Equivalent susceptance of a circular iris in a parallel plate waveguide

An approximate formula is proposed for the equivalent susceptance of a circ ular iris in a parallel plate waveguide when the TEM mode cylindrical wave is incident from the center of the iris. Schwinger's variational method for a linear iris is generalized to the cylindrical case, and an approximate c losed form formula is obtained which recovers the result of the linear iris when the radius of the circular iris is sufficiently larger than the wavel ength. For verification of the formula, an exact integral equation is formu lated and solved numerically by Galerkin's method. A comparison between the m shows good agreement.