Posted by
Edward R Cole on
Jun 24, 2010; 7:52pm
URL: http://elecraft.85.s1.nabble.com/Another-Happy-Customer-tp5205110p5219305.html
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.xlsExt 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 \(G3TCT\)" <
[hidden email]>
Subject: Re: [Elecraft] K3 PR6 preamp
To: "Edward R Cole" <
[hidden email]>, <
[hidden email]>
Message-ID: <004901cb1389$6e506180$0602a8c0@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
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