For this study two frequencies, 5.8 GHz and 11.6 GHz, were selected for sizing tradeoffs. The 5.8 GHz frequency is at the center of a 150 MHz wide IMS (industrial, medical, and scientific) band which allows users to interfere with any other users in the band. This allowable interference is important when considering the sidelobe radiation levels incident upon the Earth which will be discussed later. The higher frequency, 11.6 GHz, provides for smaller antenna/rectenna sizings and longer transmission distances. The nominal microwave transmission efficiency chain is shown in Fig. 8. The DC-RF power tube conversion efficiency of 75% assumes the use of either magnetrons or klystrons with depressed collectors. For enhanced lifetime performance, these tubes should operate at power levels of 10 kW or lower. The low antenna losses (4%) require high-quality slotted waveguides for the radiators. Rectenna collection efficiency is the ratio of the power incidence upon the rectenna to the power transmitted from the satellite. A 30-m diameter rectenna intercepts approximately 88% of the transmit power for the antenna sizes, transmission distances, error tolerances, and frequencies selected for the nuclear satellite reference system. The RF-DC conversion efficiency, 85%, in the rectenna is determined by the losses in the dipoles and half-wave rectifiers. This 85% efficiency is based upon performance tests of rectennas at JPL and other sites. The end-to-end efficiency for the microwave system is 52.8%; however, this high efficiency is dependent upon maintaining relatively short transmission distances; i.e., less than 10 km. TAPER ANALYSIS The klystron (or magnetron) power tubes of the phased array are not uniformly distributed, but are varied in spacing or power output to achieve a specified power density taper. One recent area of progress in solar power satellite microwave studies was the determination of an optimal antenna taper providing high power transmission efficiency with low sidelobes and limited antenna and ionosphere power densities. The microwave system for a nuclear satellite faces somewhat different constraints; sidelobe energies are less critical, ionospheric saturation concerns are miti-
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