Two alternative approaches as shown in Figure 6-17 were considered. One is a spacefed array in which large power converters, e. g. , 100 kW klystrons, would be centrally located to facilitate maintenance; the other is a cylindrical, active array using electrical switching to eliminate a rotary joint to a solar oriented power source. The spacefed array approach would use radiators near the converters to illuminate the larger aperture needed to launch the beam to earth as shown in Figure 6-18. The disadvantages are the need for mechanical alignment of the active elements, mechanical adjustment of the array for phase control (electrical means are too inefficient), need for an active cooling scheme to get the converter waste heat to a radiating surface, and finally, the very high microwave power density at the feed which increases in direct proportion to the reduction in area at the feed relative to the array and therefore would be on the order of X in dimension. For an F/D of about unity as shown in Figure 6-19, all elements must be in a dimension of about 3X. Intermediate feed concepts can be devised, but they approach the dimensions of the primary array and have lowered efficiency as they become physically realizable. Figure 6-17. Alternative Array Types
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