Low-loss twists in oversized rectangular waveguide

The unwanted mode conversion from twists in overmoded waveguides is calculated from numerical integration of the coupled-mode equations, considering simultaneous coupling of the five lowest-order modes coupled in a twist. Twists with tapered or linearly varying rates of twist are shown to be superior in medium- or broad-band applications to those with uniform twist rate. Measurements consistent with these theoretical calculations are discussed for uniform twists in WR90 waveguide at 60 GHz and for an electroformed twist having a linearly tapered rate of twist in WR187 waveguide from 15.7 to 17.7 GHz. The coupling coefficients needed in the calculations are derived and are compared with the results of other work, including a modal expansion of the dominant mode in twisted waveguide. The work also considers the transmission through an oversized waveguide with a mode converter generating a trapped unwanted mode, and the result for the dependence of the resonance depth on the mode conversion and the attenuation of the trapped mode is found