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Re: [AMSAT-BB] 145 MHz IF on V/s?

At 09:37 PM 5/16/2008, K & R Yoksh wrote:

>  Hello Gene (and the list),
>Your question reminds me of a question I've been meaning to ask!
>My current S-band antenna is a simple integrated antenna/downconverter with
>123 MHz IF, but I plan to install a P3E-ready dish and downconverter on my
>crossboom this summer.
>My best downconverter is a K5GNA unit with 145 MHz IF. I wonder how it'll
>play on mode V/s?  Can anyone comment on this?
>I also have some more 123 MHz IF units, but their NF is unknown. I'd rather
>use my AIDC 3731 from K5GNA, even if it means reduced performance on V/s. I
>plan to optimize for HEO operation, but I'd be happy if I could also work
>AO-51 in mode V/s.

Hi Kyle,

Just a suggestion...

All downconverters will drift with temperature. The stock AIDC 3731
is spec'd to be within +/- 25 KHz over the temp range. I don't know
what the 145 MHz units are spec'd at but I doubt that it is better.
Other units I have seen are also around this figure and some are a
little worse.

This is not much of a problem on AO-51 becuase there is a nice
strong carrier that is easy to find. On a linear passband like
P3E, it can be extremely difficult to find your own downlink
without QRMing the whole passband.

A better solution and the one that I use, is to put a low
noise preamp at the antenna and put your downconverter inside
your house where the temp is pretty stable. I have mine in the garage
which was fine for AO-40. Leaving the downconverter on all the time
is also helpful. On AO-40, my setup was more stable than the LO
on AO-40 and I could always hear my own downlink. I used InstantTune
(of course!) and loaded the parameters into the config file so it was
simple to just turn it on and work the sat. A fine tuning of around +/-1.5 KHz
was all that was needed to compensate for AO-40 LO drift.

I used an AIDC 3731 downconverter at 123 MHz which is pretty
sensitive but not as good as with the Down East Microwave 13ULNA preamp
which is really, really! good. A second advantage of using a preamp at the
antenna is that the downconverter sensitivity is then not very important
so you can use almost anything, even an old Drake unit.

If you have a really long run of coax, it might be cheaper to add a second
less expensive "booster" preamp and then use RG-6 coax from the preamp to the
downconverter. I used LMR-600 because I wanted to be able to TX on it but
that is overkill for just receiving P3E.

Tony AA2TX

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