COMETS高度移動体衛星通信実験:移動体衛星通信用コニカルビーム同軸円筒スロットアンテナの開発:衛星追尾機能を必要としないアンテナの検討

Abstract

In mobile communications, a mobile terminal can be made compact, lightweight and inexpensive by using an omnidirectional antenna, because a basestation tracking becomes unnecessary. However, the beam width at an elevation angle must be wide enough to accommodate the roll and pitch of a mobile station. A slot-array antenna on a coaxial cylinder is omnidirectional in the horizontal plane. In addition, its beam in the vertical plane can be shaped. Using this antenna, was developed a conical beam with a tilt angle of 45 degrees and a beam width of 20 deg, as an experimental antenna for the Communications and broadcasting Engineering Test Satellite (COMETS). The slot-array antenna on a coaxial cylinder is a traveling wave antenna. Whole fed energy is not radiated from the slots and is terminated at the end of waveguide, because the rate of the radiated power and traveling wave power must be lower than the upper limit so as not to disturb the traveling wave if the ideal beam shaping is desired. However, this terminated power causes a decrease in the antenna's efficiency and in the antenna's absolute gain. In satellite communications, a decrease in the efficiency can also cause an increase in the antenna temperature, because the main beam of the antenna used as a terminal is directed to the sky, whose temperature is lower than that of the antenna itself. In order to increase the G/T of a traveling wave antenna for satellite communications, it might be better to improve the efficiency of the antenna even if the beam shape is distorted a little.