Microwave tube requirements for communications satellite systems

During the past decade requirements for commercial communications satellite system components have grown significantly. Initially these requirements were for 6 GHz uplink and 4 GHz downlink. More recently the requirements have been extended to 14 GHz uplink and 12 GHz downlink. These requirements, coupled with the changes in modulation techniques, are reflected in increasing demands on microwave tubes. The purpose of this paper is to provide an estimate of the present and anticipated technological needs for microwave tubes both in commercial satellites and earth stations. Performance in satellites invariably includes premiums for combined high efficiency, low distortion, long lived devices. Typical operating examples from current systems will be discussed illustrating FM and digital signal processing characteristics. Life data will also be reviewed, as well as some of the techniques being applied to select long lived tubes. Performance requirements in earth stations are in some respects similar to those of satellites in that again good life, linearity, and efficiency are premium qualities. However, earth station RF power requirements are typically several order of magnitudes higher than in satellites and the ability to minimize maintenance/application requirements modify the overall priorities. Typical sample applications will also be presented. Longer range considerations in terms of ultimate lifetime factors, as well as RF performance, are vital to the viability of future microwave tubes. For example can these now mature and highly sophisticated devices continue to be developed where essentially unattended operation (i.e. space as well as ground) is sought with virtually perfect performance for periods in excess of 7 or 10 years?