A novel miniaturized forward-wave directional coupler loaded with periodic shunt inductors

A novel miniaturized forward-wave directional coupler loaded with periodic shunt inductors is presented. Considerable phase difference between even- and odd-mode is obtained. This contributes to enhanced forward coupling and size reduction. A simple but accurate equivalent model of a unit- cell is developed to predict the dispersion relations and the characteristic impedances for both even- and odd-mode. Besides, by controlling the total length of the coupler, the input impedances of the both two modes can be perfectly matched. A test sample is fabricated on FR4 substrate. The coupling level of 1.1 dB can be measured at 3.9 GHz with the total length 1.04 λ_g.