Dear Mal, LF Group,
You wrote:
> Most of what you say is theoritical and guesswork, in reality you could be
> at least 10 db out and that makes a big difference.
> Your figures in the past on 137 khz varied between 6 and 10 db by your own
> admission.
Actually, what I said is backed by hundreds of practical measurements by
myself and others. With care, the results are very consistent; I find that
field strength measurements are repeatable within +/-1dB for a particular
antenna.
The measured ERP with an antenna in an "open field" location will agree
quite accurately with the theoretical ERP calculations from antenna current
and dimensions. Antennas in less-than-ideal environments give measured ERP
that is less than the theoretical ERP. This is not surprising, since the
simple theoretical formulae make no attempt to model environmental effects.
For the antennas I have tested at my QTH, the reduction in ERP is about 3 -
6 dB. This is a characteristic of a particular antenna and its environment,
not any kind of error.
> Amateur radio design and applications on MF and LF are not an exact
science.
> Mal/G3KEV
No experimental science is exact, but using simple calculations and
measurements with a tape measure and an RF ammeter, you can come up with a
figure for radiated power that is within a few dB of the correct value at LF
or MF. This is remarkable when compared to antenna measurements in most
other parts of the spectrum.
Cheers, Jim Moritz
73 de M0BMU
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