Wave impedance: At a point in an electromagnetic wave, the ratio of the electric field strength to the magnetic field strength.
Note 1: If the electric field strength is expressed in volts per metre (V/m) and the magnetic field strength is expressed in ampere-turns per metre (A·t / m), the wave impedance will have the units of ohms. The wave impedance, Z , of an electromagnetic wave is given by the equation:
Where μ is the magnetic permeability, ε is the electric permittivity and σ is the conductivity of the material the wave is travelling through. In the equation, i is the imaginary unit, and ω is the angular frequency of the wave. In the case of a dielectric (where conductivity is zero), the equation reduces to:
For free space, μ is 4π × 10-7 H/m (henries per metre) and ε is 8.854 × 10-12 F/m (farads per metre), from which Z ≈ 377 ohms is obtained.
In Wave impedance a perfect dielectric (where conductivity is zero), the wave impedance is 377/n, where n is the refractive index. In a lossy dielectric or a conductor (where conductivity is greater than zero), the wave impedance will be a complex number.
Note 2: Although the ratio is called the wave impedance, it is also the impedance of the free space or the material (medium).
Note 3: The symbol η (eta) is often used instead of Z for wave impedance to avoid confusion with electrical impedance.
Source: from Federal Standard 1037C and from MIL-STD-188
Electronics Topics
The field of electronics is the study and use of systems that operate by controlling the flow of electrons or other electrically charged particles in devices such as thermionic valves and semiconductors. The design and construction of electronic circuits to solve practical problems is part of the fields of electronic engineering, and the hardware design side of computer engineering. The study of new semiconductor devices and their technology is sometimes considered as a branch of physics.
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