Re: [amsat-bb] AO-40 Reception

["Edward R. Cole" <al7eb@xxxxxxxxxxxxx>]

From: "Frederick M. Spinner" 
>Has anyone else out there used a Drake with an outboard preamp--
>I did all the mods that don't require spending $$$ on the Drake
>so far (not the MGA MMIC, Toko Filter)-- which supposedly puts the thing in 
>the 6dB ballpark (I'll find out at Dayton I suppose).. I have an old DEM 
>2400 MHz (LUA) preamp, measured at CSVHF 2000 to have 11.8 dB gain, and a 
>0.82 dB NF.
>
>Interestingly enough, the Drake behind that preamp seems more sensitive than 
>the OLD DEM downconverter behind the same preamp.
>Which is wierd as the DEM converter should have the same to nominally
>less NF.. (It does have less conversion gain, though)...
>
>I guess I'll need to pick up one of those transystems at Dayton and
>try it out..
>
>Fred W0FMS

Fred et al:

Interesting that the lineup of DEM preamp-->Drake performs better.  You
state the DEM has 0.82 dBNF and 11.8 dB gain.  The Drake has 6 dBNF? [and
if not measured that is really ??] with something like 15 dB gain if all
you did was clip coils/caps and add a cap.

The stand alone DEM convertor is probably about 2.5 dBNF and ?gain [I'll
guess 25 dB].  I ran my NF spreadsheet:

DEM preamp+Drake:
NF = 1.62
NT = 132K
signal threshold = -137.0 dBm

DEM preamp+DEM conv:
NF = 1.02
NT = 76.6K
signal threshold = -137.4 dBm

Only 0.4 dB better SNR.  I guess that says the preamp is doing its job if
the converters are performing equally.

BTW I plan on running minimally modified Drakes, myself.  One on 123 MHz IF
with only the IF mods, and the other on 433.9 MHz as modified by my freind
WL7BQM.  I have ordered the Kuhne MKU-232A preamp which is about 0.5 dBNF
with 35 dB gain.  I will drive both Drakes thru a coax tee so all I need do
is select which IF line I want.

Here is what I get from the spreadsheet:

MKU-232A+Drake:
NF = 0.51
NT = 36.1K
signal threshold = -137.7 dBm

Now 35 dB gain may overdrive the Drakes, so adding 10 dB attenuation my NF
= .61 and NT = 44K with no discernable change in threshold.

Preamp should be here Monday or Tuesday so we'll see.

Ed




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