Side-coupled-ring medium-band directional coupler

A directional coupler based on a quarter-wavelength side-coupled ring is presented. Observation on the parameters that control the response of the directional coupler is conducted. While the length of the one-wavelength ring determine the center frequency, the reflection, transmission and isolation characteristics of the coupler can be varied by varying the impedance of the ring elements, which are the even- and odd mode impedance of the coupled-lines, Z_oe and Z_oo, and the ring line impedance, Z_r. The proposed concept is tested in the design of a directional coupler at 2 GHz, using microstrip on FR4 substrate with characteristics of: ϵ _r= 5, h = 1.6 mm and tan δ = 2.10^-2. The side-coupled ring directional coupler is fabricated, and measurement results show good agreement with the ones from simulation.