Impedance of free space

The impedance of free space, Z0, is a physical constant relating the magnitudes of the electric and magnetic fields of electromagnetic radiation travelling through free space. That is, Z0 = |E|/|H|, where |E| is the electric field strength and |H| is the magnetic field strength. It currently has an exactly defined value

The impedance of free space (more correctly, the wave impedance of a plane wave in free space) equals the product of the vacuum permeability μ0 and the speed of light in vacuum c0. Since the values of these constants are exact (they are given in the definitions of the ampere and the metre respectively), the value of the impedance of free space is likewise exact. However, with the redefinition of the SI base units which are going into force on May 20, 2019, this value is subject to experimental measurement.

Terminology

The analogous quantity for a plane wave travelling through a dielectric medium is called the intrinsic impedance of the medium, and designated η (eta). Hence Z0 is sometimes referred to as the intrinsic impedance of free space,[1] and given the symbol η0.[2] It has numerous other synonyms, including:

  • wave impedance of free space,[3]
  • the vacuum impedance,[4]
  • intrinsic impedance of vacuum,[5]
  • characteristic impedance of vacuum,[6]
  • wave resistance of free space.[7]

Relation to other constants

From the above definition, and the plane wave solution to Maxwell's equations,

where

μ0 is the magnetic constant,
ε0 is the electric constant,
c0 is the speed of light in free space. [8] [9]

The reciprocal of Z0 is sometimes referred to as the admittance of free space and represented by the symbol Y0.

Exact value

Since 1948, the definition of the SI unit ampere has relied upon choosing the numerical value of μ0 to be exactly 4π × 10−7 H/m. Similarly, since 1983 the SI metre has been defined relative to the second by choosing the value of c0 to be 299792458 m/s. Consequently,

exactly,

or

This chain of dependencies will change when the ampere is redefined on 20 May 2019. See New SI definitions.

Approximation as 120π ohms

It is very common in textbooks and papers written before about 1990 to substitute the approximate value 120π ohms for Z0. This is equivalent to taking the speed of light c0 to be precisely 3×108 m/s in conjunction with the current definition of μ0. For example, Cheng 1989 states[2] that the radiation resistance of a Hertzian dipole is

( not exact).

This practice may be recognized from the resulting discrepancy in the units of the given formula. Consideration of the units, or more formally dimensional analysis, may be used to restore the formula to a more exact form, in this case to

See also

References and notes

Further reading