Substrate-Integrated Waveguide Filters Based on Dual-Mode Air-Filled Resonant Cavities

This paper presents the design and implementation of a novel class of substrate-integrated waveguide (SIW) filters, based on partially air-filled cavity resonators. Removing the dielectric in a portion of the SIW cavity allows obtaining dual-mode cavities: such cavities represent a doublet, used as the building blocks in the design of filters. A thorough modal analysis of the air-filled resonant cavity based on transmission lines' representation and a detailed study of the doublet permit to demonstrate a full control of the operation frequency, pass bandwidth, spurious-free region, and position of the transmission zeros of the filter. This paper presents the design, fabrication, and measurement of several two-pole filters based on this concept, with the aim to show the capabilities of the proposed structure and of the design technique. A five-pole filter is also presented, to describe the extension of the proposed approach to higher order filters.