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Re: AO-40 spreadsheet



>
>
>
> This the reason why the SSB units seem to hear better-- they do-- but we
can
> improve our more reasonably priced Transystems and Drakes by adding more
low
> noise gain up front.
>
> Fred W0FMS

Hi Fred, (et al) (Fred knows this stuff, I'm sure. This is just a
summarizing post)

Yes, that is what I have been "preaching" since I started reading about all
the problems people were having with the Transystems and Drakes. They
desperately need a preamp.

There is nothing to panic about....these downconverters are just what they
are...downconverters...they ain't preamps! The simple addition of a low
noise preamp in front of them and most people are going to be very happy.
(assuming the image is sufficiently surpressed in the downconverter... I
know some people are quite concerned about it, and should be).

For some reason, some people are turning this into a contest between the SSB
Units and the Surplus Units. It isn't. If either "system" is done "right",
the results will be very similar.

It's really this simple:

The surplus converter approach takes a lot more effort and attention to
modification, but will save a TON of money, even after buying a 150 dollar
preamp.

The SSB UEK's work right out of the box, but you pay for the privilege.

So...you can spend 180 bucks and do the hard work, or spend 270 more and
plug and pray. (that you don't tx thru the UEK!).

The goal is straightforward, lowest possible MDS. Here's how you get it:

1. Biggest antenna you can afford/build/keep in the air, no less than 20 dBi
2. Lowest system NF you can get, no more than 1 dB
3. Enough gain in the preamp to set the "system" NF.

It doesn't matter how you do it...but the Drake, Transystem and other
surplus downconverters by themselves simply are NOT adequate. Put a good low
NF preamp in front of them and there is no reason why they won't work every
bit as well as the SSB.....IF and only IF the proper modifications have been
done to the surplus units to make sure there are not image problems or other
anomalies cropping up from the "conversion".

The original "illusion" that these 20 or 30 buck converters with a bbq grill
antenna system were going to work just fine has been shattered. There ain't
no free lunch. One lunch is, however, much cheaper than the other, if you
have the parts, competence and test equipment to accomplish it.

I saw some initial discussion from the converter owners saying this:

"Just what kind of NF and GAIN are we really getting from these units?"
They were asking because the performance was quite mediocre, given what
values had been talked about. I've yet to see a straight answer. Ignoring
the image problems while measuring NF leads to the infamous 3 dB error.
Based on the best I've seen talked about on the sig, it seems the NF is
going to be 6 dB or higher, with no preamp.

I'm still waiting to hear from someone who can competenly measure NF,
(including eliminating the image problem) to publish accurate information
about each of these models. Then we can try to figure out why so many people
are having problems with them working anywhere near as well as "was
advertized".

It has been an interesting exercise in link budgeting and we'll learn more
as we plod through it all. Every time a new weak signal bird comes on line,
we go through it "one more time". Every time, there is a new "shortcut".
..that comes up very short.

73

hasan schiers, NØAN
schiers@netins.net

----- Original Message -----
From: "Frederick M. Spinner" <fspinner@hotmail.com>
To: <amsat-bb@AMSAT.Org>
Sent: Friday, May 25, 2001 7:49 PM
Subject: Re: [amsat-bb] AO-40 spreadsheet



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