A compact dual-band bandpass filter with high design flexibility using fully isolated coupling paths

A second-order dual-band bandpass filter using two outer folding open-loop resonators and two embedded anchor-shaped stepped impedance resonators is presented in this letter. The proposed scheme provides sufficient isolation between the two specific passbands, thus each passband can be individually turned. Electric couplings are adopted among the four resonators. By adjusting the spacing of resonators, the electric coupling of two different coupling feeding paths can be independently controlled, which allows flexible control of each bandwidth. The embedded structure which is introduced can further miniaturize the dimensions of the overall structure. In addition, the proposed filter has three transmission zeros at each passband, which exhibit high skirt selectivity. Good agreement between the simulated and measured results validates the correctness of the proposed design approach and shows that the two central frequencies of the fabricated filter are located at 2.5 and 5.1 GHz as desired, with 3 dB fractional bandwidths of 13.8 and 13.6%, respectively. The measured return losses are greater than 18.5 dB, while insertion losses are less than 0.9 dB in each of the two passbands.