Circular-waveguide backward-wave structures

Abstract

The paper describes a graphical method for determining the cut-off conditions of those modes of a circular waveguide containing an axial dielectric rod which possess an azimuthal dependence of unity. Results are presented for the six modes of lowest order. It is shown that the cut-off conditions of a pair of modes such as the H_1m and E_1m can, under certain circumstances, coincide. This degeneracy at cut-off is shown to be associated with backward-wave propagation above cut-off. The modes with a backward-wave component are designated hybrid H_1m modes, although, under certain conditions, these exhibit E-mode properties at cut-off.The propogation behaviour of the modes is investigated theoretically, special attention being given to parameters most suited to backward-wave applications. Only the mode of lowest order exhibits backward-wave properties which appear favourable for use in devices. The possible application of this mode to a backward-wave oscillator is investigated, but it is shown that with existing dielectric materials high beam voltages would be required. The application to a frequency selector is also described.The results of a study of cut-off degeneracy in other waveguide structures are given.