Topic summary
TEM mode
Extracted from the Wikipedia article Transverse mode.
Conductor-based transmission lines
In coaxial cable energy is normally transported in the fundamental TEM mode. The TEM mode is also usually assumed for most other electrical conductor line formats as well. This is mostly an accurate assumption, but a major exception is microstrip which has a significant longitudinal component to the propagated wave due to the inhomogeneity at the boundary of the dielectric substrate below the conductor and the air above it. Inhomogeneity also occurs at connectors or bends in a coaxial cable. Non-TEM modes created by connectors are usually negligible unless the signal has a high enough frequency. This is referred to as maximum extraneous-mode-free operation or simply mode-free operation frequency of the connector.