Dual-mode dual-band bandpass filter based on square loop resonator

In this paper, a dual-mode dual-band microstrip bandpass filter utilizing asymmetric square loop resonators is proposed. A pair of bent open-circuited stubs is installed to the loop as perturbation stubs. By stretching the perturbation stubs more than half-wavelength of the loop, the degenerated modes in a loop are split for dual-band operation. Two asymmetric resonators are cascaded to form two bands. Even- and odd-mode analysis method is used to deduce the resonant frequencies of two bands. Based on the transmission line theory, resonant frequencies of the two resonances are derived. According to the resonant conditions, a kind of dual-mode dual-band bandpass filter can be easily designed. Finally, a dual-band filter with two bands centering at 1.9 GHz and 2.6 GHz is designed in a comprehensive way. Measured results of experimental circuit show good agreement with simulated responses.