Efficiency enhancement and harmonic reduction of wideband helix traveling-wave tubes with positive phase velocity tapering

Using numerical simulations, a wideband (>1.5 octave) vane loaded helix traveling-wave tube (TWT) was studied, the output section of which is provided with a 'positively' phase velocity tapered helix portion, in which the helix pitch is made to linearly increase with distance towards the output end for reasonably high efficiency values, As compared to a device with a conventional 'negatively' phase velocity tapered helix portion, which gives an efficiency hike only in a narrow band of frequency, the positively tapered device provided a wideband efficiency improvement. Also, over the band, it provided a higher efficiency and a lower second harmonic content than a device with a nontapered portion. Factors such as the beginning position of the taper front the input and the amount of taper were considered in the optimum design for improving efficiency and reducing the second harmonic content of the device.