Field strength in free space
Field strength in free space, in telecommunications, is the 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
In this equation SI units are used.
Expressing the same equation in:
is equivalent to multiplying the expression on the right by . In this case,
References and notes
- Reference data for radio Engineers, Howard W.Sams co,Indianapolis, 1956, 27-7
- K.H.Kaltbeitzer: Site selection, EBU Techhnical Monograph 3104,Bruxelles,1965, p 30