Compact triple-band bandstop filter using folded, symmetric stepped-impedance resonators

A compact bandstop filter composed of eight folded stepped-impedance resonator cells for triple-band stopband is presented. Based on the analysis of unit structure's characteristic and parametric analysis, it is found that the result of input output connecting lines with proposed stepped-impedance resonator cells exhibits three transmission zeros. Through tuning the geometry parameters in a small range, the design flexibility of transmission zeros is depicted. Then, the influence of different amounts of folded, symmetric resonator cells is researched for the sake of getting the characteristic of triple-band rejection. Via optimization, a compact triple-band bandstop filter adopting eight stepped-impedance resonator cells has been fabricated and measured. Three center frequencies at 3.8, 6.7 and 9.5 GHz are reported corresponding to the attenuation level of 54.3, 37.4 and 27.5 dB with return losses of 0.11, 0.25 and 0.71 dB. Additionally, there are two transmission poles on the right and left sides of each stopbands for better selectivity. Owing to the folded shape, the totally size of the proposed structure gets reduced.