MODELING HELIX IN THE PRESENCE OF STEP REFLECTIONS

摘要

Helix is a widely used slow wave structure due to its natural bandwidth, 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 populatly used method, reflections are measured at the extreme ends of the helix. But helix is sometimes made in many steps by varying the pitch values to minimize reflections. In the present work a new model of helix has been taken using the impedance transformation approach considering that impedance values of helix changes at different steps due to variation in pitch. A software program 'SXNREF' has been developed using this theory to calculate the reflections and return loss etc. in SWS using different steps of pitches in between the input and output Using this approach the designs of the input, output and transition bodies of a C-band 60W TWT have been studied. The results obtained are much helpful in the designing of 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.