Studies have shown that a tapered gaussian power distribution of transmitted energy is most useful and efficient. A value of 9 dB has been selected in this case as the near optimum power taper level. Because of the physical limitations of discrete components and in order to standardize on some fundamental building blocks in constructing the transmit array, this 9 dB power taper is accomplished in eight steps from the center to the edge of the antenna. This approximates an ideal gaussian distribution (Fig. 7-39). The array is built up of 1652 rectangular subarrays which in turn are constructed from the 1 x 2 m basic waveguide dc-RF converter assembly unit. Each subarray contains 230 of these basic elements. Another important item in the antenna system is the structure. It relates directly to the phase control of the microwave beam and to the pointing accuracy of the overall system. A very rigid structure will be required to support the 1 arc min pointing requirement and the thermal distortion limits of the subarrays. The structure requirements are such that performance should be given priority over cost. The receiving portion of the microwave system is referred to as a rectenna because of the use of many small dipole elements. Each dipole element receives RF power and, through a highly efficient arrangement of special diodes, filters, and distributing bus system, converts the power to useful de energy with high efficiency. The baseline system characteristics are contained in Table 7-16. A further analysis of each element of the microwave power system is included in the following discussion. 7. 3. 2 TRANSMITTING ANTENNA STRUCTURE The selected structural configuration for the micro wave transmitting antenna is built from a single truss column structural element which, in turn, is assembled with other elements to form equilateral pyramids. These then are joined with other like pyramids to form a TRI/HEX structural module. The assembly sequence is portrayed in Figure 7-40, which also includes assembled waveguide/amplitron subarrays. Each of the waveguide/amplitron subarrays are mounted on the main structure by adjustable (possibly screwjack type) mounts that permit in-plane alignment of all subarrays. Structural load determination, element sizing, and material selection are still in progress with aluminum and graphite epoxy the two most promising materials. The desired characteristics and requirements of the structural system are:
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