A dual-band bandpass filter constructed by using T junctions I/O structure

This letter designs a microstrip dual-band bandpass filter (BPF) based on hairpin resonators using T junctions I/O structure. Firstly it presents the principle of T junctions based on a diplexer. Secondly, the method of designing single-band BPF based on hairpin resonators is introduced. By using a T junction to connect two single-band BPFs, the influence from each filter is reduced. Then a dual-band BPF working at 2 GHz and 3GHz with two junctions is designed and fabricated. Due to the use of T junctions, the performances of each passband could be designed almost independently unlike some other kind of dual-band BPFs. The measured results accord well with the simulated results, showing outstanding performances of the designed filter.