Two small programs for mode coupling in abrupt transitions between smooth and corrugated circular waveguides

An analytic solution to the problem of coupling a smooth and a corrugated circular waveguide of the same diameter was treated in [1] in the context of corrugated feed horns. A useful by-product is the ability to describe hybrid modes in a corrugated waveguide as a known combination of smooth waveguide modes, which allows using general-purpose codes (e.g. ANSYS HFSS, or CST Microwave Studio) for assessment of results obtained using hybrid modes. A small FORTRAN 90 program was written to compute coupling between a pure smooth circular waveguide mode and the first few hybrid modes as a function of frequency for user-specified waveguide geometry. The reflectivity in the same smooth waveguide mode is also computed. The roots of the characteristic equation for the first few hybrid modes are also computed for the chosen geometry. A second FORTRAN 90 program was written for computing the coupling between HE11 and TE11-12-13, TM11-12-13 using a mode matching formulation. The common results of both programs agree. Both programs are available in the public domain in the same location as this report. Users are kindly requested to cite this report as a reference.