Fig. 5. Thermal/silicon cell satellites A thermal Brayton system and the silicon cell system are illustrated in Fig. 5. The photovoltaic system offers the advantages of space construction simplicity and potentially higher reliability due to its lack of “moving parts.” The economic viability of the photovoltaic system, however, will depend on the ability to develop low- cost, high-volume manufacturing techniques. Power transmission The energy collected and converted to electricity aboard the satellite requires a high-efficiency system to transfer the power to Earth. The 2.45-GHz microwave power transmission link described in current studies (10) is depicted in Fig. 6. The system has direct-current-to-radiofrequency (dc-to-RF) power amplifiers feeding a 1-km-diameter (0.6 statute mile diameter) phased-array transmitting antenna. The antenna is designed to provide a tapered illumination across the array surface. It is composed of 7000 subarrays, each about 10 m (33 ft) on a side, having slotted waveguides as the radiating surface. The dc-to-RF power amplifiers are mounted on the back of the antenna. Each transmitting-antenna subarray has its own RF receiver and phasing electronics to process a pilot-beam phasing signal emanating from the Earth-based (and controlled) receiver station. The subarrays are phased together in response to the pilot-beam signal to provide a single coherent beam focused at the center of the ground rectifying antenna (rectenna) system. The rectenna has an active panel area of about 75 km2 (29 square statute miles) consisting of a series of panels constructed perpendicular to the incident beam. Each section is composed of a structural support system, a wire mesh screen ground plane which is opaque to micro wave energy but has 80% optical transparency, and many half-wave dipole antenna elements which collect the microwave energy and feed it to Schottky barrier diodes for conversion to de power. The dc-to-RF efficiency of this microwave power link has been the subject of
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