High-isolation single-layer dual-mode substrate-integrated waveguide diplexers for millimetre-wave applications

New coupling scheme substrate-integrated waveguide (SIW) diplexers based on dual-mode cavities are proposed for millimetre-wave applications. Dual-mode SIW cavities are simultaneously employed in common input and coupling cavity to produce filtering transmission poles and transmission zero (TZ). A common input SIW cavity with two orthogonal modes is employed instead of the traditional T-junction for circuit miniaturisation, insertion-loss reduction, and intrinsically high isolation. A critical dual-mode SIW cavity with diagonally distributed modes has been employed in each channel to realise a controllable stopband TZ for high selectivity of passband's skirt. To further improve channel isolation and passbands' selectivity, a SIW diplexer based on dual- and single-mode cavities is proposed, and four-pole responses for each passband channel are designed. For the verification, two millimetre-wave diplexers have been fabricated and tested.