Design of Cross-Coupled Bandpass Filters with Flexible Coupling via Half-Mode Substrate-Integrated Waveguide

In this article, a simple approach for achievement of flexible coupling is presented based on the half-mode substrate-integrated waveguide (HMSIW). Since both electric and magnetic fields vary along the magnetic wall of the HMSIW, there exists mixed coupling between two adjacent HMSIWs. In this context, only by adjusting the width of the coupling slot at specific regions, both coupling property and strength can be conveniently controlled. Besides, as the coupling slot possesses the merits of simple structure and high flexibility, it is also expected to achieve desired couplings between multimode resonators. For demonstration, three cross-coupled bandpass filters (BPFs) with different kinds of frequency responses are constructed based on the typical cascaded trisection coupling topology by mixing one SIW and two HMSIWs, including two single-band and one dual-band designs. All measured results are highly consistent with the simulated ones, validating that the HMSIW not only has the inherent advantage of smaller size than the conventional SIW counterparts but also possesses unique feature in providing flexible coupling without extra circuit.