smaller land area may be environmentally unsound, since the peak microwave intensity would increase from 23 mW/cm2 (for 2.45 GHz) to 4600 mW/cm2. If the size of the SPS were scaled down so that the peak intensity of the 35 GHz beam was 23 mW/cm2, then the power leaving the transmitter would have to be restricted to about 32 MW, because of which more SPS units would be needed to make up the shortfall, so that there would be no savings in land area. However, if the demonstration of SPS technology is the goal that is set forth, 35 GHz technology might be considered as an additional option, since the units would be smaller and might more easily be financed and constructed. However, microwave lenses in space have been suggested as a means of increasing the effective aperture of the SPS transmitting antenna.19 This would allow for smaller rectennas, while retaining the 2.45 GHz frequency. The long-term feasibility of 35 GHz power transmission is limited by rain, cloud, and atmospheric attenuation. For a typical temperate zone rainfall rate of 5 mm/hour, the transmission efficiency is only 17% (our calculation was based on Ref.
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