Directional coupler with two octaves band and high directivity using stepped-impedance coupled structure

A directional coupler using stepped-impedance broadside coupled structure is presented. The design is targeted to have 3-dB coupling with more than 30 dB directivity across a two-octave band (more than 110% fractional bandwidth). The second-order even- and odd-mode analysis method is adopted to design the device and calculate its performance. Moreover, a prototype is modelled in a simulation tool to validate the design procedure. The simulation results indicate 3-dB coupling with less than 1 dB deviation, more than 35 dB isolation and 30 dB return loss, and less than 4° of phase imbalance across the band from 1 to 4 GHz.