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Re: Re: Extra gain stage in DEM preamp?




I use an older W5LUA DEM (self biased) preamp, and noticed that I needed 
some more gain with the system, too.  I did the modification in the (old) 
Mode-S: The Book where a MAR-6 is added to the old DEM/LUA preamp.  I can 
now hear noticably better.  I can actually hear fairly well now with a 
two-foot dish instead of the four footer I was using earlier.

This makes some sense.  At 2.4 GHz, we are a little out of the ratings of 
the MAR-6, but it still works well.  Even if the NF is out to 4 dB
(Its probably 3.5 dB or so at 2400 MHz) and the gain is down to 15 dB, this 
actually improves the system noise figure a little bit.  (as compared to the 
5.5 dBNF Drake)  And in my case I now have a noise floor at about S-1/2 in a 
cold sky.  It really seems to help to have the S-meter be active all of the 
time for system comparisons, etc.  Also I can run RG-58 from the front of 
the dish to the back without much worry, even if I lose 2-3 dB by doing it.  
It is a nice mechanical solution.

I dont know if there is enough room to cut in a ERA-2 or a MAR-6 into the 
new DEM preamp, but this is where I'd consider doing it if you are driving a 
Drake with it (then you don't have to worry about stability problems with 
the Drake).  Another option is to add one of those 2 dB NF HP MMICS like 
some people are doing to the Drake.  But as a second stage amp, you don't 
gain that much going from 2 dB to 3.5 dBNF..

Fred W0FMS


>From: K5OE@aol.com
>To: <w1pa@hotmail.com>
>CC: <amsat-bb@AMSAT.Org>
>Subject: Re: [amsat-bb] Re: Extra gain stage in DEM preamp?
>Date: Tue, 23 Oct 2001 12:27:11 EDT
>
>Bill,
>
>w1pa@hotmail.com> writes:
>
> > http://ozoni.com/n4ip/DSCN0492.JPG
> > Pieter N4IP did that (an ERA2, I believe). Don't
> > remember if he was happy with the results.
>
>Yes, that is Pieter's preamp mod.  I talked to him at Atlanta 2 weeks ago 
>about installing a different device in the long strip between the FET and 
>the output coupling capacitor, and he advised me that strip was used to 
>match the output impedance.  So... where he has the MMIC mounted after the 
>cap appear the best place, even if a little "tight" (in case others are 
>tempted to go the easy route).
>
>My testing here indicates the Drake can use a little more front end gain 
>than the DEM 13ULNA provides.
>73,
>Jerry, K5OE
>----
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