Design of shorter TE/sub 11/-to-HE/sub 11/ mode converters in a highly over-moded corrugated circular waveguide using a modified Chebyshev method

A TE/sub 11/-to-HE/sub 11/ mode converter is realized by a section of corrugated waveguide which has a special profile of slot depths which vary from lambda /2 at the TE/sub 11/ end to lambda /4 at the HE/sub 11/. The design of the profile is considered in terms of an idealized model that considers the corrugated waveguide in terms of a continuous, but anisotropic, wall surface impedance having zero value in the phi direction and an average of the fin impedance and the slot input impedance in the z direction. A second assumption in the simplified design consideration is that there is coupling only between the TE/sub 11/ and TM/sub 11/ mode branches. This method has been used to design a converter with a conversion loss of 0.2 dB at a frequency of 60 GHz in a 1.094-in.-diameter waveguide. The length of the converter was calculated to be about 3.5 in. compared to about 4.1 in. for the constant coupling profile. The results of a scattering matrix computation indicates that the output of the converter is about 82.2% TE/sub 11/ mode (in terms of power) and 15.5% TM/sub 11/ mode. The phase difference between the TM/sub 11/ and TE/sub 11/ modes is about 18 degrees as calculated with the scattering matrix program, compared to the ideal case, where they should be in phase. This can be remedied by adding an about-1/2-in.-length section of smooth circular waveguide.<>