be considered. In climates having high rainfall, higher frequencies may prove impractical due to decreased transmission efficiency; in dry climates, however, there may be a considerable advantage. Two alternative frequencies may be considered for SPS utilisation, 4.9 and 5.8 GHz. If the reference system SPS goes ahead, it is likely that harmonic frequencies of 2.45 GHz may have to be allocated for SPS usage and therefore 4.9 GHz is a prime alternative transmission frequency. On the other hand, if systems operating at 2.45 GHz are not to be constructed, 5.8 GHz may be a more suitable frequency since this lies within an ISM band and does not lead to such great problems associated with its harmonics. RF INTERFERENCE, IN PARTICULAR RERADIATION FROM THE RECTENNA It is generally recognised that implementation of the solar power satellite system may result in a number of rf interference effects. There are three ways in which the SPS system can give rise to these effects: - the spacetenna - interactions of the satellite beam with the ionosphere and troposphere - reradiation of energy from the rectenna. The first two areas have received considerable attention both in the US and in the UK, however, little attention has been directed towards the rectenna which may well represent the most significant source of rf interference. The rectenna can give rise to rf interference of three types: - reradiation of energy at the fundamental frequency - radiation of harmonics generated in the rectification process - radiation of wide-band noise generated by the rectifiers. Studies have indicated that under normal loading conditions, 2% (100 MW) of the incident rf energy is directly reradiated at the fundamental frequency. In the event of a fault developing in the rectenna power processing/distribution system, a load mismatch would result, with the consequent reradiation of even higher levels of energy. It has been claimed that, for a northerly rectenna site the reradiated energy is directed upwards and southwards, ionosphere irregularities will result, however, in the incident energy not being totally coherent thus introducing the possibility that radiation will take place in other directions. Diffraction effects will arise at the edges of the rectenna panels and these will give rise to a further reradiated power rf interference phenomenon. The diodes employed in the rectenna to convert the incoming power to d.c. will undoubtedly generate interference at harmonics of the incoming signal frequency. In the reference system design, radiation of these frequencies is minimised by two mechanisms: a low-pass filter is interposed between the diode and the antenna element to reduce harmonic levels, and, the busbars are enclosed in a grounded metal shield to reduce harmonic reradiation from the busbars. Although these two measures may constrain the reradiated harmonic power to be below that of the fundamental, it may well be that harmonics present the most significant source of terrestrial rf interference from the rectenna. This is due to the fact that as frequency increases, the radiation pattern of the rectenna changes and it could be that significant energy is transmitted along paths close to the earth. It is important to remember that interference from the reradiated fundamental and
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