Microwave integrated circuits are currently available which can operate in the gigahertz band proposed for SPS power transmission, and have been demonstrated by many different laboratories for operation at the tens and hundreds of gigahertz [14]. In a possible near-term design, the microwave integrated circuits are fabricated separately and then bonded to either single crystal or thin film solar cells. Using single-crystal solar cells, a more advanced design with microwave ICs fabricated directly on the cell could be a logical next step. The future extension of the design would construct the entire integrated unit using thin film technology. Thin-film microwave electronics are a reasonable extrapolation of the union of two recent developments: solid-state microwave electronics, and thin-film transistors. Thin film transistors have been developed for other applications such as display screens, where many millions of devices have been integrated on a large-area sheet. Current technoloxy only allows frequencies in the range of kilohertz to at most megahertz [15], but this will likely increase with further research. Development of gigahertz-speed thin-film transistors would allow the transmitter elements to be deposited on the thin substrate at the same time as, and possibly using the same materials as, the thin-film solar cells. Conceptual structural designs It is important to design new, low mass structures in order to reduce the structural mass of the system proportionately to the photovoltaic and transmitter mass reductions. Many structural designs for such a system are possible. Two, the “bicycle wheel” configuration (Figure 4) and the “sphere” configuration (Figure 5), are shown. Using a phased array means the microwave antenna does not have to face directly toward the receiving station, as long as the antenna is not edge-on to the receiver.
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