Transmission of microwave beamed-power from an orbiting space station to the ground

Transmission efficiencies and surface power densities are calculated from the interaction of a 10 GW microwave beam with rain clouds. Computations are made as a function of frequency (2.45 to 10 GHz); beam nadir angle; raindrop size distribution, and cloud shape. Scattered surface power densities outside of the receiving rectenna do not exceed 10 microwatt/sq cm for frequencies of 2.45 and 3.3 GHz, even for extremely heavy rainfall rates. At higher frequencies exposure levels outside of the rectenna may reach 100 microwatt/sq cm, or two orders of magnitude less than the U.S. safety standard. From the standpoint of public health and safety, the scattering of microwaves by rain clouds is not a serious problem, with scattered fluxes outside of the rectenna much smaller than sidelobe fluxes. Beam losses due to absorption in rain clouds are significant in some cases, with absorption losses far more important than scattering losses. The amount of scattering increases with increasing microwave frequency, increasing drop size and drop concentration, and increasing nadir angle of the beam.