Synthesis and Implementation of Multiband SIW Bandpass Filters Based on In-Line Topology

This article presents a similarity transformation method for multiband bandpass filters (MBPFs) to convert the star-like topology into the in-line topology. A general theoretical technique is introduced, which replaces the traditional step-by-step extraction of <tex-math notation="LaTeX">$LC$</tex-math> circuit through filter synthesis with the similarity transformation rotation of coupling matrix, addresses the round-off errors in the parameter extraction process, and improves the accuracy of the theoretical synthesis results. The application of in-line topology significantly enhances the flexibility of filter design, reduces circuit complexity, and simplifies the fabrication of high-order MBPFs. Based on the substrate integrated waveguide (SIW) technology, a series of examples, including tri-band, quad-band, and particularly the first reported quint-band third-order Chebyshev SIW bandpass filters, is designed and implemented. Good agreement is achieved between simulated responses and measured results, validating the designed filter models and the proposed theoretical method.