Dear Luis, LF Group,
To calculate ERP, one needs to determine the radiation resistance of the
antenna, Rrad, and the antenna current, Iant. Rrad depends on the geometry
of the antenna and the wavelength:
Rrad = 160*pi^2*(Heff/lambda)^2
Where Heff is the "effective height" of the antenna, which depends on its
shape and size. For simple wire antennas, Heff is nearly equal to the
average height of the conductors, so for a plain vertical wire it is about
1/2 the overall height, and for a T or inverted L with very long horizontal
wires, it is nearly equal to the overall height. You can find some formulae
to work out Rrad for different antenna geometries at
http://wireless.org.uk/on7yd/ . For your antenna, Heff works out to about
13m, so Rrad is about 0.74ohms at 500k.
The ERP is then:
Perp = D*Iant^2*Rrad
D is the directivity of the antenna compared to a free-space half-wave
dipole, for a short vertical over an infinite ground plane D is 1.8
(2.62dB). The information we do not have is Iant. The best way to find Iant
is to measure it with a RF ammeter. Or you can calculate Iant if you know
the TX power and the RF resistance at the feed point of the antenna.
Typically antenna resistance at 500k might be between several ohms and
several 10s of ohms - let us assume it is 20ohms for your antenna. In that
case, with 35W TX power, Iant = 1.3A. Using the formula above, Perp = 2.3W.
The actual ERP is likely to be reduced by a few dB if there are many
buildings and trees, etc, around the antenna.
A more definitive ERP measurement can be obtained by measuring the field
strength, E, of the signal at distance d some km from the antenna, in which
case:
Perp = (Ed)^2 /49
Measuring E requires a calibrated antenna and receiver - but that is another
story...
Cheers, Jim Moritz
73 de M0BMU
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