An investigation of some waveguide structures for the propagation of circular TE modes

An analysis is given of various waveguide structures that support circular transverse electric modes. These structures are examined with a view to their application to mode filters and bends in circular low-loss waveguide systems. Circular waveguide is examined in which narrow longitudinal or circumferential slots are cut in the waveguide wall. Particular attention is given to longitudinally slotted guide, in which the H01 mode has a lower cut-off frequency than the En mode. A mode filter is described which transmits only the H01, H11 and H21 modes. Circular waveguide, with an outer conducting wall and an inner coaxial layer of closely spaced longitudinal conductors, is studied and found capable of supporting an Ho mode of free-space wavelength much greater than the outer guide diameter. It is also found, in principle, to support slow waves of the H1, H2, ? type at any frequency. An analysis is given of curved circular waveguide with small longitudinal and circumferential surface reactances that can with advantage be varied around the waveguide circumference. Application of this analysis to waveguide with shallow longitudinal slots shows that, for suitable diameter and slot dimensions, the H0 mode can be transmitted through slightly curved guide of any length with substantially no mode conversion. Uniform longitudinal slots in curved waveguide will restrict mode conversion, over a band of frequencies, to the H11 mode, the conversion being small for large bending radius. The proposed application of these small-diameter slotted waveguides is to relatively compact bends, with bending radius as little as five times the guide diameter. Although the wall conduction losses are increased by the slots, and by the operation close to cut-off, the overall loss of a bend should be small. The longitudinally slotted curved waveguide may be difficult to construct but it appears to provide a means of rapidly negotiating bends with little attenuation or contamination of the H01 mode.