Forked stepped impedance resonator bandpass filter with harmonic suppression

In this paper, a novel two-stage forked stepped impedance resonator (FSIR) bandpass filter is presented. The FSIR originated from an SIR has its both open ends split as a fork structure. A tunable transmission zero, which is used to suppress the second harmonic of the resonator, is created from the even- and odd-mode coupling effect of the two FSIR structures. The tape-lines feed together with the asymmetric feed orientation are adopted in this filter design to provide two additional zeros, each residing on either side of the passband edges, to improve the filter's frequency selectivity. An experiment is conducted and a good agreement between the measurement and the simulation is observed to verify the design.