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A new quad-band microstrip bandpass filter (BPF) using asymmetric stepped impedance resonators (SIRs) is proposed. The filter only employs two sets of the asymmetric SIRs. One set is designed to operate at the first and third passbands (2.4/5.2 GHz) and the other set is employed at second and fourth passbands (3.5/6.8 GHz). By tuning the impedance and length ratios of the asymmetric SIRs, a multi-band filter can be easily achieved. This study provides a simple and effective method to design a quad-band filter with low insertion loss and compact size. Experimental verification is provided and good agreement has been found between simulation and measurement.