Design of microstrip tri-band bandpass filter using square ring loaded resonator

In this paper, a microstrip tri-band bandpass filter (BPF) utilizing square ring loaded resonator is proposed. A pair of bent perturbation stubs is installed to the ring resonator so as to control resonant modes of the ring. The first three modes are used for tri-band BPF design. In order to control the second and third resonant modes, two pairs of square patches and loaded open stubs are added, and then two asymmetric resonators are cascaded for tri-band operation. Based on the transmission line theory, even- and odd-mode analysis method is used to deduce the resonant frequencies. According to the resonant conditions, a kind of tri-band BPF can be flexibly designed. Finally, two filters are designed as examples in a comprehensive way for demonstration, and one design with passbands located at 2.4, 3.5 and 5.2GHz is fabricated. Measured results agree well with the simulated ones.