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.

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 R 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 ${\displaystyle 120\cdot \pi }$

Since a half wave dipole is used, its gain over an isotropic antenna (${\displaystyle {\mbox{2.15 dBi}}=1.64}$ ) should also be taken into consideration,

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