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This paper will review research-level phased array activities and developed phased array systems in Japan. Two basic technologies for the beam forming networks will be discussed, one is the microwave processing technology using MMICs and the other optical processing one. The several optically controlled phased array antennas utilizing optical components will be described. As the commercial available systems, the airborne phased array antenna for INMARSAT-M, and the beam-switching train antenna on the Shinkansen are described. Several systems which have been investigated and not used for the commercial systems will be also discussed. These are the S-band beam fomring network for Inter Satellite Link (ISL), the Secondary Surveillance radar antenna, the multibeam forming network for future mobile satellite systems, and the mechanically steered planar antenna installed on the Shinkasen for DBS. INTRODUCTIONThere have been increasing demands for phased array antenna systems which can improve system performance as well as expand its functions. A lot of architectures using monolithic microwave integrated circuits or fiber optics have been investigated for use of radar and communication systems due to the attractiveness of beam steering without mechanical elements, multibeam and beam shaping capability, and spatial power combining with MMIC or MIC amplifiers. The technologies of MMICs and fiber optic signal distribution to the array elements will substantially increase the feasibility and cost effectiveness of phased array antennas. In this paper, the above two technologies which are investigated for satellite mobile communication systems in Japan will be reviewed. The size-reduced lumped MMICs and large scale MMICs which utilize uniform performance characteristics and higher yields of MMICs, and can eliminate bondwire connections between functional circuits are introduced for multibeam foming networks. Photonic beam forming networks using optical waveguide and spatial devices are also described. The advantages of photonic technologies are broad bandwidth, light weight, size reduction, and immunity to electromagnetic interference. These will make it possible to realize onboard photonic microwave processing networks.In addition to the research-level activities, this paper discusses the experimental antenna systems whose performance has been confirmed in the field or will be done in the near future, and the commercially available phased array antenna systems developed by Japanese companies. The S-band phased array antenna for inter satellite link, the secondary surveillance radar antenna, and the mechanically steered

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