Wave impedance

Note 1: If the electric field strength is expressed in volts per meter and the magnetic field strength is expressed in ampere-turns per meter, the wave impedance will have the units of ohms. The wave impedance, Z , of an electromagnetic wave is given by 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, i.e., 377, ohms 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