations, however, workers may be required to insert hands and arms Into the near field of the antenna (within several inches of the feed horn). A check should be made immediately adjacent to the feed horn in order to determine the power density.
d. Are personnel exposed routinely, intermittently, or not at all?
e. Protection provided: warning signs, safety lectures, use of dummy loads and blankers, periodic physicals.