Suppression of backward–wave oscillation by filter helix methods

An experimental study has been made of the filter helix properties of a periodically loaded helix, using a special traveling-wave tube. The filter helix is shown to possess filter-like frequency pass bands and stop bands and a phase velocity characteristic such that forward- and backward-wave phase velocities are separated, making the filter helix a useful circuit for traveling-wave amplification at high ka (ratio of helix circumference to free space wavelength) without danger of backward-wave oscillation. Certain difficulties associated with backward-wave oscillation frequency pushing were found to arise in filter helices, but useful gain was obtained at ka greater than 0.5 using filter helix techniques.