TABLE 2 MICROWAVE POWER TRANSMISSION EFFICIENCIES Efficiency Presently Demonstrated3 Efficiency Expected with Present Technology3 Efficiency Expected with Additional Development3 Microwave Power Generation Efficiency 76.7b 85.0 90.0 Transmission Efficiency from Output of Generator-to-Col lector Aperture 94.0 94.0 95.0 Collection and Rectification Efficiency (Rectenna) 64.0 75.0 90.0 Transmission, Collection, and Rectification Efficiency 60.2 70.5 85.0 Overall Effici ncy 26.5C 60.0 77.0 The quantity of 1 to 2 million tubes that would be needed for each SSPS is large enough to warrant large-scale, highly efficient mass production. There is substantial production experience on magnetrons, similar in many respects to the Amplitron device projected for use in the SSPS. d. Microwave Transmission A series of microwave generators will be combined in a subarray (e.g., about 5 m by 5 m) which forms part of the antenna. Each subarray must be provided with an automatic phasing system so the individual antenna radiating elements will be in phase. These subarrays will radiate through a slotted waveguide and form a phased-array transmitting antenna about 1 km in diameter to obtain a microwave beam of a desired distribution. The distribution can be designed to range from uniform to near gaussian. For this 1-km diameter antenna, the diameter of the receiving antenna on Earth would have to be about 7 km for gaussian distribution in the beam within which 90% of the transmitted energy is intercepted. The use of such a large receiving antenna area would reduce the microwave power flux density on the Earth to a value low enough so that the flux density of the edges of the receiving
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