A method of changing the dominant mode in a hollow metal waveguide and its application to bends

It is the usual practice to use, where possible, the dominant mode in a waveguide, because, by making the cross-section of the tube just sufficient to support that mode, all others can be effectively excluded. For some applications the normal dominant mode is unsuitable, and the paper describes arrangements by which another desired mode can be brought to the dominant role. Thus the H01 mode in a circular-section guide can probably be made effectively dominant by employing an anisotropic surface impedance, yielding an inductive reactance to the H modes and a capacitive reactance to the E modes. Advantage is also taken of the conversion of all H modes with the exception of the H0n family to corresponding E modes as a result of longitudinal current in the wall of the guide. The net result of this technique is to make the guide effectively larger for the H modes and smaller for the E modes, while virtually suppressing all H modes requiring for their support longitudinal current. The necessary anisotropic surface impedance is provided by transverse and longitudinal corrugations or by coating the metal with a thin layer of dielectric.