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Re: Downlink sensistivity, preamps and attenuators, LEILA



Charles,

     In general you are probably correct, but we should say "some" surplus
converters won't do the job.  I will not argue that it can't be improved,
but I will argue the TranSystem converter will work just fine out of the
box.  I have yet to trigger LEILA, but I have made many QSOs.  The only time
I couldn't hear myself was when the AGC was being hammered.

73,
Joe
ka0yos@amsat.org

----- Original Message -----
From: "Charles Suckling" <charles.suckling@ntlworld.com>
To: <amsat-bb@AMSAT.Org>
Sent: Friday, May 18, 2001 2:32 AM
Subject: [amsat-bb] Downlink sensistivity, preamps and attenuators, LEILA


> There have been a lot of messages posted recently about the need to
improve
> the performance of our downlinks to avoid the need for excessive power on
> the uplink.  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.
>
> 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 not
> worthwhile.
>
> Transponder noise can be checked by tuning to a clear channel 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).
>
> 73
>
> Charlie G3WDG
>
> ----
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