A microstrip dual-mode dual-band bandpass filter using stepped impedance resonators (SIRs) is designed for dual-band wireless local area network (WLAN) applications at 2.4 and 5.2 GHz. By appropriately selecting the impedance ratio (Rz) and length ratio (α) of the SIRs, the harmonic frequencies can be tuned for generating the dual-bandpass response. Based on SIRs, a dual-mode dual-band bandpass filter is designed with one transmission zero. To improve the selectivity, another three transmission zeros in stopbands be created by introducing two stubs in input/output (I/O) lines. Two experimental filters are fabricated. Both simulated resulted and measured resulted are presented.
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