The Skinny on FT-8 Signal Reports

Discussion in 'Ham Radio Discussions' started by N2SUB, Dec 11, 2017.

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  1. N2SUB

    N2SUB Premium Subscriber QRZ Page

    The signal reports don't make any sense, right? They are negative numbers (unless you are running QRO...but let's not go there), yet the signals are clearly audible. The number represents the strength or your FSK signal (6.25 Hz) as it compares to the signals in an entire 2.5 kHz SSB bandwidth. I was curious about the math, and I combed the internet looking for an explanation. Here's the actual math behind the report:

    Keep in mind that the noise floor is relative to the receiving station. The equations explain why FT8 signals start decoding at -26db - everything else is below the noise floor:

    SNRFSKFT8 = SNRreportedFT8 + (10 x LOG (2500 Hz/6.250 Hz))
    SNRFSKFT8= SNRreportedFT8 + (10 x LOG (400))
    SNRFSKFT8 = SNRreportedFT8 + 26 dB

    Given that 1 S-unit = 6db, I put together the following table to help understand what your 'real' signal report would be. It's surprising that the stations running QRO and getting +10 reports are really only an S6 above the noise floor. :)

    W5EBB, KM4SII, N7ZAL and 1 other person like this.
  2. KC8VWM

    KC8VWM Ham Member QRZ Page

    Interesting... :)
  3. G0JUR

    G0JUR Ham Member QRZ Page

    Hmmmm so CW hasnt been sidelined... Happy days. ;)
    N2SUB and KK5JY like this.
  4. KK5JY

    KK5JY Ham Member QRZ Page

    If the S-meter is calibrated such that the highest S-0 reading is actually 0dB SNR in the receiver, an S6 would be +36dB SNR, correct?

    If the S-meter is not calibrated that way, then making a correlation between the S-meter and the actual SNR becomes impossible, since there could be any number of (unreadable) dB between S-0 and 0dB SNR.

    Edit: For what it's worth, here are a couple of quotes from the WSJT-X manual on that subject:

    Signal reports are specified as signal-to-noise ratio (S/N) in dB, using a standard reference noise bandwidth of 2500 Hz. Thus, in example message at UTC 0003 above, K1ABC is telling G0XYZ that his signal is 19 dB below the noise power in bandwidth 2500 Hz. ...

    Signals become visible on the waterfall around S/N = –26 dB and audible (to someone with very good hearing) around –15 dB.

    If nothing else, it gives you an idea of how he is computing the SNR, and reference level to human hearing.

    Note that if you are employing appropriate operating practices (e.g., using RF gain and/or attenuator to lower the noise floor in your receiver, the computed SNR should be higher than the equivalent signal strength in a receiver run by a newbie who runs his receiver with excessive gain (e.g., all knobs to the right, S-meter moving on nothing but background noise).
    Last edited: Dec 11, 2017
  5. KK5JY

    KK5JY Ham Member QRZ Page

    Radiotelegraphy is alive and well... at least it was as of a couple of weeks ago:

  6. N2SUB

    N2SUB Premium Subscriber QRZ Page

    This has nothing to do with the S-Meter on your radio. I simply converted the signal reports to S-Units because that's how most of us think of signal strength. All signal reports are relative to the noise floor at the receiving station. If you want to convert to S-Meter readings, I suppose you would add the noise floor to the report. So if you got a -8db report and the receiving station has a noise floor of S4, the receiver's S-meter would read S7. But that's not the intention of the chart. The chart relates the received signal in db to S units above the noise floor at the receiving station based on the logarithm used my WSJT (to the best of my knowledge).
  7. KK5JY

    KK5JY Ham Member QRZ Page

    Okay, I understand now. :cool:
    N2SUB likes this.
  8. K1OIK

    K1OIK XML Subscriber QRZ Page

    Actually it isn't alive and well I rarely work the same station twice on any other mode but on CW it is common.
    I wish I there was a way to call CQ NEW on CW.
  9. K6CLS

    K6CLS Ham Member QRZ Page

    Oh gosh for that you would need some kind of free text, and the full alphabet not just call signs and numbers, and some kind of international undersanding for 100 years

    Doesn't seem too difficult, go make it happen!!

    Thanks we're all grateful
    VK3DAN likes this.
  10. KK5JY

    KK5JY Ham Member QRZ Page

    Maybe all you need is to start working these busy weekend events we call "contests?" ;)

    That screen capture was from my SDR during the recent ARRL 160m contest. In only a few hours of operating, I had 250+ unique calls, and all on one band.

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