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Satellite broadcasting in the 21-GHz band is expected to be used for advanced broadcasting services, such as ultrahigh-definition television, which requires extremely high information transmission rates. However, RF signal attenuation due to rainfall is very large in this band. A promising approach to solving this problem is to use an onboard phased array antenna to compensate for the attenuation in rain areas. We have been studying on a miniature traveling wave tube (mini-TWT) for an onboard active phased array antenna. It is necessary that we arrange radiation elements densely so as to achieve high gain of service area, and to suppress sidelobes. We developed a mini-TWT for its amplifier; the length of cross-sectional directions corresponded to the 1.5 wavelengths. This paper describes a concept of the satellite broadcasting in the 21-GHz band and the performance of the mini-TWT in the partial model of a mini-TWT array.