Novel dual-band bandpass filter with wide upper stopband performance

In traditional design of the dual-band bandpass filter (BPF), two or more resonators need to be utilized, which leads to a relatively large circuit size. For miniaturization, a novel stub-loaded square loop quad-mode resonator is proposed in this paper. Due to the symmetry of the structure, the even-odd-mode method is applied twice to analyze its resonance characteristics. All the four mode equivalent circuits are quarter-wavelength resonators, so the resonator has a compact size and the first harmonic is three times the fundamental one. The resonant frequency of each mode can be tuned freely by changing the corresponding physical dimensions, and the passband frequencies and bandwidths of the designed dual-band filter using the proposed resonator are independently controllable. To improve the selectivity and achieve a wide upper stopband, quarter wavelength hook-shape feedlines are designed to provide appropriate external coupling. Two new transmission zeros are generated by the feedlines. Finally, a microstrip dual-band bandpass filter (BPF) centering at 1.5 and 2.4 GHz with a 3 dB fractional bandwidth (FBW) of 10.7% and 9.6% is designed and fabricated. The measured and simulated results which are in good agreement are presented. © 2015, Science Press. All right reserved.