> True, but the original question was if the noise figure of a
> preamp was
> degraded by the poor input match looking into the preamp
> itself, & the
> answer is no.
Bob,
Actually, my question wasn't in regard to the NF of the preamp itself,
but rather the resulting NF (system NF) due to the mismatch(es).
I'm not in agreement with the system NF being independent of the match.
In the example I gave, where you have a non-ideal (i.e. lossy) feedline
connecting the source device (such as an antenna) to the input of the
preamp, VSWR on the line is going to increase the losses on that line,
thereby increasing the system NF. My contention is that if the preamp
were tuned to provide a better input match, even at the sacrifice of a
few tenths of a dB in NF, that the resulting system NF might actually be
less than in the un-tuned case.
Scenario 1: Preamp = 0.5 dB NF, antenna feeding 200' of 1/2" Heliax (3
dB loss @ 450 MHz @ 1:1 VSWR), end of feedline connects to a preamp with
input return loss = 6 dB. VSWR on the feedline due to the termination
mismatch would be 3:1 at the load end, decreasing to 1.66:1 at the
soruce end, with the resulting loss increasing to 3.9 dB. Ignoring the
NF of the antenna itself, the system NF would be 4.4 dB best-case.
Scenario 2: Preamp and feedline same as above, preamp input return loss
now 20 dB. 20 dB RL = 1.22:1 VSWR. Line loss would increase by only
0.03 dB. System NF would then be 3.53 dB best-case.
Double-check my math if you would...
Maybe we're saying the same thing, only differently? Or do you still
disagree with the premise?
--- Jeff
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