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A compact multilayer substrate integrated waveguide (SIW) balun bandpass filter is proposed. A 1808 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 28) over the passband are achieved for the balun bandpass filter.Introduction: As one of most important RF front-end functional passive devices, the balun bandpass filter integrating balun and bandpass filter is a necessary component for converting a balanced signal into an unbalanced one for a specified bandpass characteristic in RF/microwave communication systems. In the past few years, several techniques have been proposed to design narrow/dual-band, and wideband balun bandpass filters [1][2][3][4]. The characteristics of balun and bandpass filtering can be easily realised in these filter structures. However, the application of these filters is limited because the optimisation of the in-band balun performances is not convenient to realise.In the authors' previous works [5,6], two planar substrate integrated waveguide (SIW) magic-T structures with wide band and simple design based on E-plane power dividers were proposed. Compared with the T/Y-junction planar SIW power divider structures [7], the size reduction of the circuits is almost 50 and 75%, respectively. In this Letter, a compact balun bandpass filter with four inductive posts based on the multilayer SIW power divider [5] is designed. A high-order passband (centre frequency 12.5 GHz, bandwidth 4.8%) with high selectivity and extended stopband for the balun filter can be achieved.

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