007950671938 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 47, NO. 10, OCTOBER 1999 A Joint Field/Circuit Model of Line-to-Ring Coupling Structures and Its Application to the Design of Microstrip Dual-Mode Filters and Ring Resonator Circuits Lei Zhu, Member, IEEE, and Ke Wu, Senior Member, IEEE Abstract― A joint eld/circuit model is proposed in this paper to characterize a class of line-to-ring coupling structures for design and optimization of microstrip dual-mode lters and ring resonator circuits. The generic model is derived from eld theory and presented in terms of circuit elements by applying a newly developed numerical deembedding technique called “short-open calibration” in a deterministic method-of-moments scheme. It provides a new design strategy for characterizing and opti……