Two-port balanced dual-band bandpass filter based on stepped impedance resonators

We propose a two-port balanced dual-band bandpass filter based on stepped impedance resonators. The resonance frequency and the bandwidth of the first harmonic band can be controlled by using the impedance ratio of stepped impedance resonator and the coupled spacing between stepped impedance resonator and extra open-loop resonator, respectively. For a front-end application the four-port balanced dual-band bandpass filter was presented. In order to extensively utilize the advantages of the small-size and light-weight, it may be more useful if the four-port of that may be transformed into a two-port. The two-port balanced dual-band bandpass filter which may be transformed from the four-port balanced dual-band bandpass filter by the balun is proposed. The two-port balanced dual-band bandpass filter which has the input and output baluns and the fundamental frequency at 2.5 GHz and the first harmonic frequency at 5.5 GHz is fabricated and measured. Good agreement between measured and simulated results was achieved.