Compact substrate-integrated waveguide bandpass rat-race coupler and its microwave applications

A reduced size rat-race coupler incorporating a bandpass frequency response characteristic is presented and designed based on substrate-integrated waveguide (SIW) technology. By use of electric- and magnetic-coupling structures between adjacent SIW cavity resonators, a conventional rat-race coupler and a bandpass filter with a 200-MHz bandwidth at a centre frequency of 7.75 GHz can be integrated as one component resulting in miniaturisation. A design process based on the coupled-resonator network is introduced to synthesise such a bandpass coupler with desired bandwidth, return loss and coupling. A Butler matrix and a six-port junction with bandpass response are then developed based on the proposed rat-race coupler. They represent excellent candidates for miniaturised transceiver and direct conversion systems. The measured results are in good agreement with the simulated predictions.