Compact dual band balanced bandpass filter based on double layer SIW structure
A compact dual band balanced bandpass filter with high common mode (CM) suppression is proposed. Two layers of substrate are vertically stacked to realise the miniaturisation. The first differential mode passband can be obtained by the dominant modes and the second can be achieved by utilising the higher order modes. The desired coupling coefficients of both bands are achievable by properly setting positions and sizes of the coupling slots etched on the layer interface. Meanwhile, four slotlines are etched on the bottom layer of the substrate to improve CM suppression and band to band isolation. A four order prototype is designed for demonstration. Good agreement between simulated and measured results is obtained.