Note 1: If the electric field strength is expressed in volts per meter and the magnetic field strength is expressed in ampereturns 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 MILSTD188
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