Fig. 10.2. The antenna farm at Mt. Wilson, California. The complex consists of 27 antennas that
radiate approximately 10 MW, thereby producing ground-level power densities of up to 28,000
µW/cm2. (Reproduced by permission, from ref. 6.)
The average contribution of high-power radars to the urban EMF environment is low because
their beams are directed away from population centers. But, because of stray radiation, exposure levels
near airports and military bases can be in the range of 10-100 µW/cm2 at distances up to one-half mile
(8). Airborne radar makes a further contribution to the airport EMF environment.
Microwave-relay antennas, located at intervals of about 20 miles, are used for long-distance
telephone service and for private communications. A 10-foot diameter antenna positioned 100 feet
above the ground produces ground-level EMFs of approximately 0.03-7.5 µW/cm2 within 376 feet of
the tower (9). There are several thousand microwave-relay towers in the U.S., each with two or more
antennas.
Mobile communications equipment and hand-held walkie-talkies are relatively low-power
sources, but they account for significant exposure levels because the radiating antenna is ordinarily
close to the user. Figure 10.3 depicts the power densities in the head area that arise from a typical
walkie-talkie (10). Figure 10.4 gives the power densities inside and outside a truck with a roof-mounted
antenna (10).
Fig. 10.3. Power density (µW/cm2) in the area of the head of a Motorola HT-220 walkie-talkie
operating at 165.45 MHz with an output of 1.8 W (10). The measurements were made in the near field
where the plane wave relation between the electric and magnetic fields does not strictly apply: the
listed values are an upper limit for the actual power densities. The same comment applies to Figure
10.4.
ELECTROMAGNETISM & LIFE - 136