A compact square patch band-pass filter is proposed in this paper. The dual-mode filter is designed based on a square patch resonator with a complementary split ring resonator (CSRR) split to be used as a perturbation element. The CSRR split is properly embedded in the square patch resonator to perturb electric current distribution on this patch and thus to simultaneously excite a pair of degenerate modes. Using the proposed CSRR elements, the band-pass filter is designed with miniaturized size, and two transmission zeros in stopbands are achieved to improve the selectivity of the filter. The influence of the CSRR elements on the band-pass filter is analyzed in detail. The proposed dual-mode filter is then fabricated and measured. Good agreement over a wide frequency range is achieved between the simulated and measured results. Moreover, in order to further investigate the characteristic of the dual-mode patch filters with CSRR perturbation, a dual-mode filter with a rectangular ring slot is presented for comparative study.
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