Multi-octave planar microwave slot-coupled directional coupler up to 28 GHz with novel phase velocity compensation

A nonuniform symmetrical QTEM-mode directional slot-line coupler operating in a multi-octave frequency range from 2.5 GHz up to 28.6GHz is presented. The calculated continuously varying even- and odd-mode line impedances are realized by a 3D electromagnetic (EM) field simulation of the slot-coupled microstrip cross section. A new method of even- and odd-mode phase velocity compensation, based on a continuously structured prepreg, is suggested. A -8.34 dB coupling factor is achieved, showing a low amplitude and phase imbalance up to 28 GHz. Furthermore, a tandem configuration is evaluated by utilizing two -8.34 dB directional coupler to form a wideband -3.01 dB coupler.