Balanced BPF design of substrate-integrated waveguide cavity using hybrid microstrip/slot feed for CM suppression

A dual-mode balanced bandpass filter (BPF) design with an excellent common-mode (CM) rejection performance is presented. The balanced filter is composed of coupled substrate-integrated waveguide (SIW) cavities and a hybrid microstrip/slot feed. The cavities are of a square shape to produce two degenerated resonance modes, TE₁₀₂ and TE₂₀₁, for dual-mode operation. The hybrid microstrip/slot feed is developed to suppress the unwanted CM signals in both the in-band and out-band regions. The measured CM signal rejection levels are over 50 dB. A experiment was conducted to verify this novel balanced BPF design.