Square-Coupled Topological Filter with an Ideal Rectangular Coefficient Facilitated by Dual-Cavity Single-Mode and Single-Cavity Dual-Mode SIW Resonators

In order to achieve miniaturization and high performance in microwave filters, this paper proposes two double-layer bandpass filters with different structures, both equivalent to square-coupled topologies. These filters employ dual-cavity single-mode and singlecavity dual-mode substrate-integrated waveguide resonators. In this configuration, the upper layer comprises two single-mode resonators connected to the input and output feed lines, while the lower layer contains dual-mode resonators coupled to the upper layer's single-mode resonators through two slots on the middle metal layer. A comprehensive analysis is conducted on the impact of primary parameters on filter characteristics and transmission zero positions. The second filter is fabricated and tested, yielding results consistent with simulation outcomes. The center frequency of the filter is 4.77 GHz, with a 3 dB bandwidth of 0.16 GHz (relative bandwidth: 3.35%). Additionally, its rectangularity coefficient at 10 dB approximately equals one, an ideal value for practical applications.