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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 unitIn mathematics, the imaginary unit i allows the real number system R to be extended to the complex number system C . Its precise definition is dependent upon the particular method of extension. The primary motivation for this extension is the fact that no, and ω is the angular frequencyIn physics (specifically mechanics and electrical engineering), angular frequency ω (also called angular speed is a scalar measure of rotation rate. Angular frequency is the magnitude of the vector quantity angular velocity . The term angular freque of the wave. In the case of a dielectricMost generally, a dielectric is an insulator, a substance that is highly resistant to flow of electric current. Layers of such substances are commonly inserted into capacitors to improve their performance, and the term dielectric refers specifically to th (where conductivity is zero), the equation reduces to:
For free spaceIn physics, free space is a concept of electromagnetic theory, corresponding roughly to the vacuum, the baseline state of the electromagnetic field, or the replacement for the electromagnetic aether. It occurs in the expressions permittivity of free space, μ is 4π × 10-7 H/m ( henriesThe henry (symbol H) is the SI unit of inductance. If the rate of change of current in a circuit is one ampere per second and the resulting electromotive force is one volt, then the inductance of the circuit is one henry. The henry has dimensions V·A-1·s per metre) and ε is 8.854 × 10-12 F/m ( faradThe farad (symbol F is the SI unit of capacitance (named after Michael Faraday). A capacitor has a value of one farad when one coulomb of charge causes a potential difference of one volt across it. Its dimensions in SI units are: : Since the farad is a ves per metre), from which ≈377 Ohms is obtained.
In a perfect dielectricMost generally, a dielectric is an insulator, a substance that is highly resistant to flow of electric current. Layers of such substances are commonly inserted into capacitors to improve their performance, and the term dielectric refers specifically to th (where conductivity is zero), the wave impedance is 377/n, where n is the refractive indexThe refractive index of a material is the factor by which electromagnetic radiation is slowed down (relative to vacuum) when it travels inside the material. For a non-magnetic material, the square of the refractive index is the material's dielectric const. 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
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