From Wikipedia, the free encyclopedia
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
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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,
or by rearranging the terms
Numerical values
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Impedance of free space
is roughly
Since a
half wave dipole
is used, its
gain
over an isotropic antenna (
) should also be taken into consideration,
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
.
[2]
In this case,
See also
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]
References
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]
- ^
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