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RE: Downconverter noise



Phil, Jon

> > Should I have an S 5 reading just with the downconverter sitting on
> > the bench,
> > or should I only expect an increase in noise when the downconverter is
> > fixed
> > to the dish?
>
> I would hope not.  If the noise level of your d/c is S5 w/no antenna
> and no signal, then I think  you would have a problem.  I don't know
> why the instruction sheet says that.  Are you sure that's correct?
> These are supposed to be relatively low noise d/cs.

I might have misunderstood the question, but of the 50 or so AIDC 3731's
I've distributed here in the UK, all of them have produced a very
significant increase in noise level and S meter reading when hooked up to
the half a dozen different radios I've used them on (and no antenna, dish or
feed connected). S5 is about ball park but due to the vagueries of S meters
it might be S3 or it might be S7. Either way, when plugging in the IF output
of a powered d/c you should hear a very large increase in noise when the
radio is listening on SSB. You won't here any difference if listening on FM.

Check your radio's AGC is turned on - the AGC level is often what's used to
make the S meter move. Also check that the RF gain on your radio is up, and
that there is no attenuator switched in.

Subsequently adding the feed and dish to the d/c's antenna input will have a
relatively small effect.

You might like to check out http://www.g6lvb.com/AO40HowToRx.htm for some
practical tips.

> > Another question, at the focal point is a thick circular aluminium
> > plate which
> > has three grooves machined in it and a central hole about 70 mm in
> > diameter.
> > The fellow that I bought the dish from referred to it as a choke. I
> > have had
> > the patch aerial mounted directly to that choke, could that be a
> > problem?
>
> Depends on the frequency of the choke.  You might want to cover it over
> with some foil tape or a plate of your own.

Mmm... I wonder if this is a choke ring on the feed? But the description you
give doesn't really reflect this. If it is a choke ring these are often used
on feeds to neaten up the radiation pattern to more efficiently illuminate
the dish and/or cut down side lobes or overspill. Either way it is probably
very close to the focal point and placing your patch there should be fine.

73, Howard G6LVB

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