The effect of helix loss on noise figure in traveling wave tubes

Internal noise in a traveling-wave tube drives from two principal sources, the electron beam and the helix. Noise arising from loss in the helix can influence the noise figure of the tube in two ways. First, thermal noise in the helix propagates as a wave, that component which is synchronous with the beam being amplifier in the same way as the signal. Secondly, the amplifying properties of the tube are influenced by loss and hence affect noise figure. The latter effect can be estimated by considering the circuit loss term in the minimum noise figure expression of Haus and Robinson. An analysis of the effect of amplified thermal noise is presented and is shown to result in the addition of another term, involving helix temperature, cold attenuation per unit length, and tube gain per unit length, to the minimum noise figure expression. The results of several experiments in which TWT´s were immersed in liquid nitrogen while monitoring noise figure are described. Noise figure decreases of between 0.5 and 0.8 db were measured upon cooling. The experimental results are in reasonably close agreement with the theory. The possibility of practical application of helix refrigeration techniques is considered briefly.