In this letter, a compact microstrip bandpass filter (BPF) with four transmission zeros (TZs) is designed by using a short-stub centered loaded folded dual-mode resonator and an I/O mutual coupled open-stub loaded feedline structure. The coupling structure can realize source-load (S-L) coupling with quadratic function coupling coefficient, which can generate three TZs in the upper-stopband to improve the selectivity. Owing to the intrinsic characteristics of the dual-mode resonator, one extra TZ can be created near the lower passband edge. Finally, a compact BPF with fractional bandwidth (FBW) of 3.5% located at 2.4 GHz for WLAN application has been designed and fabricated. Good agreement between simulation and measurement verifies the validity of the design.
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