[Lowfer] DX record?

[Les Rayburn]

Thanks to Stewart for this humorous take on why
pursing weak signals will likely remain an attraction
on LF for years to come.

Years ago, I wrote an article for the Lowdown suggesting
the use of the Madienhead grid square system for
Lowfers; in an effort to encourage lowfers to move
away from QRO operation. At the time, I thought
that the VHF/UHF/SHF models for pursing weak
signal records made good sense on LF. For the record,
I still do.

On the higher bands, stations routinely go to extraordinary
lengths to extend a record by just a few miles. I can see
similar things happening on LF.

While I fully understand those who long for tools that might
make "real time" QSO's easier, there are also quite a few
of us who just want tools (like WOLF) that continue to
push the envelope on weak signal reception.

73,

Les Rayburn, N1LF






> > It's a known fact that if you integrate a signal long enough,
> > eventually it will be detectable in the noise. The trick is to
> > find the technique that best meets your own personal interests.
>
>If you don't have a trick, "eventually" will be a long time indeed.  I
>thought it might be interesting to estimate the performance of simple
>integration over an LF skywave path.
>
>If you have a signal which can barely be copied using QRSS with one
>second dots, and you cut the transmitter power in half, you can
>compensate by going to two seconds per dot, and making appropriate
>adjustments in the Rx software.  However, if you were originally using
>one hour dots, halving the power would require you to use _four_ hours
>per dot, even if propagation remained relatively constant.  That's
>because random phase variations imposed by the path limit the minimum
>bandwidth to something like one millihertz, even if your Tx and Rx are
>locked to GPS.  With constant bandwidth, although doubling the signal
>duration doubles the received signal energy, the integrated noise also
>increases by square root of 2, so it takes four times the duration to
>compensate for having half the power.  On LF, IMO, this "square law"
>requirement starts at about ten minutes per dot.
>
>Now, imagine that you just set the world DX record for a one watt
>lowfer signal, receiving a three character ID sent with QRSS-600,
>which neatly fit in a five hour window of good propagation.  You ask
>the sending operator to choose a new message, cut his Tx power to 100
>mW, repeating the message every five hours until you have integrated
>enough signal to copy.  You have an automated receive system which
>will sound an alarm when the message has been detected.  How long
>might it take?
>
>Your first thought is that since the square law applies, one would
>need 100 repeats, about three weeks.  That would not be awful, to set
>the definitive 100 mW record.  But of course, the good propagation
>won't last that long.  Let's assume, optimistically, that average S/N
>is only 6 dB worse.  The square law applies here, too, so we now need
>about 1600 repeats, roughly a year.
>
>But of course, the season doesn't last that long.  If summer noise is
>10 dB above winter levels, the average level might be 7 dB above the
>quiet season.  Again applying the square law, it looks like about 25
>years are required.
>
>But of course, lowfer beacons don't run that long.  I'll ignore
>mundane interruptions like blown-down antennas and lightning-fried
>MOSFETs.  If the sending operator loses interest in the hobby after
>say, 15 years, and his kid picks it up after another 15, then your
>great-grandson might hear the alarm sometime around 2102.
>
>He shouldn't rush to whatever has replaced the Internet to proclaim
>his success.  The record will have long been eclipsed by someone who
>had a trick.
>
>73,
>
>Stewart KK7KA
>
>
>
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Les Rayburn, director
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