A Miniaturized Ultrawideband Directional Coupler

This letter presents a miniaturized ultrawideband (UWB) directional coupler, which can offer a wide range of coupling over a wideband. The coupler uses two series and one shunt coupled arm, along with a phase-compensating transmission line section. The sum of the electrical lengths of all the arms responsible for coupling is only 48% of that of a traditional three-section directional coupler (3-SDC), while still providing comparable bandwidth performance. A comparison with other recently reported wideband planar directional couplers shows that it occupies a smaller area and can be fabricated using a single-layer printed circuit board (PCB). A lossless transmission line model is used to obtain closed-form expressions for the S-parameters. A prototype 10 dB coupler is fabricated, occupying an overall size of $0.13~\lambda ^{2}_{\text {g}}$ at $f_{0}= {2.5}$ GHz, and offers at least 105% effective bandwidth.