Nonresonant Perturbation measurements on dispersion and interaction impedance characteristics of helical slow-wave structures

Abstract

A nonresonant perturbation (NRP) theory is developed from first principles for the measurement of dispersion and interaction impedance characteristics of a helical slow-wave structure (SWS). The phase of the reflected signal from a test helical structure varies when a perturber also in the form of a helix is moved along the axis of the test structure. The variation of phase with perturber position is interpreted to find the phase velocity of the structure under test. The interaction impedance of the structure is found by measuring the change in the axial phase-propagation constant of the structure as a dielectric rod is placed along the axis of the structure. For this purpose first a "resonant" perturbation formula is derived for interaction impedance which with proper interpretation is then extended to get a "nonresonant" perturbation formula in terms of the above change in the axial phase-propagation constant. The formula shows an improvement over the first-order formula derived under "thin-rod approximations" in the form of a correction factor that takes into account the finite perturbation of the axial electric field inside the dielectric-rod perturber the presence of a radial electric field the nonuniformity of fields over the rod cross section and the space-harmonic effects. However the TEfield contributions are considered to be insignificant thereby allowing one to ignore the presence of the azimuthal electric field. Measurements are carried out with the help of an automated setup using an HP 8510 vector network analyzer (VNA) and a PC to collect the phase informations for the various precisely controlled positions of the perturber using a stepper motor which is also interfaced with the PC. The experimental and theoretical values of the phase velocity and the interaction impedance of a typical "cold" experimental helical structure for a wide-band TWT are found to be close within 0.5% and 5% respectively in an octave band of 8-16 GHz. © 1997 IEEE.