Modeling helix in the presence of step reflections

Due to its natural bandwidth, the helix is a slow wave structure (SWS) widely used in the travelling wave tube. Reflections may become a severe problem in the travelling wave amplifier, giving rise to feedback and hence oscillations. In a popularly used method, reflections are measured at the extreme ends of the helix. The helix is sometimes made in many steps, by varying the pitch values, to minimize reflections. A new helix model is taken using the impedance transformation approach considering that impedance values of the helix changes at different steps due to the variation in pitch. A software program, ´SUNREF´, has been developed using this theory to calculate the reflections, return loss etc. in the SWS using different steps of pitches between the input and output. Using this approach, designs of the input, output and transition bodies of a C-band 60 W TWT have been studied. The results obtained are very helpful in designing the SWS by predicting the number of steps necessary to achieve the minimum or specified reflections and maximum return loss. The program has been validated using the λ/2 and λ/4 transformer theory.