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

Helix loss increases the noise figure of a TWT by altering the amplifying properties of the tube, and also by acting as a source of thermal noise which is amplified by interaction with the beam. Noise power originating in the helix propagates as a wave and undergoes amplification in the same manner as the signal. This noise power adds to the noise figure of the tube by an amount proportional to the attenuation of the helix and to its temperature, and inversely proportional to the tube gain per unit length. The effect of refrigerating the helix has been demonstrated by several experiments in which low-noise TWT's were immersed in liquid nitrogen while monitoring the noise figure. The reduction in noise figure varied from a few tenths to nearly one db. One case of particular interest is that of a WJ-226 low-noise TWT, operating in the frequency band 1.4 to 2.3 Gc, which showed a minimum noise figure of 2.6 db when operating in a room temperature environment. On cooling the tube to liquid nitrogen temperature, a minimum noise figure of 1.7 db was obtained, at a frequency of 1.65 Gc.