Dual-band Bandpass Filter Design Using Stub-loaded Resonators

A novel dual-band bandpass filter (BPF) using stub-loaded resonators (SLR) is presented. The proposed filter consists of a pair of independent open-ended square loops arranged in an adjacent manner with each other. Thus, allowing electromagnetic coupling configuration to be established. In addition, an open-ended microstrip stub is integrated into each respective square loop to achieve the dual-band characteristics at 2 GHz and 4 GHz. The resonant frequencies of the even-mode can be flexibly controlled by tuning the length of the open-ended stub whilst the odd-mode resonant frequencies stay constant. Two microstrip transmission lines with 50 Omega characteristic impedance act as the input and output feedlines respectively. The proposed dual-band BPF is implemented on a FR4 substrate with dielectric constant of 4.6, loss tangent of 0.027 and thickness of 1.6 mm. It has an overall dimensional size of 53.5mm by 32.1 mm. The best measured matching return loss Sit for the first passband is observed at 2.21GHz with a value less than -10 dB. The corresponding passband response S-21 is approximately -1.76 dB with an upper and lower stopband attenuation of more than 20 dB/GHz. Additionally, the best measured matching return loss Sit for the second passband is observed at 3.81 GHz with a value less than -10 dB. The corresponding passband response S-21 is approximately -2.88dB with a lower and upper stopband rejection rate of more than 10 dB/GHz and 30 dB/GHz respectively. It is also observed that three transmission zeros with more than 30 dB attenuation are realized. Both simulation and measurement results are presented and discussed.