Bandpass Filters Based on Multimode Microstrip Resonators with a Hairpin Conductor

Half-wave and quarter-wave microstrip resonators (MRs) with a rectangular stripline conductor split at one of its ends by a slot have been studied. Capacitive and inductive equivalent circuits have been constructed for the lower three oscillation modes of resonators and a third-order filter on their basis. It has been shown that the frequency responses of three-mode MR filters and their equivalent circuits, both tuned to the same bandwidth, are in good agreement with each other. It has been found that one can use a single MR to create devices with a passband arising from resonances between two, three, or even four lower oscillation modes by varying the width and length of the gap in the hairpin stripline conductor. The boundaries of feasible passband relative widths for second-, third-, and fourth-order filters have been determined.