Compact balanced bandpass filter design using miniaturised substrate integrated waveguide cavities

The contribution of this work is to propose a compact balanced bandpass filter design using the miniaturised full/halfmode substrate integrated waveguide cavities (SIWCs) together with the microstrip coupling slot feed. The circuit area of the proposed full-mode SIWC is only 4.64% of its conventional SIWC counterpart, and the half-mode SIWC further reduces the circuit area by half. Besides the circuit compactness, the non-coupling of the microstrip and the slot in the common-mode (CM) operation provides an excellent CM signal rejection. Prototypes of two-pole full/half-mode balanced SIWC BPFs are fabricated and measured to validate the circuit design. Agreements between measured and simulated data are observed. The CM signal rejection levels are larger than 45 dB over a wide-frequency range with the passband's differential-mode insertion losses around 1.6 and 0.8 dB for the full- and half-mode designs, respectively.