Wave impedance

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 meter (V/m) and the magnetic field strength is expressed in ampere-turns per meter (A·t / m), the wave impedance will have the units of ohms. The wave impedance, Z , of an electromagnetic wave is given by the following equation, where μ is the magnetic permeability and ε is the electric permittivity:

For free space, these values are 4π × 10^-7 H/m (henries per meter) and (1/36π) F/m (farads per meter), from which 120π ohms (≈377Ω) is obtained. In dielectric materials, the wave impedance is 377/n, where n is the refractive index.

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

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