Wideband patch directional coupler based on non-uniform mushroom structure

A wideband patch directional coupler is proposed. The coupler is designed based on the non-uniform mushroom structure with unique characteristics. The current distribution within the patch can be controlled by the mushroom cells underneath for wideband property. An 8 dB directional coupler has been designed, fabricated and measured for verification. It exhibits good return loss, small coupling coefficient variation and quadrature phase difference among the output ports over a wide bandwidth of 74.6% centred at 8.45 GHz.