[Linrad] Re: Preamp WSE

[Edward Cole <acsalaska.net; kl7uw@xxxxxxxxxxxxxxxx>]

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
>

Jeremy,

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). 
http://www.kl7uw.com/emelink.xls

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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