[Elecraft] K3 - Performance on 6M?

[Joe Subich, W4TV]

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:jamesduffey at comcast.net] 
> Sent: Friday, April 02, 2010 7:35 PM
> To: Joe Subich, W4TV
> Cc: James Duffey; elecraft at mailman.qth.net
> 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
> < jamesduffey at comcast.net >
> 
> 
> 
> 
>