Table 1. Coastal North Sea Rectenna Specifications in both electronic and mechanical configurations, and thus greater cost reductions.” This would apply a fortiori to the design of marine rectennas, which would involve more different elements, would be more expensive to construct and have not yet been studied in as much detail as land rectennas. Though certain functional elements are common to all rectenna designs, different coastal sites would impose substantially different requirements on the rectenna design. Table 1 lists the rough specifications for an offshore rectenna appropriate to a coastal North Sea site. NASA's Reference Design rectenna consists of approximately 800 East-West rows of inclined dipole arrays some 10 m high, feeding de power along buses to a switching yard and Grid interface. Cost estimates for the rectenna array vary considerably. In rounded figures, Boeing's estimates for the rectenna system (13) is $3,000 m, comprising $700 m for the Grid interface and $2,300 m for the arrays. Given the beam aperture of 78.5 km2 at the ground, the array cost is approximately $30/m2, or $30 m/km2, and so we can write an approximate, linear cost equation: where Cr represents the cost of the rectenna in $ m, and A represents the rectenna aperture in km2. Although the cost of land increases in proportion to the land area of the rectenna, this represents only a few percent of the total rectenna cost. Roughly 95% of the cost depends not on the land area but on the microwave beam crosssection. For the rest of this paper we consider a site represented by Table 1, for which the surface area is twice the area of the beam aperture. 3. MARINE RECTENNA DESIGNS AND COSTS Three straightforward designs for the marine rectennas are considered below in order to provide comparative cost figures: (a) a rigid structure supported on steel and concrete piles sunk into the sea bed, with the rectenna on the top; (b) an artificial island constructed out of a sea-wall with landfill in the enclosed area, and the rectenna sited on this;
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