Its presently accepted value is[1] where Ω is the ohm, the SI unit of electrical resistance.
The impedance of free space (that is, the wave impedance of a plane wave in free space) is equal to the product of the vacuum permeability μ0 and the speed of light in vacuum c0.
However, with the revision of the SI that came into force on 20 May 2019, the impedance of free space as expressed with an SI unit is subject to experimental measurement because only the speed of light in vacuum c0 retains an exactly defined value.
[3] It has numerous other synonyms, including: From the above definition, and the plane wave solution to Maxwell's equations,
Between 1948 and 2019, the SI unit the ampere was defined by choosing the numerical value of μ0 to be exactly 4π × 10−7 H/m.
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 c to be precisely 3×108 m/s in conjunction with the then-current definition of μ0 as 4π × 10−7 H/m.
For example, Cheng 1989 states[3] that the radiation resistance of a Hertzian dipole is 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