lems involved, these countries would also have to resolve any problems arising from the environmental impacts of the system. For the densely populated countries of Western Europe the implications of this may be rather different than for the USA, with its larger land areas. Taking this into consideration may lead to different conclusions about the feasibility of the concept and its evaluation. Various different space-to-earth power transmission systems are being studied, including the use of lasers (1,2), different microwave wavelengths (3), non- geostationary microwave transmitters (4) and direct reflection of sunlight (1,5). While some of these systems offer advantages, in particular the use of much less land area, they are all less far-advanced than NASA's current Reference System design (6). This specifies the use of a microwave beam at 2.45 GHz (in the industrial waveband) transmitted from geo-stationary orbit (i.e. 22,300 mile altitude). Current theory suggests that the microwave intensity should be kept below 23 mW/cm2 in its passage through the ionosphere, which entails that, with the 10 dB Gaussian power distribution specified in NASA's Reference System, a beam delivering 5 GW,, would have a circular aperture 10 km in diameter normal to the beam direction at the ground. For non-equatorial sites the receiving/rectifying antenna (or “rectenna”) has to be elliptical; and for much of Western Europe, which is at more than 50°N latitude, rectennas would have to be about 20 km in the North-South direction, covering an area of 160 km2 or more (7). Primarily because of the high population density, finding sites of this size on the Western European mainland would be very difficult (8,9). Work currently under way at Imperial College (10), suggests that this would not be economical, in the UK at least. Thus the possibility of siting rectennas over coastal waters would be very attractive for Western Europe, if it could be done economically. Siting rectennas over sea has been proposed in several studies on SPS systems, with the aim of reducing both land use and exposure of the population to microwaves. However, very little work has been done on this concept to date. In the US Department of Energy Report on Resource Requirements for NASA's SPS Reference System (11, pp. 103-104) it is stated: “The inclusion or exclusion of rectennas sites on the sea is another critical general issue requiring resolution. . . . Unfortunately, there exists no reference concept or preliminary design. . . . The briefing literature on the reference concept does not discuss sea sites specifically. Without some discussion and even the coarsest of design parameters, any attempt to evaluate sites in the next round of site evaluation will be a largely empty exercise.” Since then, Hervey (12) has examined the question as part of a longer study by Brown & Root Inc. and Rice University. In the present paper some cost comparisons are made between offshore and land rectennas, which, though only approximate, allow a number of useful conclusions to be drawn. 2. DESIGN CONSIDERATIONS As a major cost element of the SPS system, the rectenna offers good opportunities for innovative design. It is stated in NASA's Reference System Report (6, p. A-39) “The rectenna configuration, although in many respects has had more investigations and experiments than other areas of the SPS, offers the potential for greater change
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