Analysis of Power Holes in Helix Traveling-Wave Tubes with Non-Uniform Delay-Lines

The influence of non-uniform delay lines on the parasitic effect of power holes in helix traveling-wave tubes is investigated. For the analysis, we introduce a helix geometry which favors power hole occurrence. The harmonic backward-wave is assumed to be excited through output-coupler mismatch. Whereas homogeneous delay lines can only cause a single power hole, the analysis shows that tapered helices may lead to multiple gain dips at different frequencies where each power hole can be assigned to a section of the delay line.