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

:$\mbox = \frac$

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

:$\mbox = \frac$

The following relation is obtained by equating the two,

:$\frac= \frac$

or by rearranging the terms

:$\mbox =\frac$

Numerical values

Impedance of free space is roughly $120 \pi ~ \Omega$

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

:$\mbox =\frac \approx 7\cdot\frac$

In this equation SI units are used.

Expressing the same equation in:
: kW instead of W in power,
:km instead of m in distance and
:mV/m instead of V/m in electric field

is equivalent to multiplying the expression on the right by $\sqrt$.K.H.Kaltbeitzer: ''Site selection'', EBU Techhnical Monograph 3104, Bruxelles, 1965, p 30 In this case,

:$\mbox \approx 222\cdot\frac$
:

See also

*Antenna (radio)
*Effective radiated power
*Electric field
*Field strength meter
*Radio propagation model

References

{{Radio frequency propagation models

Telecommunication theory
Antennas
Broadcast engineering
Radio frequency propagation model