Miniaturized Evanescent Mode Substrate Integrated Waveguide Filter with Mixed-Coupled Folded Complementary Split-Ring Resonators

A miniaturized evanescent mode substrate integrated waveguide (SIW) bandpass filter loaded with mixed-coupled folded complementary split-ring resonators (FCSRRs) is reported in this work. Compared with the conventional CSRR, the proposed FCSRR is with enhanced product of equivalent inductance and capacitance, which is beneficial for size reduction of the conventional CSRR. Hence, as the FCSS pair are loaded into SIW, the newly-formed resonator will exhibit evanescent mode resonance characteristic as well, and even achieve smaller physical size as compared its corresponding conventional CSRR-based counterpart. Meanwhile, it has been well-known that, in the SIW section with a pair of face-to-face FCSRRs, a transmission zero (TZ) at the upper stopband can be generated by the mixed coupling between the SIW and FCSRRs. Furthermore, by adding a slot between the splits of the two FCSRRs, a dual-mode resonator is developed, in which the cross-coupling scheme is constituted to produce an extra TZ at the lower stopband of the resonator to improve its selectivity performance notably. To demonstrate the aforementioned idea, a size-miniaturized bandpass filter based on the proposed SIW section with mixed-coupled FCSRRs is realized. Experimental results show that both miniature physical size and good performance are achieved simultaneously, which illustrates that the proposed work is promising in practical radio frequency (RF) and microwave applications.