[Elecraft] K3 PR6 preamp

[Edward R Cole]

Jim,

Your comment is a little confusing.  First you state that you believe 
the External preamp definitely improves sensitivity (when external 
noise level is low).  I agree.  I said the same in my original e-mail.

Then you state that you are not suggesting the ARR is better.  Not 
better than what?

It is definitely better than the internal preamp, but it is probably 
close to the same performance as the PR6 (regarding 
sensitivity).  Elecraft claims exceptional dynamic range for the PR6 
which the ARR probably does not have (which is important in urban 
high noise environs or in high density QRM that happen in openings/contests).

I did some checking of the K3 specs:  Sensitivity with (internal) 
preamp turned-on is spec at -136 to -138 dBm at 500-Hz bw.  -138 dBm 
is equivalent to  noise figure of 9.5 dB.  With the PR6, the K3 is 
spec at -143 to -144 dBm at 500-Hz bw.  That is equivalent to a noise 
figure of 5-dB.  Since the PR6 spec is 0.5 DBNF with 18 dBG this 
implies that the K3 receiver has a NF of 20-dB (all preamps off).

The ARR preamp is spec at 0.5 dBNF with 22-dBG so the resulting 
sensitivity should be -148 dBm.

So doing a little more reverse analysis, this implies the internal 
preamp NF is something like 5-dB if you assume the gain is 10-dB (I 
could not find what the gain of the internal preamp is).

How does this translate for the 6m operator?  In high noise environs 
the internal preamp is probably adequate since external noise 
predominates.  In low noise areas the PR6 or ARR definitely will 
improve sensitivity by 7 to 10 dB.  If you are considering doing eme 
or meteor scatter on 6m the low-noise external preamp is going to 
help quite a bit.  Antennas elevated above the horizon do not see the 
thermal noise of the ground or as much man-made noise.  Sky noise at 
6m is still quite high compared to higher VHF and UHF frequencies so 
placing the preamp at the K3 is acceptable if your coax is not lossy.

Remember my statement that I though running both internal and 
external preamps might not result in better sensitivity?  I took the 
numbers I developed for both the PR6 and internal preamp and input to 
my spreadsheet.  The result is total sensitivity of -152 dBm.  So 
maybe running both internal and external preamps is better.  (As jim 
point out) you have to evaluate this in real conditions as results 
will differ depending on your local noise environment.  One caveat is 
turning on both preamps may lead to overdriving the receiver 
resulting in reduced dynamic range or even distortion.

73, Ed - KL7UW
Final note: SSB sensitivity numbers at bw of 2.8 KHz are 7.5 dB lower 
than at bw of 500-Hz.

------------------------------

Message: 31
Date: Thu, 24 Jun 2010 13:15:30 -0700
From: "Jim Brown" <jim at audiosystemsgroup.com>
Subject: Re: [Elecraft] K3 PR6 preamp
To: "elecraft at mailman.qth.net" <elecraft at mailman.qth.net>
Message-ID: <20100624201535.EEECA5849C at gw1.nlenet.net>
Content-Type: text/plain; charset="us-ascii"

On Thu, 24 Jun 2010 11:52:46 -0800, Edward R Cole wrote:

 >Running both preamps probably only raises the noise level and
 >does not  improve receiver sensitivity in any significant
 >manner.

I suggest that you defer judgment on this until you're trying to
copy very weak signals during a band opening. The additional preamp
at the patch point (in my case an ARR) definitely DOES improve RX
sensitivity at times when the external noise level is low. I'm not
suggesting that the ARR is better -- indeed, I doubt that it is.
But it was bought and paid for when the K3 showed up, and it works
fine. :)

73, Jim K9YC



73, Ed - KL7UW, WD2XSH/45
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