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A novel compact triband bandpass filter using SIR embedded quarter‐wavelength resonators

A novel second‐order triband bandpass filter using stepped‐impedance resonator embedded quarter‐wavelength resonators (SIREQRs) is presented in this letter. By embedding a meandered stepped‐impedance resonator (SIR) into the low‐impedance section of the conventional quarter‐wavelength SIR, the proposed resonator is obtained. By coupling two SIREQRs with a common short‐ended via‐hole and feeding them through a pair of open‐loop resonators, the proposed second‐order triband filter is achieved. The first passband is generated by the quarter‐wavelength SIRs, the second one is created by the embedded resonators, and the side‐coupled open‐loop resonators are utilized to construct the third one. Seven transmission zeros are generated due to the 0° feed structure and the canceling effects between electric coupling and magnetic coupling, which improve the skirt selectivity and band‐to‐band isolation level greatly. Furthermore, the overall circuit size is very compact with only 7.2 × 12.6 mm2 or 0.08 × 0.144 λg2 due to the embedded configuration. A triband filter prototype has been designed and fabricated, and the measured results agree well with the simulated ones. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:1345–1349, 2015