Message-ID: <4BA3187E.9010801@abelian.org>
Date: Fri, 19 Mar 2010 06:23:58 +0000
From: Paul Nicholson <vlf0308@abelian.org>
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Subject: Re: VLF: TA, 300m and new PA?
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Stefan schrieb:
 > what would be the TX power i have to apply to a 300m
 > vertical to reach Canada/USA,

Well, to extrapolate so far from a single data point would be
very silly.   But silly can be fun, so ....

Given

  B = sqrt( 9.5e-21 * ERP/r) * exp( -r/a)

where a = 2.9e6 for daytime path, or 4.3e6 for nighttime
path, day and night charts are plotted here,

  http://abelian.org/vlf/prop9-day.png
  http://abelian.org/vlf/prop9-night.png

You will have to cross 5200km to Nova Scotia, or 6500km to US
east coast.   Assume a nighttime path where the rx is still
just in daylight and current noise levels.   Then 30mW ERP is
detectable in Nova Scotia and 100mW ERP is needed for east USA.

 > => antenna efficiency at 8,97kHz will be 0,0064%.

Then 500W needed for Nova Scotia, 1.5kW for east USA.
That's just to get a reliable detection with say 10-30 minutes
averaging of pure carrier in 5mHz bandwidth and assumes
the propagations doesn't spread the signal much beyond that
bandwidth.   To actually get a message across would take a
bit more.    As I said the other day, it will need every bit
of ingenuity...

And anyway, there is nobody in North America with a suitable
rx at the moment!

Remember, those propagation charts are just a guess.  If we
bother to dig into the relevant textbooks, the field strength
can just be calculated - all the basic research was done a long
time ago - lots of cold war propagation studies and scientific
research.   But often it is fun to work things out for oneself,
to enjoy the sense of discovery.   At the moment I think those
charts are inaccurate at long distance - maybe not quite enough
difference between day/night to account for observed diurnal
noise level changes.  Perhaps with some arithmetic - integrating
average worldwide lightning activity weighted by the above
formula to get an average background estimate...

For me, the power levels for this long DX are getting worryingly
high as they will at some point start to cause interference to
research activity.  The VLF band is heavily used for geophysical
research and nobody is supposed to be transmitting there -
despite what the UK and German licensing authorities may say.
Someone familiar with ITU agreements may be able to clarify
the position.  Soundings into the research community don't bring
an encouraging response, varying from "what's the point?" to some
degree of horror.  I am anxious to identify a genuine use for
amateur signals - something beyond the usual pointless beeping.
--
Paul Nicholson
--