Compact Bandstop Half-Mode Substrate Integrated Waveguide Filter Based on a Broadside-Coupled Open Split-Ring Resonator

A shunt RLC resonant circuit obtained from a broadside-coupled open split ring resonator (BC-OSRR) is proposed. This BC-OSRR cell allows a parallel connection with planar waveguides. Hence, it has been applied to a half-mode substrate integrated waveguide (HMSIW) section. The analysis of the frequency response and electromagnetic field distribution of the HMSIW structure loaded with an appropriate BC-OSRR cell have shown two main behaviors. The first characteristic is a high-pass frequency response inherent to the HMSIW and the second is a transmission zero in the passband of the HMSIW, which is due to the resonance of the BC-OSRR cell. The First- and second-order prototypes have been designed and fabricated. The measured bandstop HMSIW filters using BC-OSRR achieve a 3-dB stopband bandwidth around 10% with more than 21-dB insertion loss. This BC-OSRR cell with low undesired radiation loss can be a new alternative for the implementation of compact bandstop filters in the planar technology.