Design of planar tri-band bandpass filter

This paper presents a high-selectivity planar triband bandpass filter. The proposed filter utilizes two different kinds of resonators, i.e., stub-loaded resonators and half-wavelength resonators. The stub-loaded resonator is used to generate the first and third passbands and the half-wavelength resonator yields the second passband. One set of resonators is inside the other and thus the overall circuit is compact in size. Using this configuration, the passband can be conveniently tuned to desired frequencies by controlling the corresponding resonator dimensions. On each side of each passband, there is at least one transmission zero, resulting in high skirt selectivity. For demonstration, a prototype filter is designed, fabricated and measured. The comparison of simulated and measured results is presented.