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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.
  1. 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.
  2. Quasi-TEM Mode
    This is the mode which approximates a true TEM mode for sufficiently low frequencies.
    Description: 40389.png
  3. Waveguide Mode
    In this mode, either EzHz 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.

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