Design of Closed Coupled Line with Step Eigen Modes Impedance

Abstract

Tightly coupled lines are widely used for wide band directional couplers, filters and baluns(1)(2). Though the phase velocity of TEM modes constructed in the uniform medium takes the equal values, the coupled line made on the surface of the dielectric substrate, however, are often used to make it easy. Sometimes, the technologies are used for the band pass filter and others. In this case, the phase velocity of eigen modes such as an even mode and odd mode take the different values, and the performance of the circuits using the coupled line are quite deteriorated. This paper, therefore, proposes the technology to give the delay function for either mode such as the even mode for the symmetrical coupled line to make same values of both eigen modes. Especially, the directional coupler of two microstrip lines coupled through the aperture with step impedance of the even mode are shown with the design theory and the experimental results comparing without step impedance.