The two sidebands are”demodulated in the RF receivers in the subarrays and the carrier is reconstructed. The ionosphere constrains the frequency separation Af between the upper and lower sidebands and the downlink carrier to be greater than the maximum plasma resonance frequency (about 10 MHz). This limitation is to prevent intermodulation products between the uplink pilot beam and the downlink power beam from creating parametric instabilities associated, with overdense ionospheric heating. RFI COMS IDERAT IONS The radio-frequency interference comes primarily from the DC-RF power conterter tubes. This interference can be divided into three main categories: (1) interference from the high power downlink beam due to sidelobe and grating lobe radiations, (2) spurious noise generated near the carrier frequency by the tubes, and (3) harmonic generation within the tubes. The sidelobe and grating lobe levels were previously examined, with the results given in figures 1 to 4. The phase noise characteristics for a Varian V-58 klystron as a function of frequency is shown in figure 10. The SPS phase control system will probably have a phase lock loop (PLL) around each power tube to provide frequency and phase stability and to reduce the output noise. A representative loop would have a 5 MHz bandwidth, with a 2nd or 3rd order filter. The phase noise spectral density out of the tube would be reduced as shown. This PLL filtering will reduce the noise close to the carrier frequency, but would not affect the noise characteristics outside the 2450 + 50 MHz band. The SPS configuration has a 10 dB Gaussian taper for the aperture illumination which means that the power radiated at the center of the antenna is 10 times that radiated at the edge. However, it can be frequency from carrier (Hz) Figure 10. Klystron Noise Characteristics shown that, for noise power calculations, uniform Illumination 1s a good approximation. The average antenna area illuminated by a single klystron within the 1 Km array is it (500 )2/90,000 klystrons or 8.7 m^ per klystron. For the conditon of no mutual coupling between klystrons, the effective antenna gain is given by o where A = antenna area (8.7 m ), A = wavelength (.1225m), N = coherency factor (.5) Since there will be 90,090 klystrons in the antenna for the 5 GW system, the total power density generated within the antenna will be PN = (# of klystrcns) (signal power/klystron) (Noise Spectral Density) = (90,000) (72.009) (-160 dB/Hz) = -61.9 dBW/Hz (3) The noise spectral density at the ground is Pn_g = PN Gn/4^R2 -187.4 dBW/m2/Hz. r The CCIR (Internat'onal Radio Consultative Committee) requirements for power flux density at the earth's surface is -180 dBW/m2/Hz for S-band frequencies with an angle of arrival above 25°. The klystron tubes will have a multiple cavity design which provides additional filtering to reduce out-of-band noise. Using a 24 dB/octave attenuation characteristic for multiple cavities, the SPS noise characteristics are shown in figure 11. Summarizing, the RFI effects due to spurious noise will be below the CCIR requirements, provided the klystrons tubes are phase-locked for noise reduction and a multiple cavity design is used. References 1. 6. 0. Arndt .nd L. Leopold. “Microwave Transmission Characteristics of Solar Power Satellites," IEEE-MTT-S International Microwave Synposium, Ottawa, Canada, June 1978. t. Solar Power Satellite - System Definition Study. Vol. IV, Part 2, KASA Contract KAS 9-15196. Boeing Aerospace Co., Seattle, Wash., Dec. 1977. 1. “Achievable Flatness In a Large Microwave Power Antenna," NAS 9-15423, Kid-Term Progress Report, General Dynamics, March 1978. 4. W. C. Brown, "Electronic and Mechanical Improvement of the Receiving Terminal of a Free-Space Microwave Power Transmission System." NASA Report CR-135194, Aug. 1, 1977. 5. R. M. Dickinson, "Satellite Power System Microwave Subsystem Impacts and Benefits," Jet Propulsion Laboratory, Pasadena, Calif., Sept. 1977. 6. W. C. Brown "Knife Edge Diffraction In the Context of a Serrated SPS Rectenna." To be Published 7. "Preliminary Study of Possible Weather Effects due to Solar Power Satellite Rectenna Operation" JSC Internal Note 12519, Unpublished 8. L. M. Duncan and W. E. Gordon, "Final Report - lonosphere/Microwave Beam Interaction Study,” NAS 9-15212. Rice University. Houston, Texas, Sept. 1977. 9. F. W. Perkins and R. G. Robie, "Ionospheric Heating by Radiowaves: Predictions for Arecibo and the Satellite Power Station," J. Geophysical Research. (To be published) 10. I. H. Koiway, A. H. Katy, G. Melti, "Ionospheric Effects of a High Power Space-Borne Microwave Beam," Raytheon Technical Memorandum T-1028, Waltham, M. A., Nov. 1977. 11. "lonosphere/Microwave Beam Interaction Study," KAS 9-15212, Dec. 1977, Monthly Progress Report, Rice University, Dec. 1977. 12. V. t. Gordon, Rice University, private comunication, March 1978.
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