*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:

*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