Waveguide Coupler.

The rectangular waveguide to circular waveguide coupler includes a first section of circular waveguide spaced by a gap from a reflective-plane conducting wall, the latter having a hole for passage of an electron beam if the coupler is used in a traveling wave tube, and a second section of circular waveguide disposed external to, and coaxial with, at least a portion of the first waveguide section and extending to the wall to provide a conductive boundary surrounding the gap. The region between the first and second waveguide sections forms an input port of the coupler; the first waveguide section forms the output port of the coupler. Electromagnetic waves in a TE(01) coaxial waveguide mode are applied to the input port from a rectangular waveguide supporting the dominant TE(10) mode. The second waveguide section has a cutoff determining dimension r(e) proportioned to support both the TE(01) and TE(02) circular electric modes in the region of the gap at the frequency of the applied waves. The first waveguide section has cut-off determining dimensions r(i) proportioned to support the circular electric TE(01) mode to the exclusion of all higher-numbered circular electric modes at the frequency of the applied waves.