TEMmode

Transverse electromagnetic mode (TEM) is a class of wave in which the electric and magnetic fields are entirely transverse to the direction of propagation. In a TEM wave, there are no field components along the axis of travel; E and H fields lie in planes perpendicular to the propagation direction. TEM modes are supported by transmission lines comprising two or more conductors with a return path for current, separated by a homogeneous dielectric. Typical examples include coaxial cables and parallel-plate lines; in these structures the line supports a finite voltage between conductors without exciting longitudinal fields.

Due to the boundary conditions, TEM modes have no cutoff frequency and can, in principle, propagate from zero frequency. The phase and group velocities equal the speed of light in the dielectric (approximately c/√ε_r for a homogeneous dielectric). The characteristic impedance Z0 is determined by geometry and the dielectric; common coaxial lines yield Z0 values such as 50 Ω or 75 Ω in practice. In an ideal lossless, uniform line, the energy is stored equally in electric and magnetic fields.

In practice, the dominant mode in many transmission lines is TEM. In microstrip and other planar structures, the mode is often called quasi-TEM because the dielectric is not uniform and fields extend into air. At sufficiently high frequencies or particular geometries, higher-order TE or TM modes can be excited, introducing dispersion and mode conversion.

Summary: TEM mode describes a transverse-only electromagnetic field on a two-conductor transmission line with a homogeneous dielectric, explaining the behavior of coaxial cables and many RF transmission lines, while recognizing limitations in nonuniform dielectrics and waveguides that do not provide a return path.