Message-ID: <001701c08199$411d6a00$a328fc3e@p.cleal>
From: "Peter Cleall" <peter.cleall@virgin.net>
To: "rsgb_lf_group" <rsgb_lf_group@blacksheep.org>
Subject: LF: Receiver insensitivity
Date: Thu, 18 Jan 2001 21:07:31 -0000
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I tried to send this message 3 times last week it apparently got held up as
I had started the subject line with the word "help" the software at
Blacksheep thought it was some sort of command but could not act on it.
Hopefully with a more sensible subject line we will be alright this time.

thanks
G8AFN

I have been investigating my receive signal to noise ratio, which seems to
be about
20  to 30dB less than many people. My antenna is a 30 m longwire at about 6m
average height to a Lowe 225 rx. Before I embark on a lengthy investigation
I would like to confirm the signal to noise ratio people receive DCF39 at
around Europe in an SSB bandwidth.

After discussion with various people at the LF forum at Windsor I have.
a) fitted a G3yxm pre amp
b) added a 0 to 50 dB switched attenuator, 0.5dB steps.
c) Rewound my in-line longwire loading coils for higher Q
d) built a tapped parallel tuned circuit, similar to what is advocated for
transmit purposes, to tune the loaded antenna against ground.

Everything appears to resonate.

These changes have resulted in about 6 to 8 dB improvement.
I  can now hear DJ, PA, ON and F regularly at signals very close to the
noise floor.
Using "spectrum" on these signals shows s/n ratios of about 6dB.


It is frustrating to hear GM3YXM/p working F6CNI and find that, although I
am
500km nearer, F6CNI is still only RST 239 with me.

I still cannot see VE1ZZ as an identifiable callsign, even when G3XDV
reports it as being at a level that was nearly audible, I am still looking
at a "T" condition signal at best.

Comparing the measurements of CFH taken at ON7YD suggestion before Christmas
again shows my signal having a low dynamic range, compared with other
stations.

I have just taken Rik's reported signal to noise measurements from his top
five stations of last weekend and recalculated what they would have been if
they had been taken at this QRA using his system. Again I am deficient by 20
to 30dB in s/n ratio

These look  like  clues, but something fundamental must be wrong, I do have
a major
manufacturing plant about 850m away and an  NDB about 900m away.

I cannot however invest in any enormous antenna farms nor any fancy new
receivers

I plan to measure DCF39 several times both day and evening over the next few
days, around 0800hrs and 2000hrs using the Spectrum programme  to measure
signal to noise level and would appreciate any comparative measurements.

I shall use my rx in USB mode, set to 137kHz, which should give a mid scale
trace when spectrum is set to
frequency window 1750 Hz to 1940 Hz
waterfall scroll 250 msec
Sample rate 11025
FFT size 16384
FFT Av 3
Log output

I then plan to measure the carrier frequency which will be seen on 1830 Hz
and the background noise 20 Hz higher.
If you are able to help please e-mail your measurements, together time and
with QRA,
direct to avoid cluttering up the reflector.

many thanks

Peter Cleall G8AFN