Figure 3 shows scattered radiation power densities at the surface as a function of distance from the beam center for frequencies ranging between 2.45 and 10 GHz. At 2.45 GHz almost all of the scattered radiation still lies within the receiving antenna. However, the intensity of scattered radiation increases rapidly with frequency. At the edge of the rectenna (r = 5 km), the power density levels are about 0.4, 2.0, 10, 50, and 100 gW/cm2 at the frequencies of 2.45, 3.3, 5, 7, and 10 GHz, respectively. These are the maximum values of scattered radiation which would be found at the surface near the edge of the rectenna. At a distance of 5 km from the receiving site (10 km from the center of the beam), power densities at the surface decrease to values of 0.03, 0.15, 1.3, 4, and 10 p.W/cm2, respectively. Therefore, the Soviet safety standard of 10 ^.W/cm2 occurs about 1 km beyond the edge of the rectenna at 5 GHz, 2 km from the edge of the rectenna at 7 GHz, and 5 km from the edge of the rectenna at 10 GHz. At frequencies of 2.45 and 3.3 GHz the Soviet microwave standard is never exceeded outside of the rectenna. Superimposed on the curves shown in Fig. 3 are the Gaussian beam power densities given in Fig. 2. At frequencies less than about 5 GHz, scattered radiation outside of the rectenna exceeds that due to the beam geometry only far from the edge of the rectenna. At higher frequencies (greater than about 5 GHz) surface power density contributions from scattering in some cases may be larger than those contributions by the beam geometry outside of the rectenna. Note that even at a frequency of 10 GHz scattered surface power densities outside of the rectenna may only reach 100 /xW/cm2, or two orders of magnitude smaller than the current U.S. microwave exposure standard. Therefore, radiation power density outside of the rectenna is determined primarily by beam geometry (and sidelobe suppression) rather than by scattered radiation. Total beam efficiencies may, however, be critically influenced by the beam frequency and cloud raindrop size spectrum. Heating rates of approximately 0.05 °K/h would result if the entire microwave beam were absorbed by this 7 km thick rain cloud. For all practical purposes, cloud heating rates due to microwave beam heating may be neglected. B. Cloud Temperature Table 4b shows the effect of temperature upon cloud attenuation characteristics for a frequency of 5 GHz. The amount of radiation reaching the rectenna unattenuated remains relatively unchanged, but the amount of scattered radiation reaching the ground increases from 54.7 MW for a cloud uniformly at a temperature of -20 °C to 63.9 MW for a warm cloud at a temperature of +20 °C. Warmer clouds scatter more microwave radiation while absorbing less.
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