A novel compact 3-dB forward-wave directional coupler with periodic capacitive enhancement

A compact forward-wave directional coupler using periodic capacitive loadings has been proposed. The phase difference between the even and odd modes can be significantly enlarged, which makes the size of the coupler more compact. A simple equivalent model of the unit cell is developed to predict the electrical lengths and the Bloch impedances for both even and odd modes. A test sample is fabricated on an FR4 substrate. The coupling level of 3.4 dB can be measured at 1.7 GHz with a small electrical length of 0.19λg.