Theory and potential applications of backward waves in nonperiodic inhomogeneous waveguides

Backward waves can propagate in an inhomogeneous waveguide of circular cross-section when the geometry and the properties of the materials are suitable. The phenomenon is related to a degeneracy of the H11 and E11 modes at cutoff, and this suggests where to look for the most pronounced effect. The behaviour of the group velocity is studied in detail, in the neighbourhood of the degeneracy, for a number of values of the guide radius. A discussion is given of the possibilities of applying the phenomenon to backward-wave amplifiers, travelling-wave masers, a frequency selector, a circular polariser, switching and phase compensation.