Wide upper-stopband microstrip bandpass filter with dual-mode open loop stepped-impedance resonator and source-load coupling structure

In this article, a dual‐mode microstrip bandpass filter (BPF) with compact size, sharp skirt, and wide upper‐stopband performance is proposed using the short‐stub‐loaded open loop stepped‐impedance resonator and source‐load coupling structure. The advantageous features of the resonator are that the even‐mode resonant frequencies can be controlled exclusively by the short stub, and two transmission zeros are created near the passband by the short stub and the folded arms. Meanwhile, two transmission zeros generated by the source‐load coupling structure can be applied to suppress the first and second spurious harmonics. Finally, a microstrip BPF located at 2.4 GHz with a fractional bandwidth 4% is designed, fabricated, and measured. The upper stopband in experiment is extended up to 20 GHz with an insertion loss lower than −21.2 dB. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:1618–1621, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26881