Optimum dielectric overlay thickness for equal even- and odd-mode phase velocities in coupled microstrip circuits

Abstract

An optimum dielectric overlay thickness was found for achieving the equal even- and odd-mode phase velocities in coupled microstrip lines. The theoretical results were calculated based on a variational method. Coupled linear resonator structures were used to measure both the even- and odd-mode effective dielectric constants and the results agree very well with the theory. A directional coupler, built with overlay, has greater than 60 dB isolation as compared with 20 dB isolation for an uncompensated coupler.