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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?0 and the speed of light in vacuumc0. Before 2019, the values of both these constants were taken to be exact (they were given in the definitions of the ampere and the metre respectively), and the value of the impedance of free space was therefore likewise taken to be exact. However, with the redefinition of the SI base units that came into force on 20 May 2019, the impedance of free space is subject to experimental measurement because only the speed of light in vacuum c0 retains an exactly defined value.
The reciprocal of Z0 is sometimes referred to as the admittance of free space and represented by the symbol Y0.
Historical exact value
Between 1948 and 2019, the SI unit the ampere was defined by choosing the numerical value of ?0 to be exactly . Similarly, since 1983 the SI metre has been defined relative to the second by choosing the value of c0 to be . Consequently until the 2019 redefinition,
It is very common in textbooks and papers written before about 1990 to substitute the approximate value 120? ohms for Z0. This is equivalent to taking the speed of light c0 to be precisely in conjunction with the then-current definition of ?0 as . For example, Cheng 1989 states 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