Microstrip tri-band bandpass filter using dissimilar stepped-impedance-resonator with controllable multiple transmission zeros

A microstrip triple band filter using dissimilar quarter-wavelength stepped impedance resonator (SIR) with a source-load coupling feedlines structure is proposed in this paper. Multiple transmission zeros are generated by using source-load coupling structure. Then, multipath coupling diagrams are constructed and the relative phase shifts of each path are studied to explain the responses of the triple-band filter. The bands of the designed tri-band BPF are operated at 1.9 GHz and 2.6 GHz for TD-LTE application and 3.6 GHz (Wi-MAX band) was designed. Also, the circuit size occupies only 0.29λ_g×0.20λ_g, where λ_g is the guided wavelength at the center frequency of the first passband.