Dual-mode bandpass filter based on substrate-integrated waveguide loaded with complementary split ring resonators

A compact dual-mode bandpass filter adapted from circular substrate-integrated waveguide (SIW) is presented in this paper. The excited modes of the cavity are controlled by introducing metal vias and loading complementary split ring resonators (CSRRs) to the cavity. Higher order dual resonant modes are used to generate the passband response. Single ring CSRRs with double splits are loaded in the SIW cavity to provide independent control over the passband resonant modes. The loading of CSRRs contributes to further size reduction of the cavity. The fundamental mode of the cavity is controlled by using optimally configured metal vias. Thus, a nearly symmetric passband response and an improved out-of-band response below the dual-mode passband are achieved by suppressing the fundamental mode. The modified SIW cavity loaded with CSRRs reduces the size of the standard circular cavity by 83%. The proposed bandpass filter has center frequency at 8.7 GHz with 14% fractional bandwidth. 0.7 dB is the measured minimum insertion loss with a return loss better than 20 dB.