Coupling between two-conductor multi-layer shielded microstrip transmission line

The quasi-static TEM mode of a shielded microstrip transmission line is analyzed. The transmission line consists of two perfectly conducting coplanar strips located between two ground planes. Four different dielectric regions are considered: two layers of substrates, a rectangular notch below the strips, and an overlay or a superstrate. The effects of the structure parameters of the shielded transmission line on the odd and even modes' phase velocities and coupling capacitance are studied. Once the phase velocities are equalized, the design of distortionless lines is achievable. It is found that the presence of the shield allows the reduction of the interline spacing with minimum coupling. With the proper selection of the height of the shield and the separation distance between the strips, it is possible to equalize the phase velocities at more than one combination of the substrate heights.