Evanescent and propagating modes of dielectric-loaded circular waveguide

The paper describes the results of a theoretical study of the properties of evanescent and propagating azimuthally dependent modes of a circular waveguide containing an axial dielectric rod. It is shown that, provided that the rod permittivity exceeds the minimum value which allows backward-wave propagation, a region of the spectrum will exist in which there are no purely propagating or purely evanescent modes. In this region, solutions of the propagation equation are complex. It is shown that the range of rod/waveguide radius ratio over which complex modes can exist is larger than that for backward waves. Although the computed results refer to a rod of relative permittivity 21, the qualitative effects of varying rod permittivity are discussed.