Novel balanced bandpass filters using substrate integrated half-mode waveguide

The contribution of this work is to propose novel balanced bandpass filters (BPFs) using the half-mode substrate integrated waveguide (HMSIW) and its size-reduced form, a folded HMSIW structure. The size of an HMSIW is about half of the substrate integrated waveguide (SIW) and the size of the folded HMSIW further reduces to 25%. Besides compactness, the proposed BPFs are attractive because the thru-via array needed for providing the side-wall effect is no longer required. In addition, the common-mode (CM) rejection of the HMSIW BPF is achieved from the non-coupling slots, caused by the PMC. The CM rejection of the folded HMSIW BPF is obtained by up-shifting the CM operating frequency far away from the differential-mode (DM) passband. Prototypes of the proposed circuits are fabricated and measured to present CM rejection levels larger than 40 dB over a wide frequency range and DM insertion losses around 2 dB in the passbands. Agreements between measured and simulated data are observed to validate the circuit design and performance.