least as large as the cell blanket. A beam solid angle of 4.3 x 10 Rad or less implies a directivity greater than 84 dB and, at these frequencies, an effective aperture greater than 3.4 x 10 m . The Arecibo dish has an aperture only twice this large, and all other single dishes are smaller. Interferometers which can just resolve the 0.8 km transmitting antenna will see it become brighter than the blanket over a band extending approximately 140 MHz on each side of the center frequency. Further increases in resolution will eventually reveal the transmitting array center to be 2.5 times brighter because of illumination taper. The total integrated noise power implied by the spectrum of Figure 26 is approximately 5 x 10 W/m . The calculation reported as Figure 26 (G. D. Arndt and L. Leopold, "Environmental Considerations for the Microwave Beam from a Solar Power Satellite," 13th Intersociety Energy Conversion Engineering Conference, San Diego, CA, August 1978) implies a noise beam width approximately 100 times larger than the power beam width. Frequency scanning effects are neglected, as is any noise contribution to the klystron drive source. To date, no one has constructed a prototype SPS klystron. All noise data are extrapolated from performance of existing tubes both with respect to tube construction and with respect to frequency dependence in the wings of the noise spectrum. It is difficult for us to assess the reliability of these extrapolations. Although not currently part of the Reference System, both amplitron tubes and solid-state devices are being seriously considered in some quarters as alternatives to klystrons. Of the two, the amplitrons are inherently much noisier devices with much broader emissions, and are most convenient to apply in cascades which are significantly noisier than individual tubes. A variety of solid-state devices is being considered, and the present rapid rate of device development may well make available entirely new devices in the decade or more which will elapse before the transmitter design must be frozen. As a result, the formulation of effects statements suggested in the introduction, either as parametric in system properties or as effect thresholds, is especially important for noise effects.
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