RFI AVOIDANCE TECHNIQUES Optimization of Microwave Transmission System Background. — The feasibility assessment of the microwave generation, transmission, and rectification for the SSPS was carried out in support of the baseline configuration definition. The substantial output of microwave energy requires that the radio frequency interference (RFI) effects of an SSPS also be assessed. Discussions with personnel of the Office of Telecommunications Policy have set the guidelines for the RFI assessment; however, this investigation was limited in scope to cover only what is currently believed to be the most likely part of the microwave band of frequencies. Should the SSPS be found to have the capacity to supply a significant portion of future power needs, it would enjoy a priority in frequency allocation such that the required bandwidth at the near optimum frequency could be made available and alternative approaches identified to compensate the displaced users. It is possible to select a near optimum range for the fundamental frequency from 2 to 4 GHz. Above this range, confidence in the design of devices, such as the Amplitron and associated filtering, decreases and the attenuation of the microwave beam by water-laden cloud increases, particularly for the low-probability instantaneous meteorological events. From the SSPS point of view alone, it would be preferable on the one hand to tend toward the high end to reduce antenna size requirements and, on the other hand, to tend toward the low end where wavelengths are longer and precision in manufacturing of such elements as. filters are not as critical. However, at the lower frequencies there is high impact on existing users in many countries. The conduct of definitive trade-off studies to indicate a near optimum frequency selection requires detailed design of the devices, including filters, and estimates of associated costs. Such detailed design investigations will be required to assure that the design goals for noise and harmonic filtering can be met. Since a near-optimum frequency for the SSPS from a design point of view could be identified anywhere in the 2- to 4-GHz band, the selection of a fundamental frequency within this band should be made from the point of view of minimizing interference with other users. To carry out the RFI task a model and a set of assumptions for the microwave transmission system (37) were defined. The model included orbital and ground location, ground power transmission, device characteristics, phase-front control, efficiencies, induced RF environment, ionospheric and atmospheric attenuation, major frequency segment, specific frequency, typical users, and selected equipment. Based on a set of assumptions for filter design and recognizing existing allocated radio astronomy and fixed satellite space-to-Earth bands, a frequency of 3.3 GHz was selected as the fundamental frequency for the main power beam of the SSPS and further RFI investigations were conducted on this basis.
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