Concept for the implementation of very high directivity and decade bandwidth in compact microstrip directional couplers

A concept to induce a very high directivity and a flat coupling over a large bandwidth in compact microstrip directional couplers is presented. By introducing lumped element based compensation networks at the isolated and coupled port of a simple electrically short directional coupler, a very high directivity and a constant coupling over enormous bandwidths up to one decade can be achieved. With a design focusing on flat coupling, a high power directional coupler with a coupling length of only 50 mm, less then λ/25 at the middle of the band, was shown to have a coupling in the range of 52.11 dB ± 0.05 dB from 37.4 MHz to 200 MHz. In a further design step with focus on directivity and coupling flatness at the same, the coupling stayed within 55.25 dB ± 0.35 dB from 22 MHz to 164 MHz with a directivity of greater than 50 dB (30 dB) from 35 MHz to 84 MHz (20 MHz to 187 MHz).