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

In this work, a simple circuit model comprised only of lumped elements is developed to model the inter-resonator tapped-in coupling mechanism between symmetric resonators. This circuit model is utilized to explain the coupling phenomena. Mathematical relations are derived to express the coupling and resonant frequency in terms of the circuit-model elements. Derivatives of these expressions are used to describe the behavior of coupling and resonant frequency in terms of circuit elements. It is noted that the circuit model and the results introduced here are used in the development of circuit-based CAD tool to speed up the design of microwave combline filters. Finally, the measurement results are provided to confirm the simplicity of adjusting coupling strength between combline resonators using the proposed coupling mechanism.