industrial microwave band of 2.40 to 2.50 GHz. The output power level of the Amplitron at 2.45 GHz exhibits a near optimum value when the output is about 5 kW.6 Microwave Beam Transmission Space is an ideal medium for the transmission of microwaves; an efficiency of 99.6% is projected to be achieved after the beam has been launched at the transmitting antenna and before it passes through the upper atmosphere. Over the transmission distance of 23,500 miles, the curvature of the phase front of the beam will be very small; nevertheless, the front must be controlled with high precision to achieve high efficiency. To achieve the desired high efficiency for the transmission system, the geometric relationships between the transmitting and receiving antenna indicate that the transmitting antenna should be about 0.8 km in diameter, while the receiving antenna should be about 10 km in diameter (depending on latitude). The large size of the transmitting antenna is required to achieve a reasonable power density within the microwave beam at the receiving antenna for efficient conversion of microwaves into DC. To reduce the dimensions of the transmitting antenna, the microwave amplitudes can be tapered from the center to the edge over the range of 5 to 10 db. The advantage of transmitting-antenna amplitude taper, as opposed to uniform illumination, is that it reduces the intensity at the center of the beam to less than 50 MW/cm2 . To achieve the desired control of the phase front in the transmitting antenna, 18-x 18-meter subarrays are arranged into sectors to provide the required center-to-edge amplitude taper. Using a large number of small subarrays reduces the effect of attitude-control inaccuracies. Phase control electronics are provided for each subarray to compensate for subarray distortions which may be induced by thermal effects. The phased array wave guide approach is used for the subarrays to achieve very high efficiency, low-frequency cut-off, and reduced RFI effect. Figure 7 shows the microwave generators and space radiators installed in a typical subarray. A closed-loop phase-front control is used to achieve the desired high efficiency and safety essential for the microwave beam operation. A command and adaptive phase-front control concept is utilized. The reference beam launched from the center of the receiving antenna is sensed at each subarray and at the reference subarray in the transmitting antenna center. The central subarray transmits the reference signals to the subarrays over calibrated coaxial cables. The difference in phase between these signals which, for example, may result from the displacement of a subarray from the nominal reference plane because of thermal distortions of the structure, corrects the phase of the transmitted beam at the
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