Wide-band semivane and heavily dielectric loaded helix traveling-wave tubes

Theoretical simulation of the dispersion and interaction impedance characteristics of a semivane loaded negative dispersion helical slow-wave structure, which is a slight variant of a conventional vane-loaded helix of a wide-band traveling-wave tube (TWT), was validated for the nonresonant perturbation measurement. While the experimental and theoretical results agreed well with respect to interaction impedance for both the semivane structure and the structure without vanes, there was disagreement found with respect to the dispersion characteristics of the semivane structure, which was explained quantitatively by the deformation that had taken place in assembling the structure. The output performance of a TWT using the semivane structure fairly agreed with the prediction by an in-house one-dimensional nonlinear Lagrangian code, with respect to saturated output power (∼50 W), saturated gain (∼40 dB), and lower-band-edge second harmonic level (∼-5 dBc), in the operating frequency range of 6-18 GHz, except at the low frequency end of the operating band, where the saturated power was less and the second harmonic level higher than predicted, a finding that may be attributed to the departure of the deformed structure from exhibiting a negative dispersion at the lower band edge.