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[Linrad] Re: Preamp WSE

At 09:10 AM 2/19/2009, you wrote:

>Aloha to all:
>More elemental questions. What is a good amplifier level before 
>input to WSE RX144? If I have say LNAs at antennas and have maybe 15 
>dB gain at RX144, then inserting another 15 dB for each channel at 
>converter is working nicely.
>What amount of signal amplification should supply to the RX144 
>channels? AND what effect is NF of second RF amplifiers in shack 
>have upon NF of system?
>My interest is may I use a more economical RF amplifier at the back 
>of the converter and a good LNA at antenna to make required input to 
>WSE converter?
>This has likely been convered before?
>Jeremy w7eme


I have a spreadsheet program that does the caculation for NF for two 
preamps and a receiver (or any three stages).  The program uses this 
to determine MDS and full eme pathlink performance (SNR). 

the second preamp NF is reduced by the gain of the first preamp but 
any cable loss between the two preamps is added with the NF of preamp 
#2.  NF = noise factor = 10^(noise fig/10).  Much easier to do math 
with noise factors (ratio) than with noise figure (dB).

So if your first preamp has a gain of say 16-dB, NF(2)/40 is the 
contribution to total NF.  A 1-dBNF second preamp will only 
contribute about 0.2 dB to the overall system noise figure.

73, Ed - KL7UW

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