technology will certainly be a consequence of the nonavailability of sufficiently large land-based areas for rectenna sites adjacent to the load demand centres. This fundamental problem has led to studies of some power transmission systems which attempt to seek solutions to this difficult and sensitive area. The first, and perhaps most obvious solution is to use offshore rectenna sites, and this proposal has considerable merit. Firstly, Europe has a very long coastline, when “smoothed-out” it is twice as long as that of the US, and secondly the coastal waters are generally shallow. In the main, the waters are reasonably benign with the notable exception of the North Sea and the northern coasts of the UK and Scandinavia. Thus, to exploit the possible uses of off-shore sites requires the specialised technical expertise which is available in Europe, particularly Holland which is actively concerned with massive land reclamation schemes and construction of artificial harbours and islands, primarily for the oil industry. Unfortunately, offshore sites will have high capital costs and a significant impact on shipping lanes. The other possibility concerns the actual microwave beam configuration and possibilities exist here. It is not possible to reduce the microwave reception area by simply reducing the total received energy because this area is governed directly by the transmitted distance and inversely by the transmission dish size or the radiated frequency. It is impractical to consider reducing the SPS to Earth distance because this means the unique properties of the geostationary orbit (GEO) would no longer be applicable. The use of laser transmission with correspondingly higher transmitted power densities is a possibility under active study, but there are severe restrictions on the use of the whole electromagnetic spectrum. A means of overcoming some of these difficulties has been proposed by Henderson (3) which uses a hybrid combination of laser and microwaves, which results in a significant reduction of rectenna area. As the basic aim is to significantly reduce rectenna areas, while maintaining sight of the major reference system design guidelines, it is suggested that several microwave beams are transmitted from a single SPS phased array to multiple ground receiving sites. This solution, while being considered mainly for European needs, is also applicable to other territories which have similar problems. To enable a better understanding of this solution to be made, the major technology to be developed or extended is listed below: • Optimisation study for using multiple beams • Reassessment of beam control systems • Determine optimum aperture illumination function • Study of potential ionospheric problems when using multiple beams • Study of cost impacts on the power generation systems. There are many other critical areas of technology associated with the power transmission system such as the choices between klystrons and solid-state microwave generating systems, methods either electronic or mechanical for maintaining the flatness requirement on the transmitting antenna. These problems are fundamental to any SPS system. The transmitting antenna will require the use of tens of thousands of high-power microwave generators which could be either klystrons or solid-state devices. This is another area which will require an applied technology development programme as well as production technology to meet the very large numbers of equipment. CONCLUSIONS This brief review of some of the critical areas of technology that will be required to
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