The joint is rotated continuously by a drive motor in the center. Tilt is accomplished by six actuators at the top. The outer ball is suspended within its cylindrical cage by many small copper wires that resist translation but not small rotations, and also conduct electrical energy from the ball to the antenna. The suspension struts sense rotation of the ball and a computer converts these signals into joint drive commands to maintain the cage/ball relative orientation. Thus, the drive system overcomes friction but makes no other inputs. A substantial design and development program is required. Heat rejection from the interior of the joint will be a major problem. Brush design and materials for a 30-year life will also require research. 9. Thermal Control Passive thermal control of the transmitting antenna appears feasible, but a closely integrated design of the microwave generators, waveguides, electronics, and structure will be required. Three significant design considerations are waste heat rejection, thermal distortion from the daily solar orientation cycle, and thermal strain from eelipses. The rotary joint presents the other major heat rejection problem. It has not been analyzed in depth, but appears to be a potential application for heat pipes. D. Microwave Reception and Conversion System The MRCS includes a large rectifying antenna, or rectenna, (fig. IV-16) to convert the microwave energy to de electrical energy, a distribution system to collect the energy, and dc-ac inverters to convert the energy to a form compatible with the commercial power grid. 1. Rectenna The rectifying antenna is an integrated system for collecting microwave power and rectifying this power into direct current. The rectenna consists of about 15 billion elements. Each rectenna element consists essentially of a half-wave dipole antenna and a half-wave rectifier (Schottky barrier diode). These elements are mounted on a wire mesh ground plane and connected in series to produce the required de output voltage. The elements would cover an elliptical area typically 10 by 14 km (at 40° latitude) to produce 5-GW de output power. The efficiency of the rectenna falls off rapidly at low power densities. This, together with the power density distribution in the microwave beam, may place an upper limit on practical rectenna size. A major development item is an element that will operate efficiently at low power densities and is producible at low cost.
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