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