[Elecraft] K3 PR6 preamp

[Edward R Cole]

OK, I did as you suggested and turned off AGC and set RF gain = 50% 
and here is the result:
Meter  IntPre  ExtPre  Ant  Load
-20dbV   N        N         N     N      K3 disconnected
-19.7      N        N         N     Y      K3 connected to load
-18.2      Y        N         N     Y
-15.5      N        Y         N     Y
-3.3        Y        Y         N     Y
-19.5      N        N         Y     N      K3 connected to 3-elem 
yagi at 55-feet
-15.4      Y        N         Y     N
-1.1        N        Y         Y     N
+8.4       Y        Y         Y     N
N=off, Y=on

There is slightly more noise on the antenna than with the 50-ohm 
load, only 3.2 dbV difference using only the Internal Preamp.  The 
External Preamp shows a 14.4 dbV increase over the 50-ohm 
load.  Running both preamps probably only raises the noise level and 
does not  improve receiver sensitivity in any significant 
manner.  Note that the gain difference between preamps influences the 
meter reading as well as noise figure.

Running the ExtPreamp  by itself is likely the best configuration for 
best sensitivity.

Using Y-factor between the load and the antenna one could determine 
the G/T ratio and maybe overall noise temperature (and noise figure).

Theoretically, T = Tsky + Tant + Trx
Tsky at 6m is high, perhaps 2000K  (but this was the question that 
started this thread)
Tant is unknown, but probably a significant portion of the 290K 
ground noise, assume 150K  (a factor of F/B ratio and sidelobe levels)
Trx should be determined primarily by the preamp:
          Tpre = 290*10^(NF/10)-1 = 290*10^(0.5/10)-1 = 290*(10^.05 - 
1) = 290*0.122 = 35K
The K3 noise figure will add a little but is reduced by a factor of 
the preamp gain (if NF(K3) = 5 dB then Tk3 = 290*2.16 = 676K
          The cascaded noise temp, Trx = Tpre + Tk3/158 = 35 + 676/158 = 40K
Sensitivity looking at the load is -139 dBm using the 2.8 KHz SSB bandwidth*
Looking at a 2000K sky the noise floor is -130 dBm*, assuming antenna 
noise to add 150K

One would need to calculate P = KTB from the noise voltage readings 
where P = E^2/R to arrive at actual noise power.

I wonder how linear the dbV meter readings are with this approach?

73, Ed
*Note calculations using my eme pathlink sw: http://www.kl7uw.com/emelink.xls
Ext Preamp P50VDG: 0.5 dBNF, 22-dBG - I would expect similar results 
from the PR6



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

Message: 36
Date: Thu, 24 Jun 2010 11:38:45 +0100
From: "Graham Kimbell \(G3TCT\)" <g3tct at lineone.net>
Subject: Re: [Elecraft] K3 PR6 preamp
To: "Edward R Cole" <kl7uw at acsalaska.net>,      <elecraft at mailman.qth.net>
Message-ID: <004901cb1389$6e506180$0602a8c0 at a>
Content-Type: text/plain;       charset="iso-8859-1"

>Ed
>There's a more reliable way of doing this. Turn on the dBV measurement
>facility on the K3 and turn off the agc, reduce rf gain a bit. Arrange for
>50ohm resistor noise to indicate 0dB and then substitute the 6m antenna and
>see the difference.
>
>BTW the preamps won't be "below a 290K source" - they're already at 290K or
>more!
>
>73
>Graham


73, Ed - KL7UW, WD2XSH/45
======================================
BP40IQ   500 KHz - 10-GHz   www.kl7uw.com
EME: 144-600w, 432-100w, 1296-60w, 3400-fall 2010
DUBUS Magazine USA Rep dubususa at hotmail.com
======================================