A novel compact backward-wave directional coupler design with defected ground structure

A novel compact backward-wave directional coupler with defected ground structure has been presented. The proposed coupler operates in the frequency band where both even- and odd-mode are in the stopbands. The even- and odd-mode Bloch impedances become purely imaginary numbers and the coupling level is now related to the attenuation length (ad) along the line. A simple but accurate equivalent model of a unit-cell is developed to predict the attenuation constants as well as the Bloch impedances for both even- and odd-mode. A test sample is fabricated on Duroid substrate with the coupled-line length equal to 0.34λ_g. The coupling level over 1.3 dB can be measured from 3.3 GHz to 6.7 GHz, which corresponds to a fractional bandwidth of 68%.