The difference between phase velocities of the odd and even modes in inhomogeneous microstrip coupled lines makes the microstrip directional coupler a poor device regarding directivity. In this work, this issue is successfully compensated by the addition of a matching network at the ports of coupled lines. Furthermore, the proposed matching network, composed of a single segment of microstrip line, was extensively studied providing significant insights for the design of a compact and broadband device. As result, simple design equations based on coupled lines S-parameters are obtained, as well as the achievable operation limits of the proposed matching network. Finally, in order to validate the proposed design methodology, a compact microstrip directional coupler is constructed to operate with directivity higher than 30 dB and coupling of 17 dB over a 600-MHz bandwidth.

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