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RF Preamp setting

John, G3WKL
John, G3WKL Member ✭✭
edited June 2020 in SmartSDR for Windows
On the topic  "FT8 Receive Meter Control" Tim, W4TME usefully reminded us of Gerald's help topic on setting the RF Preamp gain.  

We've got some Es today so not having been on 10m for a while, and as there were some signals for SNR assessment, I thought that I would follow Gerald's step-by-step process for setting the RF Preamp Gain.  This is what I recorded (Flex 6700, Butternut HF6VX vertical, and about 90m of RG213 feeder): 

No antenna (XVTR): -135dBm

RF Gain (dB)       Noise in 400Hz BW (dBm)
0                          -119
+10                      -127
+20                      -130
+30                      -132

So, following the quoted article, +10dB of preamp gain would seem to be optimum as it gives an 8dB increase in the measured noise floor. 

The reason for posting this question is why isn't the gain of +30dB better, as that gives only a 3dB increase in the measured noise floor, i.e more RX sensitivity.  Is it sub-optimum purely from the aspect of dynamic range?

73 John

Answers

  • Steve-N5AC
    Steve-N5AC Community Manager admin
    edited June 2019
    John, can you provide the same measurements with the antenna disconnected?
  • John, G3WKL
    John, G3WKL Member ✭✭
    edited June 2019
    Ah, I get it now!  

    0            -119
    +10        -126
    +20        -136
    +30        -143

    So the +20dB setting gives 6dB and +30 a +11dB increase.  So the +30dB gain is closest to Gerald's optimum setting

    Thanks Steve
  • John, G3WKL
    John, G3WKL Member ✭✭
    edited June 2019
    Steve,

    Thanks for the detailed reply, which I've just scanned as my slot for amateur radio has gone...  just quickly I should remind you that this is 10m and using a Butternut HF6VX vertical and about 90m of RG213 feeder.  In a "residential" ITU type environment...

    Catch up later

    Regards

    John
  • Steve-N5AC
    Steve-N5AC Community Manager admin
    edited June 2019
    OK terrific, John -- not sure where I got 6m. I had probably just read something else about 6m!
  • John, G3WKL
    John, G3WKL Member ✭✭
    edited June 2019
    Thanks, again. All makes sense now. I've got quite a lot of loss in the feeder....

    73 John
  • Ken - NM9P
    Ken - NM9P Member ✭✭
    edited June 2019
    Another great post, Professor Hicks.

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