Intrinsic impedance

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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 (σ > 0, e.g. seawater), the equation becomes

\eta = \sqrt{{j \omega\mu } \over {\sigma + j \omega \varepsilon} }

where ω is the frequency of the radiation, μ is the permeability of the medium and \varepsilon is the permittivity of the medium.

In a region with no conductivity (σ = 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.

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