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Re: AMSAT-BB-digest V2001 #343

In a message dated 5/26/01 2:10:34 AM GMT Daylight Time, 
owner-AMSAT-BB-digest@AMSAT.Org writes:

<< I have considered adding an integrated preamp inside a Drake 2880.  I 
happen to have a couple MMIC preamps with 14 dB gain and a 2.5 NF:
 This little (and I do mean "little") device will fit easily between the 
input N-connector and the RF amp.  All it needs is +5 Vdc, which is available 
right from the 7805 inside the Drake, and one 1,000 pF decoupling capacitor.  
I know this would not be an "ultimate" mod, but it looks relatively simple 
(and cheap) and should help with both the NF and gain limtiations of the 
Drake.  It calcs to a 4.3 dB Beacon S/N improvement on the W3PM calculator 
(thanks Gene!).  That's better than adding another antenna!
 I intended to do this 3 months ago and just simply have not had time to get 
to it.  If any of the more experienced/knowledgeable RF-gurus out there know 
of any show-stoppers with this mod, please chime in.  Now where are my 
tweasers and magnifying glass...
 Jerry, K5OE >>

Hi Jerry.

My only thought is how do you earth the device?  Long earth leads can cause 
instability in MMICs and it's a pain to have to remove the board and fit 
either pins or a couple of 'wrap rounds' for less than 1 dB advantage over 
the simple mod's performance.
I wonder if there is a better performance device?  It may be possible to find 
a 1.6dB device similar to the MGA86576 but with lower gain, which hopefully 
would give better stability with a non-ideal earth connection.

Alternatively, The ATF36077 PHEMT is cheap and can give 0.4dB NF on good PTFE 
material. I wonder what would happen if the existing Drake GaAs FET was 
replaced with a 36077?  you would have to lower the voltage a little and 
probably replace the input inductor with a less lossy piece of wire, but it 
could be spectacular.


David  G0MRF
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