Compact bandpass filters based on a new substrate integrated waveguide coaxial cavity

In this paper a new approach for the design of very compact bandpass filters (BPFs) with transmission zero generation is proposed. The proposed filters are based on a dual mode substrate integrated waveguide (SIW) coaxial cavity. This filtering building block provides two coaxial modes performing a doublet filtering configuration. The proposed dual-mode SIW coaxial cavity is studied in detail and guidelines for the filter design are given. As will be shown, the proposed building block presents a high degree of design flexibility, which allows for the design of multiple kind of bandpass filter responses, including both narrow- and wide-band bandpass filters along with TZ generation. Two proof-of-concept filters are implemented and tested: a wide-band BPF with a fractional bandwidth of 20% centered at 8 GHz, and a quasi-elliptic type narrow-band BPF formed by cascading two dual-mode SIW coaxial cavities.