Backward directional coupler for weak coupling of double-sided microstrip lines using longitudinal double-slots

A backward directional coupler for double-sided microstrip lines is presented. Aligned microstrip lines are coupled via two parallel slots in the common ground plane. This design works well for weak coupling smaller than 20 dB and has some advantages over the single-slot coupler. Geometrical dimensions of the proposed coupler design are presented for coupling values of 20, 25 and 30 dB, together with measurements for a 30 dB coupler.