To achieve maximum uniformity of illumination of the antenna aperture, a resonant slotted waveguide array is used. The basic antenna element is illustrated in figure IV-12. The components are an input feed guide that distributes power from the tube to the radiating waveguides, a back face that contains slots for coupling power from the feed guide to the radiating guides, vertical walls that separate the radiating guides, a front face that contains the radiating slots, and end walls. Aluminum is used because of its conductivity, density, and cost. Close tolerances (about 0.001 in.) are required in slot dimensions and location and should be a factor in planning space versus ground construction of this element. Waveguide technology is well established. However, manufacturing and on-orbit assembly techniques will require development if the necessary production rates and efficiencies are to be achieved. 4. Phase Control The coherence and direction of the transmitted beam are maintained by reference to a pilot beam transmitted from the rectenna to a reference receiver in the center of the antenna and to each subarray. Loss of phase control immediately diffuses the transmitted beam (see fig. IV-4), so that the concentrated beam cannot wander away from the rectenna. Two methods of comparing the received phasing signals appear promising. The first method (the "transmission line" approach) employs a separate transmission line from the central reference receiver to each small group of subarrays. This requires that transmission delay times be calibrated and maintained, requiring additional control circuitry. The second ("sequential") method transmits the reference phase from one subarray to the next; this can result in a large buildup of phase error. The preferred approach appears to be a combination of these, in which several subarrays (perhaps 8 to 16) are connected in series as a group, and each group receives the reference phase via a separate transmission line. The rf cable distribution has been used for weight estimating purposes, but a fiber optics system is potentially lighter. Areas of concern include phase control system requirements for each tube, phase stability of components outside the phase control loop, and transmitter rf interference with the phase reference system. 5. Pointing Control In an eccentric orbit, the antenna must vary from constant velocity in order to track the rectenna; this requires a cyclic torque with a period of 1 day to be applied to the antenna. A more or less
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