Propagation in curved and twisted waveguides of rectangular cross-section

The propagation in curved and twisted rectangular waveguides is investigated by putting the wave equation in a form in which the co-ordinates in a waveguide cross-section are also the independent variables in the differential equation. The wavelength is found for the E-curved and H-curved guides, and for twisted guides. In the latter case there is a degeneracy when the guide sides become equal. Curves are presented for the correction to the wavelength in a straight guide caused by the bending or twisting.