The feasibility of siting rectennas in some of the chosen areas would depend on the exact size and shape of the rectenna. At 50° Latitude the ‘footprint* of the SPS microwave beam is elongated by a factor of two in the North-South direction, which increases the area required and rules out some otherwise attractive areas, such as East-West coastlines. Strategies for reducing this problem include 1) specifying a longitude offset between the transmitting antenna and the rectenna to alter the direction of the long axis; 2) altering the beam cross-section (e.g. to an East-West ellipse) to reduce the North-South extension of the rectenna; 3) using a smaller rectenna with a lower power output; 4) reducing the size of the rectenna for the same power output through design advances in the microwave beam formation. Conclusions The siting of rectennas in WE coastal waters looks very promising. A dozen shallow water coastal areas have been identified which merit more detailed study and comparison — particularly of likely costs. There is thus a real possibility of establishing with high confidence a number of feasible WE rectenna sites in the near future. Though offshore rectennas would require additional construction expenses, they could be sited so that no human habitations were within 15 km or more of the microwave beam centre. This would allow greater freedom in the choice of transmitting antenna aperture illumination function, as the near sidelobe levels could be higher than on land. This could be particularly valuable if the rectenna output were subsequently to be increased by transmitting power to it from a second microwave beam, as appears possible (R. V. Gelsthorpe, ERA Technology Ltd., Private Communication). If feasible, this would be a critical innovation, as it could reduce the number of sites required by 50%. There is some mismatch between the availability of offshore sites and demand for electricity in WE countries — particularly in connection with the large conurbations in central France and West Germany. However, the national electricity grids in WE are becoming progressively more interlinked, and it seems likely that this will not pose any serious problems over the period 20 to 50 years in the future. It is recommended that detailed cost studies for construction and integration of rectennas at particular sites should be performed, as well as a comparative study of deeper water sites.
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