Intrinsic impedance

In problems of electromagnetic wave propagation in a transmission medium, the electromagnetic impedance, also known as the intrinsic impedance, is defined as the ratio of the electric to magnetic field amplitudes::$eta\; =\; \{E\; over\; H\}$For a region with slight electrical conductivity ($sigma\; >\; 0$, e.g. seawater), the equation becomes::$eta\; =\; sqrt$j omegamu } over {sigma + j omega varepsilon} }where $omega$ is the frequency of the radiation, $mu$ is the permeability of the medium and $varepsilon$ is the permittivity of the medium.

In a region with no conductivity ($sigma\; =\; 0$, e.g. free space), the expression simplifies to::$eta\; =\; sqrt\{mu\; over\; varepsilon\}\; approx\; 377\; Omega$ (for a vacuum)

Electromagnetic impedance is not to be confused with electrical impedance.