Comparative study of two integral-equation formulations for inductive irises in rectangular waveguides

A comparative study of two formulations based on a combination of the mode-matching technique and the method of moments is presented. Basis functions, which include the edge conditions as well as mirror images in the walls of the waveguide, are used to accurately determine the properties of an infinitely thin inductive iris. For small irises, d/a<0.1, the formulation based on the induced surface current converges with only one basis function. Two basis functions are found sufficient for all values of the width of the iris. However, the formulation based on the tangential electric field at the gap of the iris converges with only one basis function for d/a>0.1. Two parametric functions involving Bessel functions of the first kind of order 1/2 , J/sub 1/2 / and J/sub 1/, which approximate the iris´s surface current and the electric field in the gap are introduced. Numerical results for the susceptance of the iris are compared with the analytical results; excellent agreement is documented.