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Re: LF: Re: Transatlantic beacon tests

To: [email protected]
Subject: Re: LF: Re: Transatlantic beacon tests
From: "Peter Dodd" <[email protected]>
Date: Wed, 26 Apr 2000 20:53:27 +0100
Reply-to: [email protected]
Sender: <[email protected]>
Larry said

 What is needed is a suitable very narrow bandwidth receiver design to go with 
the DDS effort we
are making over here with the AD9835 DDS device.
I am also prepared to share in the Transmitter effort with alternate nights
from this side with a 240 ft high tower with a partial umbrella antenna on
it.  My Sulzer Labs standard at the transmitter will easily keep 2 parts in
10 -10th.  A reliable large final amplifier is critical to this effort, one
that does not need any baby sitting and can be left running for weeks on end
in a dark, cool transmitter building.

Some of Andy's comments

For the narrow bandwidth Rx, if you want to start from scratch what
about this as an outline scheme ?
Start off with a DDS derived LO locked to a standard oscillator, and
downconvert the LF band directly to 32765 Hz.  Make up a ladder filter
using watch crystals - I have a first cut design for a five crystal
device with a bandwidth of 1.1 Hz.  Subsequently convert the output down
to 1 - 2 Hz  'baseband', again with a locked LO.   A/D convert directly
at a sampling rate of, say, 10 Hz to 16 bit accuracy, again with the
sampling clock locked to the frequency standard and feed the output to a
PC via the serial port or even the parallel port.   At this data rate
soundcards are irrelevant and any 16 bit language running in DOS can do
some very advanced signal processing.
Phew!
All this high-tech, big-gun approach is going to leave me behind.


Could we look at it in a wider perspective?

Most of the main achievements on the LF bands have been acomplished using relatively low-tech and modest equipment. What you might call an amateur approach.
Long distance marginal links appear to be open for short durations.
This means that the transmissions cannot be spread out over a long
time, as would be necessary for very narrow band techniques.
On the other hand narrow band techniques, as currently employed by
most operators, can give a an improvement of between 15 and 20dB over
aural CW methods and seems the best compromise.
The currently available FFT S/W and the soundcard also offers the
added bonus of simplicity, which enables lots of non-techies like
myself to participate.
A lot is made of frequency stability. Spectran displays 18Hz of the
band in the horizontal mode when set to 0.064Hz resolution. A signal
with a frequency stability of + or - 5Hz can be tracked long as it
doesn't bump into one of the many Loran lines. (if it drifts into a
Loran line it will drift aout again!). The worst situation you can
have is a very stable amateur signal on top of a Loran line.
In practice most QRSS signals on the band appear to have a frequency
stability better that +/- 0.5Hz and encounters with Loran lines are rare.

At the moment, I understand that there are two beacons operating in
the USA but I am not sure if they have any keying that would
identifiable with Spectran set at say, 0.064Hz.
So let us go for the amateur approach to exploring transatlantic
propagation. We can start anytime.


--
Regards, Peter, G3LDO

<[email protected]>






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