A Compact And High-Selectivity Tri-Band Bandpass Filter Based On Symmetrical Stub-Loaded Square Ring Resonator

A compact tri-band bandpass filter is presented in this article, whose three passbands are realized by the lowest three resonant modes of a new structure of symmetrical stub-loaded square ring resonator. The filter features a compact size of 0.049 lambda(2)(g) (lambda(g) is the guided wavelength) as well as a high selectivity with nine transmission zeros. In addition, by using the odd- and even-mode analysis, its center frequencies can be easily tuned to the desired frequencies by the identical-number parameters, namely, the lengths of short-, open-, and square ring-stubs. To validate the proposed design, a tri-band bandpass filter with center frequencies of 2.4, 3.5, and 5.2 GHz was designed, simulated, and measured. The measurement results agree well with the simulation results.