TWT delay lines—Part I: Signal propagation on a multivelocity electron beam

An electronically variable microwave delay line uses an extended low-velocity drift region between the input and output helices in a TWT configuration. In this paper, theoretical limits on the maximum delay obtainable from such delay lines are calculated. For a given velocity dispersion, the insertion loss is predicted to rise sharply to prohibitively high values at a critical electron-drift velocity. This sets a practical upper limit to the delay obtainable from such devices; detailed calculations of this maximum delay as a function of the beam parameters (size, current, shape, velocity distribution) are presented. Important factors in the generation of velocity dispersion in the beam and experimental results are discussed in a companion paper [7].