Compact and High-Selectivity Microstrip Bandpass Filters Using Triple-/Quad-Mode Stub-Loaded Resonators

In this letter, a class of triple-/quad-mode stub-loaded resonators is proposed and analyzed for application in bandpass filters (BPFs). First, a pair of two identical open-ended stubs, shorter than λ/2, are tap-connected to a λ/2 resonator at two symmetrical positions with respect to its center. It results to excite three resonant modes closely to each other while producing a transmission zero above its resonant frequencies. Next, a single open-ended stub, longer than λ/2, is introduced at the center of the resonator. This arrangement is made to bring out a fourth resonant mode approaching the above three modes while causing a transmission zero below all of them. As such, a quad-mode resonator, self-contained with two transmission zeros, is theoretically constituted. Finally, the triple- and quad-pole BPFs with miniaturized size and sharpened roll-off skirts are designed and fabricated to provide an experimental validation on the predicted performances of the proposed filters.