A compact dual-band bandpass filter using triple-mode stub-loaded resonators and outer-folding open-loop resonators

A compact dual-band bandpass filter with high-selectivity is proposed using triple-mode stub-loaded resonators (TMSLRs) and outer-folding open-loop resonators (OFOLRs). The proposed TMSLRs, with two identical folded shape open-stubs located at the symmetrical sides of a microstrip line, are used to generate the upper band while the OFOLRs are utilized to form the lower passband. Two resonators are independent control of passbands, frequencies, and bandwidths. Besides, high selectivity and stopband suppression can be easily obtained due to the fact that four transmission zeros are realized near the passband edges. Measured results show that two central frequencies occurred at 2.4 GHz and 5.4 GHz for WiMAX and WLAN applications with a wide fractional bandwidth of 13.9% and 16.7% and a low insertion loss of-0.25 dB and-1.21 dB, respectively.