On the limitation of the weak-coupling assumption for crosstalk analysis based on quasi-TEM propagation

The limitation of the weak-coupling assumption for the case of two transmission lines with a common ground is examined. The analysis is based on the mathematical structure of solutions to the telegrapher's equations, and, therefore, quasi-transverse electromagnetic (TEM) propagation is assumed. All loss is neglected, and the lines are assumed to be symmetric. Requirements that are necessary for the weak-coupling assumption to hold in a homogeneous medium as well as in a medium that possesses certain types of inhomogeneities are derived by examining both the fully coupled and the weakly coupled forms of the telegrapher's equations. It is shown that terminating impedances influence the validity of the weak-coupling solution locally to resonances but have little effect elsewhere. Theoretical and experimental results are given in both the frequency and time domains for several geometries. A solution to the weak-coupling form of the telegrapher's equations is provided.>