For both land-based and sea-based sites a number of possibilities exist by means of which the transmission system may be optimised: 1. The use of a single receiving antenna simultaneously powered by two or more solar power satellites. This technique is an attempt to increase the power handling capability of a given rectenna. Unfortunately, it is not possible to increase the power input to a rectenna simply by increasing the power transmitted from the spacetenna— this is due to the fact that the ionosphere imposes an upper limit on the transmitted power density of 23 mW/cm2. The difficulty may be overcome by employing two satellites which transmit power to a common rectenna through two different “windows” in the ionosphere. If the two satellites were to operate on the same frequency, they would give a systematic power density distribution across the face of the rectenna and this would impose an unnecessarily stringent set of limitations on the performance of the rectifying elements. Two ways are envisaged of overcoming this difficulty, the most acceptable of which appears to be to operate the satellites on different frequencies; this results in a time-average power flow which is twice that of the single satellite case and thereby relaxes the constraints on the rectifiers. 2. The design of transmitting antennas having specifications different to those of the reference system. There exist at least three possible ways in which the transmitting antenna may be redesigned to give a more acceptable performance: (i) The antenna may be increased in diameter and illuminated with the same shaped power distribution to produce a narrower transmitted beam than that of the reference system. This offers the advantage that a smaller rectenna site may be utilized; on the other hand, the ionospheric power density limitation of 23 mW/cm2 implies that reduced power levels must be transmitted. Implementation of this type of antenna redesign involves an understanding of the trade-offs between increased transmitting antenna complexity, decreased power handling capability, and the decreased requirement of rectenna site area. (ii) An alternative approach is to consider the use of a larger diameter transmitting antenna in which suitable phase and amplitude variations are employed across its face to produce a beam having a more-or-less constant power density across the face of the rectenna. This technique would ensure that any rectenna site would operate at maximum efficiency instead of only a part of it as is the case with the reference system. (iii) Another alternative which may be considered involves a complete change of concept for the transmitting antenna. Instead of employing a slotted array antenna dedicated to each solar power satellite, it is proposed that the transmitting system should employ a reflector antenna. This reflector could be fed by a number of satellites and the resulting power beams could be directed to a number of different sites on earth. A number of considerable advantages are evident in this system, among which are (a) the costs of the reflector may be shared between a number of satellites, and (b) small satellites may be utilized economically. 3. The use of alternative power transmission frequencies. Since the power beam can be formed by use of a smaller transmitting antenna if higher frequencies are employed, the use of alternative frequencies should
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