Low-noise travelling-wave tubes

Following the development of the earliest low-noise travelling-wave tubes, some 20 years ago, there was a period of great activity in this field. Until about five years ago, a great deal of work was published both on the theoretical aspects of low-noise behaviour, and on the attainment of low noise factors under laboratory conditions. Since then, the major effort has been directed towards translating these laboratory models into reliable commercial devices, and, in the paper, a review is made of current work on low-noise travelling-wave tubes. After discussing, in some detail, the design requirments of a minimum-noise-factor t.w.t., the paper shows the interrelation between the noise factor and the other prime requirements of a t.w.t.,¿gain, power and bandwidth¿with particular emphasis on the practical considerations involved. The implications of the choice of permanent-magnet focus mount and r.f. coupling system on the optimisation of the size, weight and performance, are considered, together with the compromises that are available. The final sections of the paper are devoted to cosideration of some of the other characteristics of low-noise t.w.ts. which are of interest to a systems designer. These include the dynamic range, gain and phase tracking, environmental performance and life behaviour.