[TowerTalk] Noise figure in cascaded networks

[alsopb]

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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