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Downlink sensistivity, preamps and attenuators, LEILA

There have been a lot of messages posted recently about the need to improve
the performance of our downlink receiving systems.  It seems now that a
modified surplus downconverter with a high noise figure and low gain
receive antenna such as a small helix is just not good enough.  Not only do
you need too much uplink power and will suppress those of us with more
modest uplink systems, but you will also cause QRM to QSOs you cannot hear. 

If the period of experimental transponder operation has taught us
something,  then in my opinion it has been very worthwhile.  Amateur radio
is supposed to be about self training and this new bird is teaching us all
a lot!  We can't expect to be told everything up front!

Some time ago on this bb there was a debate about the supposed
disadvantages of having too much gain in the downconverter and various
articles in QST and Feedpoint were quoted in support of this.  There were
recommendations given to insert attenuators between the converter and the
radio to reduce the "filter broadening" effects in the radio which were
going to worsen our receivers.  In the light of this advice, there may be a
temptation to add an attenuator in the system to "compensate" for the
higher receiver noise level arising from the extra gain of a preamp fitted
to reduce the noise figure. 

While the jury is still out on whether an "S7" noise level really degrades
receiver sensitivity as has been suggested, I believe that you should NOT
rush into fitting ANY attenuators ANYWHERE in the system, without either
doing a system noise budget calculation (there have been some very
informative postings on this subject here on the bb recently), comparing
system noise figure by ground/sky tests with and without the attenuator, or
ideally by comparing receive performance on the downlink with and without
the attenuator.  There is a risk that fitting the attenuator will undo some
of the improvement you just got by fitting the preamp! 

I have also been listing to LEILA in operation, and can confirm that it
does go off by itself sometimes, or there is someone on the satellite doing
a good impression of LEILA!!  It has appeared on top of my downlink signal
on occasions where a) I was about at a level of about -12dBMB b) I was
uplinking on 1269MHz!  Neither of these situations should have triggered
it.  I have also  heard it go off on an empty channel with no-one on the
downlink (I can be sure there was no-one there having adequate downlink
sensitivity to hear plenty of transponder noise). 

One last thought - transponder noise could be used as a useful low-level
"beacon" for checking out your system.  If you can hear transponder noise,
then you can be fairly sure your system is working reasonably well. If not,
then you still have some way to go.  The ideal situation to aim for,  is to
have enough transponder noise (say 10dB above your receiver noise floor) so
that the limit on your receiver sensitivity is not your receiver's noise,
but that from the satellite.  Improving your system beyond that is then not

Transponder noise can be checked by tuning to a clear frequency when the
transponder is in operation  and moving the antenna off the satellite to an
area of clear sky and looking for a change in receiver noise level (just
like measuring sun noise). 


Charlie G3WDG

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