Dipole field strength in free space

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
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
Numerical values.
Impedance of free space is roughly formula_5
Since a half wave dipole is used, its gain over an isotropic antenna (formula_6 ) should also be taken into consideration,
In this equation SI units are used.
Expressing the same equation in:
is equivalent to multiplying the expression on the right by formula_8. In this case,