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A compact multilayer substrate integrated waveguide (SIW) balun bandpass filter is proposed. A 180° reverse phase characteristic between the input/output ports can be easily achieved by the middle metal-ground of the SIW power divider. Four inductive posts are added to the multilayer SIW power divider to realise a passband. Good in-band balance performance (amplitude and phase imbalance are less than 0.35°dB and 2°) over the passband are achieved for the balun bandpass filter.