kilometres in these conditions, the North Sea Island Group's cost estimate is 180G/m2 (1975), which includes the perimeter wall and land infill. Allowing 30% for inflation since 1975, and using current exchange-rates, that is $120/m2, or $240 m/km2 of beam aperture, which gives: For an area of 11 km2 this gives a cost per square metre of approximately 60% of the estimated cost of Kansai airport (19), which is perhaps not unreasonable in view of the more demanding requirements of the latter. (c) The third alternative is a flexible, floating structure which, in contrast to a fixed structure, would have lower tidal stresses to survive, and would not need to be placed high above the water. Nevertheless, the need to sustain large wave movements, and the strains of tethering the rectenna — even if split into segments — would require heavy duty construction. These problems could be alleviated by surrounding the rectenna with a sea wall, which would eliminate tidal strains, greatly reduce wave motion beneath the rectenna and also provide tethering points along the perimeter. Wave movements inside the wall would be only those that developed in spite of the damping effect of the rectenna and its floats, which could be enhanced by suitable design. A relatively small number of additional moorings, provided by steel cables and either piles or sea-floor anchors, should suffice to counteract those movements that remained. The optimum spacing of the flotation chambers would be determined by economic considerations. However, a rectenna mass of 15 kg/m2 (6), would require only 1.5 m3 of displacement for every 100 m2. Fibreglass is a cheap and durable material with a well-established technology, with which such buoyancy could be provided for about $l/m2 of rectenna. This is about the same as the cost of the concrete footings of the Reference System land rectenna. Even allowing for several times this figure from wind loading (Table 1) would not raise the cost substantially. Although the rectenna arrays in the Reference System are perpendicular to the incoming microwave beam, this is not necessary since they are almost completely non-directional (20). Hence, making the array horizontal would reduce the structural strength required by greatly reducing wind loading, which would otherwise be greater than for a land rectenna. The main additional cost in a horizontal rectenna placed over land would be the increased area of structure, which would be compensated to some extent by the reduced strength requirements. The extra strength required in a horizontal structure floating over the sea would depend on the design, and on the dynamic loading that was expected. The costs of structural materials and of installation of the land rectenna are estimated by Boeing (21) at about two thirds of the total array cost in the Reference Design. A cost of $50/m2 of aperture area for the floating rectenna—i.e. an increase of 66% over the land case—would thus cover a 100% increase in these. For sea wall construction in 20-25 m of water, NSIG's costs are between $25,000 and $40,000 per metre of perimeter. If we take the upper figure of $40m/km we can write for the floating rectenna:
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