The Inhomogeneous Dielectric Loading Effects of Practical Helix Supports on the Interaction Impedance of the Slow-Wave Structure of a TWT

Abstract

The helical slow-wave structure of a traveling-wave tube is usually supported by wedge-shaped bars or rods of circular or rectangular cross section. Such supports present an inhomogeneous loading of the helix caused by the variation of the effective permittivity of the helix surrounding with the radial coordinate. Out of the approaches to the analysis of such structures for dispersion characteristics has been the one in which the discrete supports are azimuthally smoothed out into continuous dielectric-tube regions, the number of such tubes being increased for converging results. This method has been extended here for the interaction impedance of the helix supported by a number of dielectric bars/rods of rectangular or circular cross section, the whole enclosed in a metal envelope. The method gives more accurate results than that given by the typical one in which the discrete supports are simulated by a single tube, instead of many, particularly for situations in which the relative permittivity and radial thickness of the dielectric supports and the frequency of operation are high. The analysis developed here is rather general and is not restricted to the structures with only rectangular or circular bars/rods, and a metallic envelope but also applicable to the structures with any arbitrarily shaped supports as well as with a ceramic envelope. © 1992 IEEE