Calculations of wave propagation on a helix in the attenuation region

It is a well-known fact that the attenuation region of the traveling-wave tube is very important to the performance of the tube, particularly with respect to gain and efficiency of the tube which is relatively short in terms of wavelengths. Characteristics of the attenuator in the passive tube have been measured frequently and variations of attenuation and wave velocity with respect to frequency and thickness of coating are known. In the interests of trying to understand the effect of the attenuator on efficiency, we have attempted to make certain calculations of performance of a theoretical attenuator such as is frequently used on certain spirals and the results of such calculations are presented here.