Transmission Line Meander Dual-Mode Resonators and Bandpass Filters

In this paper, the problem of synthesizing and studying the properties of transmission line dual-mode resonators characterized by a single dimension is solved for the first time. The initial samples for the synthesis are stepped-impedance resonators (SIR) with different numbers of composite sections N, and characteristic impedances Z0i, i = 1, 2, …, N. Four dual-mode resonators were synthesized, which represent a meander with alternating sections of high Z0max and low Z0min characteristic impedance. In two resonators, dual-mode oscillation arises from the mutual convergence of the resonant frequencies f01 and f02, forming a meander with three sections. The other two resonators represent a meander with five sections. In these resonators, dual-mode oscillation arises from the convergence of the resonant frequencies f02 and f03, enabling operation at higher frequencies without reducing their length. Both ends of these resonators can be either open-circuited or short-circuited. A simple analytical dependence of the even fe and odd fo modes on the parameter m = Z0max/Z0min of synthesized meander dual-mode resonators was established. The larger the parameter m, the closer fe and fo modes are. Their convergence can be as low as 4%. This dependence made it possible to express the coupling coefficient K between the even fe and odd fo modes in terms of the parameter m, significantly simplifying the design of meander dual-mode bandpass filters (BPF). Design examples of such BPFs and their microstrip implementation are considered.