4. Technology Assessment Two major areas which require extensive development are high efficiency microwave generators and rectennas that can operate at 35 GHz. In the past, the development of rectennas was concentrated at 2.45 GHz. Rectenna efficiencies greater than 50% have been demonstrated [4] using a thin-film etched-circuit technology. Efforts to extend this technology to 20 GHz were not fully successful [5] due to a lack of funding. The candidates for microwave generators are gyrotrons, klystrons, and magnetrons. At 45 GHz, a gyrotron with a peak power output over 1200 kW and an efficiency of 35% has been demonstrated [6]. Klystrons with output power of 1 kW [7] and gyrotrons with peak power of 200 kW at 35 GHz [8] are commercially available. With sufficient funding and development in the next decade, we feel confident in meeting the system requirements of 80% DC to RF efficiency for a 35 GHz microwave generator and 75% RF to DC efficiency for a rectenna. ACKNOWLEDGEMENTS This work was supported in part by the NASA Center for Space Power. The constant encouragement of Dr A. D. Patton is acknowledged. The authors would also like to thank Mr J. H. Suddath for helpful discussions. REFERENCES [1] Brown, W.C. (1974) The technology and applications of free-space power transmission by microwave beam, Proceedings of the IEEE, 62, 1, January, pp. 11-25. [2] Arndt, G.D. & Kerwin, E.M. (1986) Applications of low-earth orbit power transmission, Space Power, 6, 1986, pp. 137-155. [3] Suddath, J.H. (1980) Solar power satellite microwave antenna system, NASA Johnson Space Center Memorandum, EH2-80-104.
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