Institute for Unmanned Space Experiment Free Flyer (USEF) has been studying wireless power transmission(WPT) from the beginning of 2000. In this study, we have focused on the microwave power transmission as a keytechnology for the realization of future Space Solar Power System (SSPS). In 2009, Japanese New Space basic planwas issued, that selected SSPS as one of the research and development program to be promoted. Though SSPS wasselected as an important project, Large Space System is not declared yet. USEF group is now conducting the groundWPT experiments project which includes kW level electrical power, under the contract of Ministry of Economy,Trade and Industry (METI). For the SSPS system, the lightest weight and highest performance transmission panelwith accurate beam steering is required. In this program, we are seeking for the development and research of higheffective and high performance technologies. We are developing thin phased array antenna by four subpanels. Morethan 60% efficiency high power amplifiers with high efficient GaN HEMT and class-F circuit are applied to producethe microwave power. The received microwave at array of rectenna, antenna with rectifier, is converted into theelectrical energy again with the distance of tens of meters. The rectenna will use existing Schottky Barrier Diode.Also, the diode for rectenna rectifier with target efficiency 80% has been under development in this program.Moreover, a software retrodirective technology is applied to the microwave beam steering control, and themicrowave is controlled in precisely and high accuracy to the direction of the pilot signal sent from the rectennapanel. Japan Aerospace Exploration Agency (JAXA) is responsible for the development of this precise beamdirection control in this program. We know it is necessary to apply a further technology to divert it for the spaceexperiment. However, this project is at an important position to proceed to the next development step aiming to theSSPS achievement. The experimental system's design and element development are carried out now. In this report,outline of the project is explained.
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