Using Mixed Coupling to Realize Wide Stopband Multilayer Substrate Integrated Waveguide Filter

Mixed coupling has been used in substrate integrated waveguide (SIW) filters to eliminate the transmission of spurious modes. However, the elimination only applied to the first spurious mode. Here, we propose a new mixed coupling structure that consists of slots for magnetic coupling and holes for electric coupling. Using it as the inter-coupling structure, the total coupling/transmission of multiple spurious modes can be zero. This brief presents a prototype that is a multilayer SIW filter working in
${\mathrm{ TE}}_{101}~(f_{0})$
. Theoretically, all the spurious modes below
${\mathrm{ TE}}_{404}$
of this prototype can be eliminated efficiently, which is verified by an experiment result showing a stopband extended to
$4.02~f_{0}$
. The proposed technique not only extends the stopband of SIW filters but also the application scope of mixed coupling. It should facilitate the development of high-performance SIW filters in wireless/microwave circuits and systems.