Extremely Low-Complexity Negative Coupling Structure for SIW Filters and Approach to Adjust Rejection Next to Transmission Zeros

In this work, we present a single-layered substrate-integrated waveguide (SIW) resonator bandpass filter structure with a negative coupling that is simple to manufacture. The proposed filter structure specifically targets to have the advantage in terms of manufacturing complexity as a negative coupling can be realized simply by creating a single slot on one side of the substrate without increasing the number of substrates in use. By virtue of the proposed negative structure, a single-layered SIW filter with transmission zeros can be implemented without increasing the difficulty of fabrication. This work also presents a direct approach to adjust the level of the humps next to transmission zeros. For demonstration, we have designed, fabricated, and measured three types of quadruplet SIW filters, each with a pair of transmission zeros. Two of them have asymmetric humps, while one exhibits symmetric humps next to the transmission zeros. Sixth-order filter designs are also presented to illustrate the application of the proposed structure and approach to higher order filters.