Compact Dual-Band Microstrip BPF with Multiple Transmission Zeros for Wideband and WLAN Applications

In this letter, a novel compact dual-band microstrip bandpass filter (BPF) with multiple transmission zeros is proposed using the third-order interdigital structure and dual-mode short stub center-loaded resonator (DSLR) for wideband and WLAN applications. The high impedance feedline for the filter with the folded DSLR can function as the quarter-wavelength resonator (QWR) for the third-order interdigital filter. Meanwhile, the folded DSLR can be adopted without an evident increase of the size of the compact interdigital filter. Three transmission zeros between two passbands and in the lower- and upper-stopbands can be created due to the cross coupling between two high impedance feedlines as well as between the input and output, and the intrinsic characteristic of the DSLR. Further, two inverse QWR coupling short stubs with different size loaded in the 50 Omega feedlines can generate four transmission zeros to improve the isolation and deepen the stopband. Finally, a compact dual-band BPF prototype is designed, and good agreement can be obtained between measured and simulated results.