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From: "Talbot Andrew" <ACTALBOT@dera.gov.uk>
To: rsgb_lf_group@blacksheep.org
Subject: LF: RE: DFCW
Date: Tue, 30 Jan 2001 11:12:47 -0000
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The only problem with two tone signalling is if using very narrow
bandwidths where signalling information is lost, such as I do from time
to time to look right down in the noise.  

In the event where Rx bandwidth is less than the signalling rate,
coherence between signalling elements is necessary to see the carrier
frequency, as the switching now becomes sidebands that are filtered out.
Therefore, one FSK  tone will not be visible in narrow bandwidths where
on-off keying would have been.   The only solution, and not possible
with a DDS source, is to use two independent frequency generators, one
for each tone, and switch between them such that each oscillator stays
running at a single frequency continuously.  That way each tone can
separately be observed at ever narrower bandwidths until it eventually
pops out of the noise.   The thing I ask is that the DFCW be transmitted
at a rate slower than the QRSSSSSS (sic)  was.

Provided the source and propagation medium is stable enough, it will
_always_ be possible to see a signal at a narrow enough bandwidth and
this does appear to be the case.    I have a raw data file recorded at
1.95Hz sampling rate of last Friday / Saturday's transmission over a 20
hour period.   This has not been processed yet, but while it was being
recorded, on the real time spectrum display I'm almost convinced I saw
Larry's signal several times during the day as well as most of the night
time period. It was very very weak, but by sighting sideways  across the
screen a line appeared to be there in the noise in exactly the right
place.  That was in a 0.008Hz bandwidth with the trace across the screen
width showing 11 hours of recording.

In the original representations to the Radio Agency around 1995 about
getting an amateur LF allocation and setting an ERP level Julian, G3YGF,
made some back of envelope calculations and, assuming 1W ERP in 1mHz
bandwidth with standard propagation curves, showed that a transatlantic
QSO was feasible - hence the request for 1W ERP based on this evidence,
which the RA agreed to.    
It looks as if at long last his calculations are being shown to be
reasonably accurate.  It has only taken 6-7 years.

Andy  G4JNT


> -----Original Message-----
> From: Larry Kayser [mailto:kayser@sympatico.ca]
> Sent: 2001-01-30 10:13
> To: rsgb_lf_group@blacksheep.org
> Subject: LF: DFCW, my error
> 
> 
> Greetings:
> 
> A while back I incorrectly stated that DFCW as a two tone 
> system was not
> useable for the weak signal work I am doing.
> 
> My assumption, and it can be no less than that was wrong, 
> Rik, ON7YD has
> gone to a two level FSK system that is fully usable in the 
> model I proposed.
> 
> Sorry for the screw up on my part Rik.
> 
> Larry
> VA3LK
> 
> 
> 


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