The inhomogeneous dielectric loading effects of practical helix supports on the interaction impedance of the slow-wave structure of a TWT

The helical slow-wave structure of a traveling-wave tube is usually supported by wedge-shaped bars or rods or 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. In one of the approaches to the analysis of such structures for dispersion characteristics, the discrete supports are azimuthally smoothed out into continuous dielectric-tube regions, the number of such tubes being increased for converging results. This method is extended 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 a single tube, 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 is rather general