Date: Sat, 21 Jun 2003 12:41:31 +0100
From: "james moritz" <J.R.Moritz@herts.ac.uk>
To: rsgb_lf_group@blacksheep.org
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Subject: Re: LF: BEACONS
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Dear Mal, LF Group,

--On 21 June 2003, 09:15 +0100 hamilton mal <g3kev.ham@virgin.net> wrote:r

> Recently 137.770 freq area has been a complete shambles with several
> transmissions simultaneously in beacon mode all at 60 sec dot length and
> very close together generating other frequencies, resulting in
> considerable frequency spread.  I imagine there could be difficulties
> even at a distanct location trying to resolve and identify anyone in the
> mess.  73 de Mal/G3KEV 

Several stations very close together is of course what we are trying to do
- it would make life more difficult for Laurence in Ghana if we were
distributed widely all over the band 

Looking for a weak QRSS signal in an otherwise unoccupied part of the band
is easy - almost any adjustment of the RX gain will work, provided the
receiver noise remains below the band noise band noise . But receiving
multiple QRSS signals with widely different strengths requires a more
careful approach. Remember that the filtering that defines the RX bandwidth
does not take place until the signal is inside the computer, so as well as
the normal intermods in the front end, one must also be concerned about
distortion occuring in the IF, product detector and audio stages of the
receiver, and the sound card - this is what gives rise to the intermod
products you were seeing on the spectrogram. Few if any receivers are
designed with this mode of operation in mind, so some optimisation of gain
distribution is required. The procedure I use is like this:

-Ensure AGC is completely disabled
-Adjust RF/IF gain level so that the output level produced by the strongest
signal is well below the level at which the RX output starts clipping
-Adjust the audio level so that the AF output is undistorted, and that the
sound card is not saturated by the strong signals.

This will probably end up with much lower gain settings than you usually
use - certainly, if intemods appear on the spectrogram, or blocking effects
where the strong signals darken the rest of the display, the gain is too
high at some point. The fact that you can no longer hear the weaker signals
or the band noise in the RX audio is not a problem - the spectrogram
software can still pick them out. It also helps to reduce the contrast of
the spectrogram display compared to what you would usually use, to
accommodate a wider range of signal levels. Sometimes, it also helps to
reduce the frequency resolution of the spectrogram - eg. use Argo in 30s
mode rather than 60s - this has the effect of making the "key clicks" on
the strong signals narrower, so they obscure less of the weak signals .
Spectrum Lab is more flexible in regard to the settings you can use than
Argo is for this type of operation, allthough it is more complicated to set
up.

Observing these precautions, I have had no problem copying  CT1DRP and
RN6BN in the presence of other UK stations over the last few nights - all
the signals were quite clean, so your "frequency spreading" problem is
definitely at the RX end. During previous T/A tests, it was not difficult
to copy the DL, I, etc stations in the presence of several big signals from
the UK that were 50 - 60dB stronger.

Cheers, Jim Moritz
73 de M0BMU