Dipole field strength in free space, in telecommunications, is the electric field strength caused by a half wave dipole under ideal conditions.
The actual field strength in terrestrial environments is calculated by empirical formulas based on this field strength.
Let N be the effective power radiated from an isotropic antenna and p be the power density at a distance d from this source[1] Power density is also defined in terms of electrical field strength; Let E be the electrical field and Z be the impedance of the free space The following relation is obtained by equating the two, or by rearranging the terms Impedance of free space is roughly
Since a half wave dipole is used, its gain over an isotropic antenna (
) should also be taken into consideration, In this equation SI units are used.