Fig. 1. Imaginary photo of a solar power transmission satellite system

   The demand for power usage will increase at least 2.5 times in the next 50 years, according to a prediction. Based on the need for clean, non-polluting sources of energy, and a trend to abandon nuclear power generation, the solar power source appears to be an attractive alternative energy source. However, earth-based solar cell power generation would require an unrealistically large earth surface area (about 1/5 of the entire United States). Instead, the concept of solar power satellite (SPS) systems has been presented so as to solve this problem. One of the key technologies for the SPS system is the wireless power transmission system, which includes DC to RF converters in the transmitters, the retrodirective beam control system and the receiving rectenna system. In this work, the overall transmitting system has been analyzed in a system perspective.

   The techniques for generating and transmitting a high intensity microwave beam for solar power transmission can be divided into the centralized and the distributed approaches. The centralized approach relies on microwave tubes that are capable of generating kilo-Watts of power at microwave frequencies. However, microwave tubes are heavy, require extensive cooling system, are not as reliable as solid state devices, and therefore the system is prone to single point failure.

   A more practical approach for solar power transmission via microwaves is to adopt a distributed approach with high power solid-state devices. The distributed approach will provide a lightweight, fault-tolerant system. Moreover, it will not need active cooling systems and power distributed networks. In this approach, the dc power collected from solar cells would be converted into the form of RF power using a distributed oscillator array employing high power and efficient oscillators. Microstrip patch antenna arrays with low cost and profile would transmit the converted microwave power to the earth.

   This work addressed the theoretical and practical aspects of designing a very large, low profile, printed, self-oscillating injection locked active antenna array to generate a powerful coherent microwave beam. For the overall transmitting system, a beam scanning scheme to direct the microwave beam to a precise target area was presented. For the design of the coherent and stable coupled oscilator array, the extended resonance approach was considered in conjunction with the external injection locking technique.

         [1] J. Choi, A. Mortazawi, "Free-space power combining oscillator array for solar power transmission," Radio Science Bulletin, no. 311, pp.47-54, Dec. 2004.  [PDF]

         [2] J. Choi , A. Mortazawi and D. Pavlidis, “An approach for solar power conversion to high RF power for wireless transmission,” , Invited Talk, presented in Japan-United States Joint Workshop on Space Solar Power System (JUSPS'03) , Kyoto University, 2003 [PDF]
























      Fig. 2. Proposed schematic of the subharmonic injection locked large array containing numerous active antenna-oscillator subarrays