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Re: AO-40 spreadsheet



OK.. well the Drake doesn't have the image noise problem, unless you create 
one by modifying it incorrectly.  But its nigh NF unless you fix it, and I 
don't think the MGA MMIC is enough.  A outboard preamp is, though..(1.4 dB 
NF system) the combo would probably be super (0.6-0.7 dBNF System).

The Transystem does have an image problem (but it's not ugly! :-) )-- but 
that can be easily fixed.

And actually a Drake w/a preamp or a Transystem with the proper mods (1.4 db 
NF system) can work AO-40 fine, just 3 dB more deaf than the SSB.

I should stress under most circumstances the Drake w/preamp and the 
Transystem with proper mods will work fine with the AO-40 downlink with >20 
dBic antennas.  But at the 1.4 dB or so of System NF, there will be a 3 dB 
higher MDS.  This isn't itself a problem when you have enough gain in front 
of the converter to cover it.  The SSB setup gives you more margin.

It'd be a wrong assumption to believe that all MMDS guys will be alligators 
(hell, I'll be one!!)... at 3 dB difference, there is a noticible 
difference, but not night & day-- I.E.  you may hear and understand 
something at the noise floor with your SSB whereas a MMDS guys couldn't-- 
but I think the MMDS guy would hear it, just not understand it.  It'd be 
closer than you think. 1 dB is a barely perceptable difference for a trained 
communicator.  3 dB is perceptable..

Fred W0FMS

>From: "hasan schiers" <schiers@netins.net>
>To: "Frederick M. Spinner" <fspinner@hotmail.com>, <amsat-bb@AMSAT.Org>
>Subject: Re: [amsat-bb] AO-40 spreadsheet
>Date: Fri, 25 May 2001 21:35:20 -0500
>
> >
> >
> >
> > 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, NĜAN
>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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