gain, active element impedance, and active elements pattern that are characteristic of a large phased array arise from the fact that the elements are phase coherently combined by a microwave network. In the rectenna there is external mutual coupling but no internal RF connections. Thus, the external mutual coupling affects the impedance of each element. But an array like this where each element is connected to an independent resistive load does not have the narrow beamwidth or high gain etc., of the phase array. This is fortunate for it allows realistic tolerances to enable satisfactory performance. There is, however, a corollary. The internal mutual coupling which increases element capture area as spacing increases, up to grating lobe appearance, is not operative. Thus for a half-wave slot or dipole over a ground plane, the effective area appears to be that of the isolated element which is 0.261 times the square of the wavelength. Thus, the rectenna lattice should be square with a spacing just above a half-wavelength. However, it is not necessarily true that the element effective area is square, which may require an adjustment of the lattice. Clearly more work should be done on this problem, but conventional unit cell active impedance approaches appear to be useless. Cost Effectiveness The rectenna is presently a major cost factor in the total SPS system. As such, it should be subjected to careful cost effectiveness sensitivity studies which might point towards a slightly less efficient system, but with substantial cost savings. For example, at the outer edge of the rectenna, the cost in dollars per watt appears to be higher than that for surface deployed solar cells at the SPS solar cell cost. In other words, it would be more cost effective to replace a substantial fraction of the outer area of the rectenna with solar cells. 8. SUMMARY In summary, it is the consensus of the MPTS workshop revie1. panel that a 5 GW SPS microwave power transmission system is probably technically feasible. However, a large amount of work will be necessary in a number of areas to establish certainty and to determine system efficiency, reliability, rf compatibility, security, safety, longevity, and cost. The panel believes that the final system will not look much like the present reference system and urges NASA to recognize this in all future planning. The GBED appears too heavily locked into the reference system. The panel recommends more attention to systems engineering and failure analysis. There should be more sensitivity studies to optimize cost effectiveness. Attention should be paid to system security and anti-jamming features. There should be periodic overall design reviews to update critical design parameters. In view of the magnitude and potential importance of the SPS, the panel recommends major program management status and a single program office within NASA for greater coordination of the contractor effort. Finally, the panel applauds the outstanding effort to date by the SPS team, and is impressed with the very good progress which has been made thus far.
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