[Linrad] Re: Preamp WSE

[Jeremy Alexander <w7eme.org; jeremy@xxxxxxxxxxxxxxxx>]

Thanks Ed!

Jeremy


--- On Thu, 2/19/09, Edward Cole <kl7uw@xxxxxxxxxxxxx> wrote:

> From: Edward Cole <kl7uw@xxxxxxxxxxxxx>
> Subject: [Linrad] Re: Preamp WSE
> To: linrad@xxxxxxxxxxxxxxxx
> Date: Thursday, February 19, 2009, 10:05 PM
> 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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