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Re: Best pre-amp for Mode S




>From: "Karl Sandstrom" <k5man@cox.net>
>I am in the process of building my AO-40 receive station and am considering
>the purchase of a DB6NT pre-amp to complement a Cal-Amp downconverter (50db
>gain) and a 90 cm dish.  The folks at DB6NT offer a single and dual stage
>pre-amp.  They have 15 db and 35 db gain, respectively.
>
>Are there any thoughts on which one would be better for this installation?
>Is there such a thing as too much gain?
>

Karl,

I am using the two-stage db6nt preamp with my stock (unmodified) Drake
converters, and I would recommend it.  The proper way to determine this
would be to use AO40v2.0 or later to analyze your whole system.  If the Cal
amp has good NF (less than 2 dB) you may be OK with the single-stage amp.
But take care making assumptions about these surplus converters.  My Drake
probably is only 6-to-8 dBNF and having 15 dB gain.  It definitely benefits
form the extra preamp gain (calculations using AO40v2.0 indicate that I
need a min of 25 dB preamp gain to minimize system NF).

You can overdrive succeeding amplifiers with excess gain but not sure these
converters are that succeptible.  My Drake isn't negatively affected by the
40 dB (listed in the accompanying spec.) gain from my preamp!

I don't think you will do any harm by chosing the two-stage preamp.

Ed
mode-S:  85cm offset-feed dish, 5-turn helix feed, db6nt preamp, Drake,
FT-847 (internal preamp off)
I hear the transponder noise floor for squint<10 degrees.
Not currently QRV due to U-band maintenance needed; L-band not installed
pending improvements.
Roof has three feet snow!  

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