[TowerTalk] Noise figure in cascaded networks

[Michael Tope]

Brian,

The formulas your a referring to only apply to systems 
where the dominate noise contribution is from the receiver's
internal thermal noise (kTB contribution). Where there is
significant noise contribution is from the atmosphere, they 
must be modified to include the sky temperature. If the sky
temperature is very large, it will effectively swamp out 
contribution noise from the receiver and the feedline loss.

    e.g. Tsys = Ta + Tr

Where Tr is the receiver noise temperature (including feedline
loss), and Ta is the antenna noise temperature which includes
the sky contribution.  

While you are correct that any feedline loss will degrade overall
S/N ratio, the question is by how much. In cases where the 
system noise temperature is dominated by atmospheric noise
(Ta>>Tr), degradations due to feedline loss are negligible.  

73 de Mike, W4EF.........................

---- Original Message ----- 
From: alsopb <alsopb@gloryroad.net>
To: <towertalk@contesting.com>
Sent: Thursday, April 13, 2000 5:30 AM
Subject: [TowerTalk] Noise figure in cascaded networks


> 
> Guys,
> 
> There is a lot of misinformation being stated here about noise figure
> in
> cascaded systems.
> 
> According to Skolnik "Introduction to Radar Systems" the total noise
> figure in a cascaded system of two components (read antenna preamp and
> coax if you want) is:
> 
> F = F1 + (F2-1)/G1
> 
> Where F1 is the first component's noise figure,  F2 is the second
> components noise figure and G1 is the gain of the first component.
> 
> Looking at this equation he states:
> "The contribution of the second network to the over-all noise figure
> may be made negligible if the gain of the first network is large."
> 
> In otherwords, putting a high gain, low noise pre-amp at the antenna
> can  mitigate the loss of the coax (receive only) in total noise
> figure. This is contrary to what some people are trying to state here.
> 
> This shouldn't be a suprise to anybody who has a mast mounted TV
> preamp or VHF mast mounted ham pre-amp.  They really work.
> 
> There is a more general expression involving cascaded multi-network
> systems (e.g. preamp, coax, each of the receiver stages).  I refer you
> to the formula for this.  However, one can see from it that the loss
> in the coax will introduce additional noise into any cascaded system.
> Coax loss does introduce additional "noise" which affects the S/N
> ratio.  Again, this contrary to what some people are stating here.
> 
> Now who want's to vote on the correct answer?  It isn't a matter of
> opinion.
> 
> 73 de Brian/K3KO
> 
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