Figure 19 Command and Adaptive Phase Front Control Concepts similar retrodirective technique is used in other applications [IEEE Trans., 1964 j, although not with the primary purpose of compensating for structural deflections, which requires that a reference signal be distributed to each subarray. A number of possible approaches to subarray designs are shown in Figure 20. The slotted waveguide approach is selected because it has very high antenna efficiency while also serving as an efficient means to distribute the microwave power from the converters to the radiating elements. The spacefed array shown in Figure 21 represents a radically different approach to the overall antenna mechanization that was devised to simplify converter repair or replacement by centrally locating them. However mechanical complexity, lower efficiency and need for active heat transfer to a radiator (not detailed) are important disadvantages. A second alternative also shown is a cylindrical array using electrical switching to eliminate the rotary joint to a solar oriented power source. It is too heavy, costly and complex to compete with the recommended planar approach. Having selected the waveguide approach to antenna design, we proceeded to select material wall thickness and subarray dimensions. The thermal interface between converter and waveguide, shown in Figure 22 for the amplitron, was analyzed to obtain the deflection data shown in Figure 23. This is for a wall thickness of 0.5 mm which is believed to be the minimum produced to date. The aluminum deflection over 5 meters is sufficient to produce a 1% beam power loss at the subarray, while graphite composites can extend out to about 18 meters for a 1% loss. The graphite polyimide is a potentially attractive candidate, being 0.6 the density of aluminum and having a higher maximum temperature (290°C) than either aluminum (175cC) or graphite exoxy
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