Wide-Stopband Diplexer With Small Frequency Ratio Based on SIW Dual-Mode Resonators for Millimeter-Wave Applications

A substrate-integrated waveguide (SIW) diplexer has been designed based on the dual-mode resonator operating with TE301 and TE103 modes in this letter. Techniques of harmonic interleaving, negative excitation and planar coupling structure, orthogonal transmission, and slot lines etched on the ground plane are exploited synthetically to extend the range of the out-of-band rejection. The diplexer with lower and upper channels centered at 34.95 and 36.44 GHz is fabricated and measured to verify the proposed concept, which exhibits a wide stopband better than 20 dB up to 82.92 GHz and a high isolation better than 28.9 dB over the whole band with three transmission zeros (TZs) nearby two passbands. Measurements accord well with the simulation results.