the rectenna at a beam frequency of 5 GHz; at a frequency of 7 GHz surface power density increases only to 1 /zW/cm2 at r = 5 km, and to 7 /zW/cm2 at 10 GHz. For moderate rain using the Diermendjian Rain 10 drop size distribution function, surface power density levels are obtained which are very different from those obtained using the Rain M drop size distribution function. This is due to the fact that the Rain M distribution has much larger scattering coefficients, due to larger drop sizes, and is more similar to the MP drop distribution functions. Note, however, that at the larger rainfall rates even the MP drop distribution functions may underestimate the number density of large drops, and, therefore, scattering. Using the Rain 10 drop distribution, surface power densities were calculated to be 0.07, 1, 8, and 20 /zW/cm2 at the edge of the rectenna (r = 5 km) at beam frequencies of 2.45, 5, 7, and 10 GHz, respectively. For heavy rain of 50 mm/h, the Deirmendjian D-50 drop spectrum produces surface scattering patterns similar to, although slightly larger than, those obtained using the Rain M drop distribution function. At the edge of the rectenna, surface power densities are 0.8 gW/cm2 at beam frequency of 2.45 GHz, and 15 /zW/cm2 at 5 GHz. The surface scattering pattern at beam nadir angle of 60° is also similar to that obtained using the Rain M distribution. However, the Rain 50 drop distribution absorbs more of the microwave beam within the cloud, absorbing 0.24, 0.51, 1.50, 3.57, and 7.24 GW at beam frequencies of 2.45, 3.3, 5, 7, and 10 GHz, respectively (compare with Table 4a). Increasing rainfall rates (drop sizes and drop concentrations) has a large impact upon the microwave optical properties and upon the resulting surface power density levels. The surface power densities at beam frequency of 2.45 GHz for heavy rain are larger than those at 5 GHz for moderate rain. This behavior can be seen more clearly by assuming a progression of MP drop size distributions, MP-25, MP-50, MP-100, and MP-150, representing rainfall rates of 25, 50, 100, and 150 mm/h, respectively. For the same rainfall rate the MP drop size distribution provides larger scattering coefficients G8S) and larger single scattering albedos (w0) than do the corresponding Deirmendjian drop distribution functions. In any case, these variations support observations that drop size distributions are sufficiently variable to account for a factor of three in attenuation (at 16 GHz) for the same observed rainfall rate. Figure 6 shows surface power density levels as a function of distance from beam center for a cloud 7 km thick and 24 km in diameter, for the MP-25, MP-50, MP-100, and MP-150 drop size distribution functions, for beam nadir angle of 0°, and for beam frequencies of 2.45, 3.3, 5, and 7 GHz. At beam frequency of 2.45 GHz, maximum surface power densities at the edge of the rectenna (r = 5 km) are 0.3, 1.0, 3.0, and 6.0 /zW/cm2 for rainfall rates of 25, 50, 100, and 150 mm/h, respectively. These values are well below the Soviet microwave exposure safety standard. At a beam frequency of 3.3 GHz, corresponding surface power densities at r = 5 km are 2, 5, 9, and 20 /zW/cm2, respectively. Only at very large rainfall rates (150 mm/h) do the surface power densities exceed the Soviet safety standard. At 5 GHz, corresponding levels are 8, 20, 50, and 100 /zW/cm2. These values are at or above the Soviet safety standard, but two orders of magnitude lower than the allowable exposure levels under the U.S. microwave safety standards. It would appear that even at a beam frequency of 5 GHz, scattered radiation power density levels do not exceed the Soviet safety standards at distances greater than 5 km from the edge of the rectenna even under extremely heavy rainfall rates. Comparison with Fig. 2 shows that even for heavy rainfall rates surface power densities due to beam shape (and sidelobes) dominate those densities due to scatter-
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