0191 -9067/80/040265-03$02.00/0 Copyright ® 1980 SUNSAT Energy Council LETTERS TO THE EDITOR To the Editor: The recent SPS conference explored numerous configurations and design concepts for solar power satellites. One concept which was not addressed and which may turn out to have a major impact on system feasibility is the use of a single space antenna transmitting to multiple receivers. Conceptually, this is similar to the capability of phased array radar to focus beams on several targets simultaneously. Although several distinct transmitting arrays could be placed on a satellite, transmitting multiple beams from a single array should allow a simpler structure, greater operational flexibility, and quite possible a larger effective aperture than would be the case if multiple antennas are used. There are certain constraints; the individual phase-controlled elements must have a relatively low gain so that the beam can be steered over a reasonable angle (several degrees) by phasing. This would seem easier in a solid-state concept with individually excited elements than in a klystron concept with large high-gain subarrays. Second, the simultaneous transmission of multiple beams from a single array creates interference patterns across the array — some transmitting elements are more heavily loaded than others. Provided the various beams are transmitted on slightly different frequencies, however, the interference fringes will move radpily across the array, and mean power levels will be uniform. Thermal overload should therefore not be a problem, although peak power levels would be higher. How would such an approach impact the system? Since increasing the size of the transmitting array would allow many rectennas to be reduced in size, the optimal spacetenna might be larger and the optimal rectenna much smaller. The optimal power output of the satellite would be increased since its output would be marketed to several areas. When additional power is needed, it might well be more cost- effective to increase the size of the solar and microwave arrays of an existing satellite than to build another SPS. Similar considerations might apply to the rectenna — it might be useful for a given rectenna to receive power from several satellites (i.e., to increase rectenna power density without increasing power density in the ionosphere). The problem of thermal overload, at least, would be minimized if the beam frequencies were not identical. Daniel Woodard 420 Market St Galveston, TX 77550 (R. H. Dietz of the Johnson Space Center was asked to reply to the above letter. Mr. Dietz’s reply follows.-Editor) To the Editor: Per your request, I have reviewed the letter to you from Daniel Woodard. The
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