Rectennas will be built with different sizes and power outputs. For example, a circular rectenna some 500 metres in diameter will receive approximately 2 MW of power during reception periods. However, because the satellite is not stationary, the power reception lasts only a few minutes per orbit. So, by using storage, a 2 MW rectenna will be able to deliver a continuous output of some 50 kW. In areas where electric power is not currently available, such a level of output can have great social and economic value. Each rectenna site will be different, both in its geography, and in the local economy. Planning the best design for a rectenna, and the best means of using the power produced, will involve discussions with the local community, the local government, and the SPS 2000 project staff. Each rectenna will be a unique story. The possiblity of future evolution of the SPS 2000 system to high power levels and longer power transmission times will also require careful local planning. Like the satellite, the rectenna must be made as cheaply as possible. However, for the SPS 2000 project, the rectenna will be less economical than for a future commercial SPS, for two reasons. First, power will be received for only about 200 seconds in each orbit, or about 3 % of the time. Second, the intensity of the microwave beam will be very low. Nevertheless, both the power and the information produced by SPS 2000 will be valuable. The microwave intensity at the rectenna is very low, and will be below the strictest safety standards in the world, so the operation will be entirely safe. Using no fuel, the rectenna operation is entirely clean. Thus by designing the rectenna to use local methods of construction the environmental impact will be minimized. At some sites it will be best to place the rectenna over water, and one attractive construction method in the warm seas near the Equator will be to use artificial coral. When a weak electric current is passed through a metal structure in the sea, it is covered in a tough layer of concrete. This process has recently been developed under UNDP funding in Colombia, and could use power from the rectenna itself to extend the area of the rectenna support structure without pollution. For rectennas sited over land, local construction methods will be used. For example, in countries where forest has been cleared, there are plentiful supplies of durable wood, which could be used to support the rectenna surface above agricultural land. In other places bamboo may be a good construction material. In areas where the land is less valuable, a simpler design of rectenna might be used that would be laid directly on the ground, which we call the magic carpet. The rectennas for SPS 2000 must be sited within a few hundred kilometers of the Equator. Also, in order to receive power for as long as possible, they must be separated by some 1200 km. The range of countries which might have SPS 2000 rectennas includes countries in South America, Africa and Asia, and a number of islands. No decisions have been made yet on sites, as they will involve many delicate
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