Study the effect of positive dispersion in input circuit of broadband helix Traveling Wave tubes

An analysis of the effect of positive dispersion in input circuit of broadband helix Traveling-Wave tubes (TWTs) is put forward. Using the positive dispersion at the start of the input circuit, the frequencies at high band is synchronized while the low band is not. Thus, the interaction length of high band frequencies is extend, while the low band wave is delayed to be interaction, and the parasitic harmonic wave is also to be delayed. Then the effect can be improved at high band frequencies. A 6~18GHz TWT is designed and simulated with a positive dispersion at the start of the input circuit (slow wave structure without vane) and followed with the negative dispersion (slow wave structure with deep vane), the efficiency in band is shown slightly improved.