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The wave impedance of an electromagnetic wave, is the ratio of the transverse components of the electric and magnetic fields (the transverse components being those at right-angles to the direction of propagation). For a transverse-electric-magnetic (TEM) plane wave travelling through a homogeneous medium, the wave impedance is everywhere equal to the intrinsic impedance of the medium. In particular, for a plane wave travelling through empty space, the wave impedance is equal to the impedance of free space. The symbol Z is used to represent it and it is expressed in units of ohms. The symbol η (eta) may be used instead of Z for wave impedance to avoid confusion with electrical impedance, although η is also the symbol for electromagnetic impedance, the light wave equivalent of wave impedance.

The wave impedance is given by

Z = {E_0^-(x) \over H_0^- (x)}.

where E_0^-(x) is the electric field and H_0^-(x) is the magnetic field.

In terms of the parameters of an electromagnetic wave and the medium it travels through, the wave impedance is given by

Z = \sqrt {j \omega \mu \over \sigma + j \omega \varepsilon},

where μ is the magnetic permeability, ε is the electric permittivity and σ is the electrical conductivity of the material the wave is travelling through. In the equation, j is the imaginary unit, and ω is the angular frequency of the wave. In the case of a dielectric (where the conductivity is zero), the equation reduces to

Z = \sqrt {\mu \over \varepsilon }.

As usual for any electrical impedance, the ratio is defined only for the frequency domain and never in the time domain.


Wave impedance of free space

In free space, \scriptstyle \mu=4\pi \times 10^{-7} H/m and \varepsilon\approx 8.854\times 10^{-12} F/m. So, the value of wave impedance in free space is

Z_0 \approx 377\,\Omega \approx 120 \pi\,\Omega.

Wave impedance in an unbounded dielectric

In a perfect dielectric, \scriptstyle \mu=4\pi \times 10^{-7} H/m and \varepsilon = \varepsilon_r \times 8.854\times 10^{-12} F/m. So, the value of wave impedance in a perfect dielectric is

Z \approx {377 \over \sqrt \varepsilon_r }\,\Omega.

In a perfect dielectric, the wave impedance can be found by dividing Z0 by the refractive index. In anything else, the formula becomes larger and a complex number is the result.

Wave impedance in a waveguide

For any waveguide in the form of a hollow metal tube, (such as rectangular guide, circular guide, or double-ridge guide), the wave impedance of a travelling wave is dependent on the frequency f, but is the same throughout the guide. For transverse electric (TE) modes of propagation the wave impedance is

Z = \frac{Z_{0}}{\sqrt{1 - \left( \frac{f_{c}}{f}\right)^{2}}} \qquad \mbox{(TE modes)},

where fc is the cut-off frequency of the mode and for (TM) modes

Z = Z_{0} \sqrt{1 - \left( \frac{f_{c}}{f}\right)^{2}} \qquad \mbox{(TM modes)}

For a waveguide or transmission line containing more than one type of dielectric medium (such as microstrip), the wave impedance will in general vary over the cross-section of the line.


PD-icon.svg This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).

See also



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