0191 -9067/82/040301-15$03.00/0 Copyright € 1983 SUNSAT Energy Council MULTIPLE BEAM MICROWAVE SYSTEMS FOR THE SOLAR POWER SATELLITE G. D. ARNDT and E. M. KERWIN NASA Johnson Space Center, EH2 Houston, Texas 77058, USA Abstract— An antenna optimization technique is applied to the environmental requirements pertaining to SPS (solar power satellite) systems to create high efficiency, multiple beams from a single antenna. Effects of rectenna spacings, subarray sizing, multiple beam degradations, antenna electrical and mechanical errors, and satellite motion are investigated. Advantages of multiple beam SPS systems include operational flexibility, lower power per rectenna, fewer satellites, smaller rectennas, and adherence to a 0.01 mW/cm2 environmental guidelines for sidelobe levels. These advantages suggest multiple beam systems are attractive alternatives to the present single beam system. INTRODUCTION The reference SPS microwave system had a 1-km antenna illuminated with a 10 dB Gaussian taper which delivered 5.8 GW of power to a 10 km rectenna at the ground (1). Recently, this system was updated to accommodate optimal antenna illumination tapers which allow greater transmit powers at reduced electricity costs (2). The new sidelobe levels were minimized to meet a 0.01 mW/cm2 environmental standard. However, these optimal tapers were for single-beam antenna systems. Multiple beam phased arrays are presently used in many commercial and military systems and are applicable to SPS usage (3). The purpose of this paper is to describe the generation of high efficiency, multiple beams from a single antenna, i.e., one satellite antenna will produce multiple simultaneous beams to widely-separated rectenna sites. The advantages are numerous: fewer satellites (and associated geosynchronous slots), operational flexibility, smaller rectennas and lower power per rectenna. The 0.01 mW/cm2 environmental guideline can also be achieved with the multiple-beam systems. THEORY OF MULTIPLE BEAMS The principles of single-beam antenna theory can be applied to the development of multiple SPS beams. The ground (far-field) electric field pattern for a single-beam microwave antenna is given mathematically by the Fourier transform integral of the antenna electric field (4):
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