Traveling wave tubes for communication satellites

Traveling wave tubes (TWT´s) have contributed markedly to the development of communications satellites. As the prime-power consuming and transmitting device, the major transponder gain element, and the largest contributor to transmission nonlinearities, the TWT has been the focal point for continuous but carefully measured evolutionary improvements. Efficiency improvements continue to be made without compromising desired communications characteristics or tube lifetimes. These improvements have been made primarily in the RF circuit through loss reduction and phase-velocity tapering techniques, and in the spent-beam region through better multielement collector designs. Traveling wave tubes developed for satellites at 4 and 12 GHz are used as examples. Since TWT´s ate life-limited devices, emphasis has been placed on techniques ensuring long life in satellite applications. Both oxide- and dispenser-type cathodes are discussed and data on life characteristics are presented. During the past decade, while generally demonstrating excellent space lifetimes, operating TWT´s continue to approach their potential cathode wear-out life, which is theoretically of the order of 10⁵h.