5.4 MICROWAVE LENS To make a Solar Power Satellite (SPS) more commercially feasible, a smaller unit size than 2.5 GW is needed. This would provide a better interface to the utility grid, reduce investment for the first unit, and allow more sites for the rectenna. A microwave lens of large aperture can be placed in front of the transmitter to achieve good coupling between the microwave transmitter and the rectenna. The product of rectenna and lens apertures is a constant, and the rectenna area is proportional to the power level; thus as satellite power level and rectenna aperture decrease, the microwave lens grows in size as the inverse of the power level. At 1 GW the lens constitutes 4% of the satellite mass; at 200 MW the lens is 50% of the satellite. Finding the economic limit on how small the SPS can be before the increasing cost of the lens offsets the advantages of low power was outside the scope of this study. An example design is detailed below for 1 GW size. Note that above 2 GW the lens is not needed since the size of the microwave antenna is sufficient. FIGURE 5.4-1, MICROWAVE LENS DIAGRAM
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