Fig. 1. Cost of rectenna structures. Equations (1) to (4) are all clearly simplifications and, in particular, do not include allowances for the fact that for a marine rectenna there would be no costs for land acquisition and land preparation. For North Western European sites the cost of land acquisition (including population relocation) would be substantially more than the $2,5OO/acre allowed for in Boeing's estimate of the cost of a land rectenna in the USA (21). There would, however, be additional costs from fouling and for corrosion protection, probably through the use of suitable alloys. These factors are assumed to affect the three designs equally, and Fig. 1 plots the four cost equations. Even allowing for substantial errors in the cost estimates, a floating structure surrounded by a sea wall would seem to be the cheapest design of those considered. This design is therefore considered below. 4. OPTIMUM ANTENNA/RECTENNA RELATION An alteration in the cost of a major SPS system component can affect the design of other components through altering the overall system optimization. The relationship between antenna (i.e. transmitting antenna) and rectenna is one such area. The sizes of the transmitting antenna at the satellite, and the rectenna on the ground are connected by the relation that, for a particular system the product of their areas is a constant for a given efficiency of transmission: i.e. AaxAr=K (where Aa and Ar represent the areas of the antenna and rectenna aperture respectively). Different authors have used very different figures: Goubau (22) discussed a system
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