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.
Power density
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,