Analysis of tapped-in coupling between combline resonators applicable in wideband filter designs

In this work, the comprehensive analysis of coupling for combline resonators coupled through tapped-in TEM transmission line is presented. The analysis and discussions are based on developed equivalent circuit modeling the physical structure. The effects of lumped elements in this model on coupling value and resonant center frequency are studied. The results confirm the possibility to achieve a wide range of coupling varying from zero to unity. Strong coupling results in the dramatic increase of resonant center frequency. The measurement results for two-coupled planar resonators based on tapped-in coupling method verify the concepts. Therefore, this tapped-in coupling between resonators is an asset in designs of wideband filters. The developed circuit model is useful for early stages of design. The lumped elements are functions of physical design parameters hence the circuit model speeds up the realization process of finding the initial physical dimensions of resonators which satisfy the required coupling matrix and resonant center frequency. Following, full-wave analysis is performed for the final optimization of total filter structure.