Transmission Line Modes
Transmission-line mode is the distinct pattern of electric and magnetic field induced on a transmissionline under source excitation.
Assuming the transmission line is oriented with its axis along the z-axis (direction of wave propagation), transmission line modes may be classified as follows.
- Transverse Electromagnetic (TEM) Mode
In this mode, the electric and magnetic fields are transverse to the direction of wave propagation with no longitudinal components [Ez = Hz = 0].
TEM modes cannot exist on single-conductor guiding structures. TEM modes are sometimes called transmission-line modes since they are the dominant modes on transmission lines. Plane waves can also be classified as TEM modes.
- Quasi-TEM Mode
This is the mode which approximates a true TEM mode for sufficiently low frequencies.
- Waveguide Mode
In this mode, either Ez, Hz or both are nonzero. Waveguide modes propagate only above certain cut-off frequencies. Waveguide modes are generally undesirable on transmission lines such that we normally operate transmission lines at frequencies below the cut-off frequency of the lowest waveguide mode. In contrast to waveguide modes, TEM modes have a cut-off frequency of zero.