Stepped impedance microstrip couplers with improved directivity

This paper presents the design of microstrip couplers with improved directivity. In order to compensate for the unequal phase velocities of even- and odd-mode in inhomogeneous media, the coupler is subdivided into multiple sections. It is shown that by properly choosing the lengths, the impedances, and the coupling coefficients of the corresponding sections, the coupler's directivity can be optimized. The design procedures can easily be applied using commercially available simulation software. Measurements confirm the design methodology and show good agreement with the simulation results.