Accurate calculation of coupling coefficients of irises/slots in combline filters

A methodology is presented for the computationally efficient and accurate prediction of the coupling coefficients of electrically thin irises and slots in combline filters. The proposed methodology utilises a rational function approximation with adaptive polynomial order identification to fit the frequency-domain response of the structure. Thus, the calculation of the coupling coefficients reduces to a polynomial root-searching problem. The key attributes of the proposed algorithm are its efficiency, its high frequency resolution, and its avoidance of spurious roots due to improper choice of polynomial orders. Examples are presented to demonstrate the validity and accuracy of the proposed approach.