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Adjusting RF Gain on the Antenna -- which 'noise floor' do I use?


Hi guys (and gals, of course!)

I'm trying to adjust the RF Gain on my new to me 6700 and dutifully following the instructions Tim provides in his 2019 article. I'm kind of stuck tho, on trying to determine the 'no antenna noise floor' I'm supposed to be using as a reference.

Here's a screenshot of what I see on 80m with no antenna connected, CW, 500hz bandwidth, no noise reduction, etc.

My query regards the apparent difference in noise floor measurement/display between the panadapter (looks like about -120dBm) and the S meter in the flag showing -110dBm. Which one should I use in determining the noise floor? And which one should I use in determining if there is a 8-10db difference when I add an antenna and the RF Gain?

Thanks for your help.


  • KD0RC
    KD0RC Member, Super Elmer Moderator

    Hi Don, technically, you should use the noise floor in the passband of the slice (-110 dBm in this case). I find it easier and faster to just look at where the noise floor intersects the scale on the right-hand side of the screen (-120 ish dBm). Since it is a relative thing (8 - 10 dB), I find it plenty good to eyeball it that way.

    I don't think it makes much practical difference, especially since the RF Gain is adjustable in 8 dB increments (at least that is how it is on my 6400). You might want to try both ways to see if you see a material difference.

  • Stan VA7NF
    Stan VA7NF Member ✭✭✭

    Don, as per Len, use the 500hz receive signal.

    The Panadapter readings are for each bin in that panadapter (there are 4096 bins in the panadapter width so take the panadapter total width / 4096 will be the size of each bin). As you zoom in each bin will become smaller and the dbm reading will drop accordingly. At the extreme zoom in range each bin may be less than 1Hz.

  • Al K0VM
    Al K0VM Member ✭✭✭

    Adjusting the noise floor with RF gain is only useful when the antenna is connected. Adjusting without an antenna serves no useful purpose.

    AL, K0VM

  • VE7ATJ_Don
    VE7ATJ_Don Member ✭✭✭

    Thanks guys...that's what I thought (using the slice's passband i.e. the Flag's S-meter).

    But here's my quandry... I did that on 160m (as an example) and:

    1) with no antenna, the noise floor registers as -112 dBm

    2) with antenna connected, it rises to -106dBm. Not bad, but a little below the 8-10db difference suggested. So I start adding RF gain

    3) with +10db, I can 'hear' more noise, but the S-meter shows -107 dBm (now 5db difference)

    4) with + 20db, again, hearing more noise, but S-meter shows -105/6 dBm

    5) +30db, more audible noise, but S-meter shows -108dBm.

    Not the results I was expecting....

    The same thing happens on 80m -- no antenna = -118dBm

    With Ant = -110dBm

    Changing the RF Gain makes no difference from that -110dBm reading. But maybe that's normal...

  • Ken Wells
    Ken Wells Community Manager admin
    edited July 2023

    You likely "Hear More noise" because it has popped above your AGC-Threshold.

    After changing your Preamp Gain, or bandwidth (Or many DSP settings) it is often necessary to re-adjust the AGC-T for best signal to noise balance.

    This is why I have AGC-T as one of my AUX settings on my FlexControl Knob. It makes it much easier to adjust the AGC-T settings "on the fly."

  • VE7ATJ_Don
    VE7ATJ_Don Member ✭✭✭

    Hi Ken...yes, it makes sense that the increased 'heard' noise is due to the AGC-T threashold.

    However, the focus of my followup was not that, but the lack of change in the noise floor when I increase (or decrease) the RF Gain. If there's essentially no change in the noise floor when I add/subtract RF Gain, how am I supposed to use this to get the 8 to 10 dBm difference recommended?

    Or, should I just leave it off, since it doesn't seem to making any appreciable difference in the noise floor?

    Or... is there something wrong with the pre-amp section of the rig????

  • brianbruff
    brianbruff Member ✭✭
    edited March 7

    I notice the following on my rig 6400, increasing the rf gain drops my noise floor

    Is this normal, (I seem to recall this happens on most radios?)

    I'm out in the country side, and i've just done the following reading on 160M @5PM local time

    Noise floors as follows on SmartSDR

    SSB (2.9khz)

    Gain full left (-8) = S6 -91dBm

    Gain full right (32) = S3 -102dBm

    CW (500hz)

    Gain full left (-8) = S4 -100dBm

    Gain full right (32) = S3 -110dBm

    CW (100hz)

    Gain full left (-8) = S3 -108dBm

    Gain full right (32) = S1 -116dBm

    CW (50hz)

    Gain full left (-8) = S3 -109dBm

    Gain full right (32) = S1 -120dBm

    (i will add that i got right up in that AGCT slider after each movement and noticed no impact)

  • brianbruff
    brianbruff Member ✭✭

    Replying to my own comment to those who follow:


    Behaving as expected

  • WX7Y
    WX7Y Member ✭✭✭✭

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