Wave impedance: At a point in an electromagnetic wave, the ratio of the electric field strength to the magnetic field strength.

Note 1: If the electric field strength is expressed in volts per metre (V/m) and the magnetic field strength is expressed in ampere-turns per metre (A·t / m), the wave impedance will have the units of ohms. The wave impedance, Z , of an electromagnetic wave is given by the equation:

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

For free space, μ is 4π × 10^-7 H/m (henries per metre) and ε is 8.854 × 10^-12 F/m (farads per metre), from which Z ≈ 377 ohms is obtained.

In a perfect dielectric (where conductivity is zero), the wave impedance is 377/n, where n is the refractive index. In a lossy dielectric or a conductor (where conductivity is greater than zero), the wave impedance will be a complex number.

Note 2: Although the ratio is called the wave impedance, it is also the impedance of the free space or the material (medium).

Note 3: The symbol η (eta) is often used instead of Z for wave impedance to avoid confusion with electrical impedance.

Source: from Federal Standard 1037C and from MIL-STD-188