TABLE 3 DC-RF CONVERTER FEATURES Space is an ideal medium for the transmission of microwaves; a transmission efficiency of 99.6% is projected after the beam has been launched at the transmitting antenna and before it passes through the upper atmosphere. To achieve the desired high efficiency for the transmission system while minimizing the cost, the geometric relationships between the transmitting and receiving antenna (14) indicate that the transmitting antenna should be about 1 km in diameter, while the receiving antenna should be about 10 km in diameter. The power density at the receiving antenna will be a maximum at the middle and will decrease with distance from the center of the receiver. The exact size of the receiving antenna will be determined by the radius at which the collection and rectification of the power becomes marginally economical. The transmitting antenna is divided into a large number of subarrays. A closed- loop, retrodirective-array phase-front control is used with these subarrays to achieve the desired high efficiency, pointing accuracy, and safety essential for the microwave beam operation (15). In the retrodirective-array design, a reference beam is launched from the center of the receiving antenna and is received at a phase comparator at the center of each subarray and also at the reference subarray in the transmitting antenna center. 4.1.2. Microwave power reception and rectification. The receiving antenna is designed to intercept, collect, and rectify the microwave beam into a de output with high efficiency (12, 13). The de output can be designed to either interface with high-voltage de transmission networks or be converted into 60-Hz alternating current. The receiving antenna consists of an array of elements which absorb and rectify the incident microwave beam. Each element consists of a half-wave dipole, an integral low-pass filter, diode rectifier and bypass capacitor. The dipoles are dc-insulated from the ground plane and appear as rf absorbers to the incoming microwaves. The collection efficiency of the array is relatively insensitive to substantial changes in the direction of the incoming beam. Furthermore, the efficiency is independent of potentially substantial spatial variations in phase and power density of the incoming beam that could be caused by non-uniform atmospheric conditions.
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