three arcminutes if phasing was extended to the smaller power modules (4). The grating lobes for a two-beam system with rectenna separations of 50 km and 150 km are given in Table 1. The associated subarray size is 4.2 m. Since the first grating lobe peaks exceed 0.01 mW/cm2 for a 150 km rectenna separation, mechanical randomization of the subarrays or power modules will be required in order to diffuse the grating lobes (6). Although this randomization was not required in the reference system and could impact the assembly of the antenna, it is not considered a major impact for multiple beam systems. MULTIPLE BEAM TAPER OPTIMIZATION The SPS microwave system is constrained by thermal, ionospheric, and sidelobe limitations. An antenna design technique has been developed to generate optimal antenna illumination tapers which meet the environmental constraints for a single beam system (2). This technique, which involves optimizing multivariable functions with inequality constraints and penalty limitations, can be applied to multiple beam systems. Since the exact ionospheric limit (/„) and thermal limit (7J are not presently known, candidate multiple beam systems with several /„ and T„ limits will be presented; however, the maximum sidelobe limit, So, will remain fixed at 0.01 mW/cm2 for all systems as environmental issues strongly favor this lower level. (The original SPS reference system with a 10 dB Gaussian taper had 0.08 mW/cm2 for the first sidelobe peak.) The multiple beam optimization procedure involves first calculating the optimum single beam taper with the ionospheric and sidelobe constraints multiplied by N, the number of desired beams, i.e., the sidelobe constraint becomes 0.01 mW/cm2 x N since the resultant will eventually be divided by N when the multiple beams are
RkJQdWJsaXNoZXIy MTU5NjU0Mg==