Walter et al :-
Doesn't specifying a dipole as the radiator automatically take antenna
aperture into account, and cancels out frequency from the equation ? So
if you take just the spreading out of the total power over the area of a
sphere :
Watts per m^2 = P . / (D ^ 2 x 4.pi)
and E ( V/m ) = SQTR( Power x. Resistance of free space )
so Volts / m = SQRT(W/m^2 * Z free space) = SQRT(P x 377 /
4.pi / D^2)
= SQRT(P) * 30 / D
A dipole has a gain of 1.64 (power) so its "voltage gain" could be said
to be 1.28
so I reckon the formula should read E (V/m) = 38.4 . SQRT(P) / D
(all in SI units of course)
Some way off a value of 7, but in the right region. Can anyone spot if
I've slipped a digit or something ?
Andy G4JNT
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From: Walter Blanchard[SMTP:[email protected]]
Reply To: [email protected]
Sent: 2000-06-07 10:25
To: [email protected]
Subject: LF: Formula
I found the following in an article recently.
Quote:
"The intensity E (known as the field strength) of a transmission
at a distance D from a source transmitting P watts of RF power via
a half-wave dipole in a free, unobstructed space, can be estimated
using the formula:
E=(7*sqrt (P))/D.
Thus, for a 2 watt transmitter, the theoretical field strength in free
space at one metre distance is approximately 10 V/m and at 100 metres
distance 0.1 V/m"
This formula is new to me and I can't find it in any
textbooks I have. Since it ignores frequency shouldn't
it be power flux per unit area and not field strength?
Can anyone tell me where it comes from?
Walter G3JKV.
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