Broad-band slot-coupled microstrip directional couplers

The paper describes a new design technique for directional couplers in a printed strip-above-ground microwave transmission system (microstrip). The two transmission lines are mounted back to back and coupled through slots in the common ground plane. The coupling factors of isolated slots of different shape are calculated and are found to be in good agreement with measured results. A specific design of 3dB coupler is described in which the coupling mechanism consists of 50 transverse slots graded in length. By grading the phase velocity in one of the two lines a power split equal within 1 dB over the band 2800¿4300 Mc/s is achieved.