At 2/12/2007 03:05 PM, you wrote: > > 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
Your math looks sound, Jeff, except I thought 1/2" Heliax wasn't quite so lossy @ 450 MHz (~1.5 dB/100 ft IIRC?). Realistically though, not many people connect the input of their preamp directly to 200 ft. of line. Usually there's a filter ahead of it that has loss & can be tuned to somewhat tune out the mismatch. All this sort of muddies the waters a bit, but theoretically in your example a preamp with a matched input would in fact yield lower system NF given the same NF specs, due to additional induced feedline loss ahead of the preamp as a result of the mismatch, not due to the reduced power transfer between the feedline & preamp. Bob NO6B
