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 , TE 102 and TE 201 , 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.