Filter Helix for Harmonic Suppression in Traveling Wave Tubes

Increasing the efficiency of traveling wave tubes (TWTs) is a basic development strategy. In this paper, second harmonic suppression is introduced as a possible mean. For this purpose, a filter consisting of periodic pitch discontinuities is implemented in the helix transmission line (TL) of a tube. A method is developed to obtain the reflection coefficients of adjustable pitch discontinuities. A filter design for a low frequency TWT is tested in a helical TL without electron beam and implemented in the tube. Both simulation and measurement confirm a stopband at the second harmonic and thus a significant reduction of this harmonic at the output. An additional filter is designed and tested at S-band. It is shown to improve both the output power and the linearity of the tube.