Message-ID: <2000042620532768199@zetnet.co.uk>
Date: Wed, 26 Apr 2000 20:53:27 +0100
To: rsgb_lf_group@blacksheep.org
From: "Peter Dodd" <g3ldo@zetnet.co.uk>
Subject: Re: LF: Re: Transatlantic  beacon tests
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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

<g3ldo@zetnet.co.uk>