Effect of the finite thickness of the helix wire on the characteristics of the helical slow-wave structure of a traveling-wave tube

A helix, supported by a number of longitudinal dielectric support rods of circular cross section enclosed in a glass/dielectric tube envelope, is field analyzed for dispersion characteristics, treating the support rods as a continuous homogeneous dielectric of an effective permittivity suitably interpreted in terms of the support parameters. Measurements are made on two typical structures in two different frequency bands for dispersion characteristics. The experimental observations agree very closely with the theoretical prediction when the effect of the finiteness of the helix-wire thickness is taken into consideration in the analysis. A calculation of interaction impedance adds a relevance to the problem.