A compact dual-band bandpass filter using asymmetric T-shaped resonators

Two compact dual-band bandpass filters with T-shaped resonators are designed in this paper. The proposed filters are four-stage design composed by four T-shaped resonators in conjunction with the microstrip lines in the I/O ports. The location of the two passbands can be adjusted by suitably changing the impedance ratio and the length of the T-shaped resonators. For comparison, two different feeding structures are employed, saying, coupled-line feeding and tapped-line feeding patterns. For better energy input in the second pattern, the two resonators connected with the tapped-lines are asymmetric. For demonstration, one dual-band filter was designed at 2.45 GHz and 5.8GHz. The measured results show that the filter has return loss of -22 dB at 2.51 GHz and -18.6 dB at 5.81 GHz, and the 3-dB fractional bandwidth of the two passbands are 6.8% and 4.48%, respectively.