Re: [amsat-bb] How much 2.4GHz Preamp Gain?

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

>From: "Allen Fugelseth" <allen@cruzio.com>
>How much gain should be used  in a 2.4GHz preamp between a 20 turn helix and
>a Downconverter?
 
Allen,

I have submitted a paper on the topic of System NF and Noise Temp Calculations
for presentation at the SETI-League Symposium to be held end of April in
Trenton, NJ:
<http://www.setileague.org/admin/meet2001.htm>
The paper discusses the formulae, and how to use an Excel spreadsheet I
developed to perform calculations using up to two preamps and a receiver with
feedline between each.  This spreadsheet will be part of a program that
calculates system performance in terms of G/T ratio using sun noise.  It will
be placed on the SETI-League web site: 
http://www.setileague.org/software/spreadsh.htm

Currently an old version of the G/T program <g-t.xls> resides there, but
the NF
algorithm is incorrect.  It will be updated sometime soon.  In the meantime if
you can provide me with the following, I will give you a suggested gain for
best performance:
1.  Feedline type and length [or loss] from antenna to preamp; if directly
connected then none.
2.  Preamp NF in dB (if known).
3.  Feedline type and length [or loss] from preamp to the downconvertor.
4.  Downconvertor NF and gain in dB.
5.  Feedline type and length [or loss] from downconvertor to receiver.
6.  Receiver model and SSB sensitvity and bandwidth.

With this info I can analyze your system NF as function of preamp gain.  As a
quick rule you should have 15 dB excess gain after subtracting the feedline
loss from the preamp gain.

Ed 

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