to reduce the North-South length of the rectenna at higher latitudes. (4) Comparison of land and sea rectennas. This would necessitate the identification of candidate rectenna sites on land, and would involve comparisons of economic, social and environmental impacts of both rectenna construction processes and their costs, and rectenna operation and maintenance, as well as the comparative costs of electricity transmission from rectenna to demand centres. At the current stage of SPS work, where no single approach is clearly superior to all others, it is important that all possible lines of enquiry be examined. Thus two further matters warranting examination are: (a) The possibility of receiving power from more than one SPS transmitting antenna at a single rectenna. The main factors determining the feasibility of this are: i. The two or more beams would arrive from different angles, so that the efficiency of energy reception would depend on the non-directivity of the rectenna. ii. The East-West diameter of the microwave beam at the receiver would increase, and the shape of the beam's “footprint” would change, with longitudinal offset of the antenna and rectenna (7), which would require a larger rectenna structure if power were not to be lost. For small angles (up to about 5°) this would not be very important. iii. It would probably be impossible (or impossibly expensive) to keep the wave fronts of two different microwave beams in phase at the ground. Thus the power reception mechanism would have to be relatively insensitive to this. iv. Receiving power from more than one SPS antenna at a single rectenna would create even greater concentration of power at a single facility. v. The requirement to keep the beam intensity below a certain maximum (23 mW/cm2) during its passage through the ionosphere could be accommodated if the incoming beams were longitudinally separated by more than about 6°, for which the physical separation of the beams at the height of the ionosphere (about 100 km) would be more than the width of the beam so that no overlap would occur. The feasibility of such an arrangement would depend largely on economic considerations. The transmission of power from two SPS antennas to a single rectenna would save the cost of an entire rectenna (i.e. $2-5 bn) less the extra cost of accommodating a second beam. Thus, if the technical problems could be solved, there could be a significant advantage in a single rectenna receiving twice the transmitted energy of the Reference Design, even with lower efficiencies of reception, conversion and distribution. (b) The marine rectenna design favoured above could be an attractive site for a number of different industries whose potential value should be estimated. Concrete walls could be built to form ponds of required size in the enclosed volume of water (Fig. 5). These, together with access to the open sea, waste heat, and abundant electric power would probably be suitable for fish farming and other forms of aqua-culture. Other industries that might find the site favourable are mineral extraction plant and synthetic fuel production. Such spin-offs might help to improve the cost-benefit ratio of a marine rectenna. While it is perhaps too early as yet to expect international or transatlantic agreement on study programmes, it is desirable that the work on SPSs that is done in
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