A Flexible Design Cross‐Coupled SIW Bandpass Filter With Different Path Loads Using Mixed‐Coupling Topology

This paper presents an innovative coupling design for a bandpass substrate integrated waveguide (SIW) filter, enabling high selectivity and facilitating flexible design. The proposed filter design has two parts: one is a mixed coupling structure, and the second part is a cross‐coupling path. Both parts contribute to transmission zero (TZ) near the passband. Cross‐coupling creates TZ, while mixed‐coupling offers TZ and filter design flexibility. The cross‐coupled path is analyzed in three configurations: without a load, with an inductive load, and with a capacitive load. To demonstrate this, three filter designs are simulated and fabricated based on the type of path load. The filter achieves center frequencies near 27 GHz with a fractional bandwidth exceeding 3%. Four to five TZs are introduced to enhance selectivity. Key performance metrics include a return loss exceeding 15 dB within the passband and an insertion loss between 1.65 and 2.1 dB. The measured results align with the simulated results, validating the structure and theory of the cross‐coupled load.