c. DEFINITION There is a great deal of electromagnetic wave propagation art and analysis required in arriving at an effective slot array design. The design definition advanced here is a combination of textbook approximations, antenna experience and "rule-of-thumb11 estimates. The objective was to obtain a configuration which would be adequate for weight estimation and the parameters determined in this study are suitable for this purpose. The basic antenna element is depicted in Figure IV-C-3-1. The element components are: "The Input-Feed Guide" which distributes power from the tube to the radiating guides; the Back Face which contains slots for coupling power from the Feed Guide to the radiating guides and form the back wall for these guides; Vertical Walls which separate the radiating guides; Front Face which form the front wall of the radiating guide and contains the radiating slots; and End Walls which close out the ends of each guide. The material chosen for all parts is aluminum which performs quite well electrically and which is both cheap and available as compared to the better electrical performers, copper and gold. The light weight and superior electrical characteristics of aluminum, lead to its choice over brass which is often used for waveguide applications on earth. The proposed waveguide width is a nominal 10 cm (9.8 inside) for 2.45 GHz which leads to a slot spacing of 7.25 cm center to center. Errors in the location and length of these slots are very critical and tolerances will probably need to be held to a 0.001 in. or so with a surface accuracy of 0.1 in. in order to achieve a 95% element efficiency (worst case). However, these tolerances should be considered in conjunction with the various other loss factors for the antenna, i.e. phase control, pointing errors, surface distortions, generator phase and amplitude control, etc. The reference configuration used for sizing and weight calculations may be a little conservative, however, caution should be taken not to remove weight from the antenna to the point where even more weight will be required in the total structure. Therefore, the antenna elements which lend themselves to structural support, feedguide and walls are sized at a thickness of 0.05 cm and the face sheets are carried at 0.025 cm. d. OPERATING CHARACTERISTICS The SPS antenna as conceived in this study is configured to operate as a large number of subarrays and elements (sub-subarray), each operating in parallel. The concept of parallel operation is important from the standpoint of controlling error buildup (correlation) and managing the attendant reduction in efficiency and sidelobe levels. All mechanical errors are translated into phase errors which are added to the electrical phase errors of the system. Power density is accomplished by adjusting the size of the element to obtain an appropriate area while holding each generator at a constant power output level. This lends itself to a single tube design and allows the tube to operate at its maximum efficiency. However, if a
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