I have measured the 6m sensitivity of my K3 #1198 using a sig gen and two
methods of measuring the output signal level of the K3. I measure the noise
floor of the K3 (50.015, preamp on, usb, 2.3kHz bandwidth, agc off)
at -127.1dBm. Using the calculation provided by Ian GM3SEK (RadCom Oct 1999) I get from this a noise figure of 13.5dB, agreeing with Ian's measurement.
This is disappointing and a lot worse than my current rig, a 19 year old
IC575 which measures 5.6dB. I've found that an external preamp with the
IC575 to bring the overall NF down to 4dB helps with weak dx.
I have also measured the antenna noise here (a rural location) and in some
directions it is only 1.8dB above thermal. So there is quite a lot to be
gained by reducing the noise figure, and I wonder if Elecraft have any
response to this. I'm interested in using the subreceiver on 6m and this
would currently require two preamps if I want to use two antennas.
Regarding preamps, my measurements suggest that to get an overall system
noise figure of 4dB, you need a preamp with NF 3dB and gain 16dB, or NF 2dB and gain 13.7dB.
Graham
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