Design of Cross-Coupled Duplexers Based on Substrate Integrated Waveguide Hexagonal Resonators

To address the trade-off between the poor integration of traditional waveguides and the severe high-frequency radiation loss of microstrip lines, a novel bandpass duplexer is proposed based on substrate integrated waveguide (SIW) hexagonal resonators. The design integrates the flexibility of a rectangular resonator with the high quality factor characteristics of circular cavities. By constructing SIW resonators with a hexagonal geometric configuration and combining a three-cell cascaded structure with cross-coupling theory, transmission zeros are introduced at the passband edges to regulate the electromagnetic field distribution. A duplexer is designed and fabricated for center frequencies of 6.8 and 7.9 GHz, with relative bandwidths of 3.5% and 7.5%, respectively. The measured results show that the isolation between passbands reaches -55 and -43 dB, the out-of-band rejection is better than -40 dB, and the measured results are in good agreement with the simulation results. This scheme combines the advantages of low radiation loss, high power-handling capability of SIW structures, and the integration advantages of planar technology, providing a new path for the design of high-performance duplexers in microwave and millimeter-wave frequency bands.