Many transmission lines are composed of thin metal layers, or traces, patterned on a dielectric substrate. Variations of the line over ground plane.

Note that other types of lines are possible they would simply have different shape and/or distributions of dielectric material in the cross section. The correspondences between true TEM lines and several common quasi-TEM lines are shown in the figures below. However, if this component of the fields is relatively small compared to the components in the cross-sectional plane, then even a quasi-TEM line can be modeled using the same excitation types as used for the TEM cases. Once any loss is introduced, either via an Impedance boundary condition or a lossy material, or if there are different dielectrics present, the transmission line becomes quasi-TEM, meaning that there will be some electric, or magnetic, field component in the direction parallel to the line. Bottom right: Parallel plate, with the green lines representing Perfect Magnetic Conductor symmetry conditions. The electric field (red) and magnetic field (blue) are purely in the cross-sectional plane between the two perfect electric conductors (black) and the dielectric within (light blue) is homogeneous. Images of the four true TEM transmission lines. The electric and magnetic fields lie purely in the cross-sectional plane of the line, and the Poynting flux is everywhere parallel to the line. For the other cases, the fields extend infinitely far into the surrounding space, but fall off to zero very quickly.

For the cases of the coaxial cable and parallel plate, the fields are entirely confined between the two conductors. The space between the PEC boundaries is a homogeneous and lossless dielectric. For a true TEM line, the metal is assumed to be an ideal conductor of infinite conductivity, and it is modeled as a Perfect Electric Conductor boundary condition. There are four kinds of perfect TEM transmission lines, shown in the image below, including the:Īll of these consist of two metallic domains, arranged in various configurations. Modeling TEM and Quasi-TEM Transmission Lines

Transmission line geometry