Microwave Power Transmission Using Tapered Beams SETH D. POTTER1 SUMMARY The safety and efficiency of microwave power transmission from solar power satellites (SPS’s) is constrained by the existence of beam sidelobes. These sidelobes contain power that is wasted, and that may be hazardous to surrounding populations. The amount of power in the sidelobes can be minimized by varying the intensity of the beam across the face of the transmitting antenna, a process known as beam tapering. A family of beam tapers is examined, and recommendations are made on the basis of tradeoffs involving maximizing the amount of power in the main lobe, minimizing the peak beam intensity at both the transmitting antenna and the rectifying antenna (rectenna), and minimizing both the rectenna size and the size of the exclusion zones (unsafe areas) around the rectennas. The effect of frequency scaling is discussed, and several different microwave safety standards are considered. Introduction Transmission of power using microwaves has been proposed for a variety of uses in space and on the Earth, notably for solar power satellites. The physics of beaming power from an SPS to a rectenna on the Earth is a major constraint on the design of the SPS. The spreading of the beam due to diffraction necessitates the transmission of a large amount of power in order for the SPS to be economical. Previous work at New York University [1], [2], [3] considered the use of square and circular transmitting antennas beaming power at frequencies of 1 to 100 GHz. It was concluded that frequencies below about 10 to 15 GHz should be used for best penetration of rain and air [1], and that circular antennas have lower sidelobe levels than square antennas [2], allowing for more efficient power transmission, and smaller exclusion zones in the vicinity of the rectenna. This analysis was carried out for transmitted power levels that are constant across the face of the transmitting antenna. It was seen that the sidelobes of the beam were a source of two difficulties: wasted power (since the power in the sidelobes is too spread out to rectify economically); and "wasted" land (since the first few sidelobes may nevertheless have enough power to present a danger to people [though perhaps not crops], thereby necessitating large exclusion zones around the rectenna). Although the choice of a circular antenna provides some sidelobe reduction compared to a square antenna, even further reductions are possible if the amplitude of the transmitted beam is varied, or tapered, across the face of the transmitting antenna. Circular apertures using tapered beams will be considered as a means of concentrating more power in the main lobe with less power in the sidclobes. t Department of Applied Science, New York Univ., 26-36 Stuyvesant Street, New York, NY 10003.
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