If we consider now a frequency of 35 GHz, this minimum power density increases up to 640 W/m2. This problem of parasitic capacitance becomes then dominant and critical as the frequency increases. Different Kinds of Rectennas Different kinds of rectenna receiving arrays may be found, depending on the technology used for the rectenna elements. The early arrays were using the kind of rectenna illustrated by the Figure 7.30 [Brown, 1984a]. The manufacturing cost of such arrays is unafordable for a large scale receiving area as foreseen for the future Solar Power Satellite systems. Some simple rectenna structures have been developed, which consist of a single flexible layer in the form of an open mesh that could be installed simply by unrolling long sheets and interconnecting them appropriately [Collins, 1991], The mesh would consist of 2 mm diameter plastic cable carrying RF-DC printed circuits and covered with a protective layer. Such an open mesh would pass rain and sunlight, but would have a very low efficiency (10%), as the energy collection would be very poor. A metallic reflector plane could be added behind the mesh to improve the collection efficiency. Obviously, this ruins the advantages mentioned before, and would increase the cost. The efficiency could probably be increased to perhaps 50%. This structure (with or without the reflector plane) is sometimes called a "magic carpet". Despite its relatively low performances, this remains an interesting solution for low cost, low power, quick and easy use rectenna sites. For fixed and high efficiency (85%) sites, other ideas can be investigated. In order to increase the power density incident on the rectenna element, and to limit the surface of the actual rectenna receiving array, some concentrators can be used to focus the energy. This idea is extensively used with various rectenna technologies. [SPS, 1991] Nevertheless, the high efficiency provided by this simple idea is paid back through a significant increase of the cost. These reflectors can be realized with a plain metallic sheet or with a mesh or wires. Another interesting kind of rectenna array has been investigated these last 10 years. A thin-film technology is used to provide a very light weight, low cost, reasonable efficiency (70%) and flexible rectenna array. The antennas, the rectifier diodes and the filters are all realized in a kind of microstrip technology that reduces tremendously the manufacturing cost and increases the reproducibility. These rectenna arrays are particularly well suited for uses in space, or wherever their qualities are vital. Again, an efficiency problem could arise if the considered power density is insufficient. An improvement could be found in grouping several antennas before the rectification of the signal, increasing the collection surface instead of increasing the power density itself. By doing that, an other drawback arises as the directivity of the rectenna array increases. The trade-off must then be found depending on the application. Finally, some other problems will have to be faced for the development of the large scale and high power SPS. The classical rectenna concept is limited in handling high power. Moreover, some reliability problems could be encountered as the number of elements increases. Some new technologies are investigated in order to solve these weaknesses, and are briefly introduced under the "new technologies" point. New Microwave Technologies Four mechanisms for space based power transmission include: tethers, microwave beams, submillimeter wave beams, and lasers. Tethers have a simple mechanism for transmitting power. However, due to physical limitations for transmitting space power, their use is restricted to platforms sharing the same orbit and within a tether distance. Currently, space based power transmission and reception technologies are primarily focused on microwave (about 10 cm wavelength), submillimeter waves (wavelengths less than 0.1 cm) and laser systems (micron wavelengths). This section will concentrate on micro wave and submillimeter technology research. Space based laser systems will be discussed in the laser section (7.2.2). Microwaves are the most developed and widely used technology. Submillimeter waves, however, allow the use of much smaller transmitting and receiving antennae than microwave antennae. Technical developments are concentrated on improving these systems or changing the method of incorporating lasers and microwaves into the total solar power satellite system. This new technology summary of power
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