Simulation design of 2-18GHz ultra-broadband helix traveling-wave tubes

Broadband traveling-wave tubes (TWTs) commonly have a helix pitch profile which incorporates a section with increased phase velocity followed by a negative phase velocity. To design 2–18 GHz ultra-broadband (UBB) TWT, the second harmonics are saturated output power, based on 3-D large signal software MTSS, are simulated using normal (unloaded T-metal) and abnormal (loaded T-metal) dispersion helix circuits, respectively. It is shown that structures may reduce the generation of the second harmonic while maintaining the high frequency saturated output power. Adding a positive phase velocity taper may further reduce the second harmonics of most frequency points and improve the saturated output power of high frequency band.