Design of compact directional couplers for UWB applications

This paper presents a simple design method for a class of compact couplers, which offer coupling in the range of 3-10 dB over an ultra-wide frequency hand from 3.1 to 10.6 GHz. The proposed couplers are formed by two elliptically shaped microstrip lines, which are broadside coupled through an elliptically shaped slot. Their design is demonstrated for a 3-, 6-, and 10-dB coupling assuming a 0.508-mm-thick Rogers RO4003C substrate. Results of simulation and measurements show that the designed devices exhibit a coupling of 3 +/- 1 dB, 6 +/- 1.4 dB and 10 +/- 1.5 dB across the 3.1-10.6-GHz band. This ultra-wideband coupling is accompanied by isolation and return loss in the order of 20 dB or better. The manufactured devices including microstrip ports occupy an area of 25 mm x 15 mm.