Several reports which were reviewed assume that the primary radio frequency interference problem associated with the SPS is susceptibility to the fundamental operating frequency of the microwave transmission by other services users, due to side lobe radiation of the transmitting antenna. The validity of this assumption is dependent on the degree to which the output of the SPS can be filtered to remove unwanted spurious and harmonic frequencies. Given a transmitting antenna design, the structure and amplitude of the side lobes will vary with amplitude and phase errors on the transmitting aperture and the antenna side lobes are determined by the transmitting aperture distribution. In the case of a large antenna, additional contributing errors may be: flexing of the support structure, amplitude deviations in the outputs, and the possible failure of some of the microwave power generators. (4) EARTH BASED CONSIDERATIONS There is an acute lack of information on the RFI characteristics of cw high power amplifiers. Much testing is needed to measure the spurious output and harmonic output of several 5-10 kW cw power amplitrons or 50 kW klystrons. Measurements on several tubes are needed to determine a typical output spectrum since this spectrum will vary from tube to tube depending on the tolerances of the physical dimensions, the tube's operating voltages, and the vacuum in the tube. This testing, which could be performed in laboratories on earth, would determine if filtering is needed for the amplitron or klystron and what suppression engineering approaches should be examined. High power filter techniques need to be explored and tests need to be performed to determine the feasibility of implementing the techniques in hardware. (5) CONCLUSIONS The Solar Power Satellite as presently envisaged presents a possible radio frequency interference situation to other conventional service users of the electromagnetic spectrum. This will result from energy being radiated through the side lobes of the antenna at the operating frequency, as well as emission of spurious and harmonic frequencies. It is recommended that this problem continue to be studied. Extensive testing of DC to microwave power converters, high power filters and phased array antennas is necessary to fully evaluate methods of controlling interference. Finally, it would be desirable to perform interference tests on a pilot model of an SPS in low earth orbit to determine the total system effectiveness of interference control techniques applied to the elements comprising the SPS.
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