----- Original Message -----
From: "James Moritz" <[email protected]>
To: <[email protected]>
Sent: Saturday, February 16, 2008 1:56 AM
Subject: LF: Re: NOV UPDATE
> 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.
Again Jim by your own admission you could be between 3 - 6 db either way,
in reality ie 2x to 4x the permitted 1w, ie 2w - 4w or 0.5w- 0.25w.
>
> > 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
When a tolerance is specified, you mention a few db, that means as I have
stated above a mere 3db doubles the power or 6 db increases it by 4 times.
The point that I am making is except calculations and field results are the
exact, which they seldom are especially in an amateur radio context, the
experiment only puts one in the ball park.
Having said that even if there is an anamoly of 3 - 6 db in ones favour it
only makes the received signal a max of 1S point stronger, hardly
discernable amongst the general noise on LF/MF, however one should aim to
achieve the permitted power and not exceed it.
I have covered other considerations before, like field strength near/far
field measurements.
Mal/g3kev
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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