Compact Diplexer With Wide Stopband Based on Stacked Dual-Mode and Single-Mode SIW Cavities

This letter proposes a compact diplexer with a wide stopband based on stacked substrate-integrated waveguide (SIW) cavities. By vertically stacking the common dual-mode resonator (DMR) with multiple single-mode resonators (SMRs), the design achieves flexible bandwidth control for both channels while maintaining compactness. The proposed diplexer achieves enhanced wide-stopband performance through strategic suppression of TE₁₀₂, TE₁₀₃/TE₃₀₁, TE₃₀₂, and TE₃₀₃ modes in SMRs. A third-order diplexer is fabricated with a size of $1.02\lambda _{g}^{2}$ , extending the stopband up to $3.57~f_{1}$ with a better than 20-dB rejection level.