[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

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

Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
Not an AMSAT member? Join now to support the amateur satellite program!
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org