Propagation in corrugated waveguides

It is possible to determine the nature of the modes which propagate in corrugated, cylindrical waveguides, by imposing a nonisotropic, surface-reactance, boundary condition at the corrugated walls. Such a condition, in ignoring periodic effects, implies corrugated gaps much less than a wavelength separated by even smaller fins, and leads to simplified expressions for the fields in the waveguide. Two types of propagation occur: fast waves, characterised by low electric-field intensity at the walls, and slow waves by strong electric field at the walls. Both types are hybrid in nature, and degenerate to combinations of TE and TM modes when the corrugation slot depths are equivalent to multiples of half a wavelength. The dominant mode is a slow wave, with a cutoff equal to that of the dominant mode in a similar, uncorrugated waveguide. Mode charts, which distinguish the fast- and slow-wave regions as a function of normalised frequency and slot depths, may be used to design waveguides suitable for either fast or slow waves, and for matching such guides to uncorrugated launching sections.