Jim, > Joe - I went back and did the calculation again for both > conditions. I had made a small mistake in calculating the > combination previously.
Thanks. The PR-6 only should be good for most activity and the cascaded configuration saved for times when there are no strong signals on the band. I haven't calculated it but I suspect unless one is boresight to an EME array or one of the big contesters, the cascaded configuration (internal preamp) will probably not overload. 73, ... Joe, W4TV > -----Original Message----- > From: James Duffey [mailto:[email protected]] > Sent: Friday, April 02, 2010 7:35 PM > To: Joe Subich, W4TV > Cc: James Duffey; [email protected] > Subject: Re: [Elecraft] K3 - Performance on 6M? > > > Joe - I went back and did the calculation again for both > conditions. I had made a small mistake in calculating the > combination previously. > > K3 external preamp on, internal off, Noise Figure for > combination = 3.3 dB > > Which should be good for most work. > > K3 external preamp on, internal on, Noise Figure for > combination = 1.5 dB > > Which is probably overkill for most people. > > > Here are the details of the calculation. I hope there are no > more mistakes: > > The noise factor for stages in parallel is > > Ftotal = F1 +(F2-1)/G, where these are all ratios, not expressed in dB > > where F is the noise factor, F1 the PR6, F2 the K3, and > Ftotal the noise factor for the total system. The noise > factor is a measure of how much noise a stage adds to a signal > > F= SNRin/SNTout > > Where SNRin is the signal to noise ratio (power) of the > signal into the stage and SNRout is the signal to noise ratio > (power) out of the stage. So F is a measure of how much noise > a stage adds. > > NF=10log(F) > > so F =10**(NF/10) > > for the K3 with the preamp off, NF =18 dB, F2 = 63.1 > > for the PR6 NF=0.7 dB, F1= 1.17, G= 63 (18 dB) > > so Ftotal = 1.17 + (62.1)/63 = 1.17 + 0.98 = 2.15 > > which is 3.3 dB Noise figure > > with the internal preamp on, NF = 12 db; F2= 15.8 > > F = 1.17 + 15.8/63 = 1.42 > > NF= 1.52 dB > > The 0.7 dB noise figure is the preamp spec, it also says that > typical values run 0.5 dB which would improve things even more. > > Sorry for the miscalculation in the combined noise figure the > first time. - Duffey > > > > On Apr 2, 2010, at 4:58 PM, Joe Subich, W4TV wrote: > > > > > > >> The Elecraft external preamp with 0.7 dB noise figure and 18 > >> dB gain helps a lot: > >> > >> K3 with PR6 external preamp > >> > >> preamp on 0.9 dB > >> > >> And the preamp is pretty robust to strong signals. > >> > > > > Is this calculated with both preamplifiers on (cascaded) or > > with the PR-6 only? > > > -- > James Duffey KK6MC > DM65tc > Cedar Crest NM > < [email protected] > > > > > > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
