E-band filters based on substrate integrated waveguide octagonal cavities loaded by complementary split-ring resonators

In this paper, substrate integrated waveguide (SIW) octagonal cavities in conjunction with octagonal complementary split-ring resonators (CSRRs) are used to propose compact narrow bandpass filters at E-band. Short circuited slots are employed as coplanar waveguide (CPW) to SIW transitions as well as matching elements between adjacent stages of filters. Parameterized models with full-wave accuracy are extracted for three different SIW-CSRRs unit cells. Each unit cell is comprised of an SIW cavity, a pair of octagonal CSRRs etched on the top plate, and a CPW to SIW transition with a certain slot length for bandwidth adjustment. These unit cell models are used to design and optimize multi-stage E-band filters having five, six and seven stages. Measured results of filters show insertion loss, ripple and group delay variation on the order of 7-dB, 1-dB and 30 ps over the passband of 70-73 GHz while about 30-dB rejection at 66.5 GHz and 76.5 GHz is achieved.