Recently, I saw a newcomer's post on Twitter regarding his first WSJT-X session. I noticed right away that he only decoded a handful of stations, but those with supposedly extremely good reports. This is what happens when you set your receive bandwidth extremely low. This means that you only receive signals that lie within this range.

If we remember again our education, we find out that a normal report refers to a bandwidth of 2.5kHz. So, if I tighten my filters on the TRX (as is often done for CW, for example), the SNR values displayed there are of course no longer comparable to 2.7k or even 3k bandwidth.

For clarification I have set my bandwidth on the IC-7300 to 50 Hz. The first try I did somewhere, it came out S3 on the S-meter

S3 10m 50 Hz

and I was already positive at 30db SNR-ratios:

50 Hz S3

Then I looked for the peaks on the spectrum

10m 50 Hz looking for a Peak

and turned it to S8-S9.

S8 S9 10m 50 Hz

I came out at 45 db positive (related to 50 Hz). I also decoded +51 (unfortunately not on the screenshot).

50 Hz S8 S9

These values say nothing at all without the reference to 50 Hz. By the way, my antenna was 2 x 1.5m wire inside the apartment, so for sure it was NOT my antenna.

Back-calculation to true reports can then be performed using the "Eb/N0 (energy per bit to noise power spectral density ratio)" formula. Who is interested in this can check out Wiki.

0 # Thomas March 2024-01-20 19:19
Das ist der beste Beitrag zu dem Thema. Danke John für die Erklärung viele denken man hört "unter dem Rauschen"
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0 # Phil Karn KA9Q 2023-09-10 07:28
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0 # Phil Karn KA9Q 2023-09-06 21:39
In my opinion we shouldn't be using SNR at all, for exactly this reason (and others). We should be using Eb/N0, the ratio of the received energy per bit to the noise power spectral density. Both have units of joules (watt-seconds) so their ratio is dimensionless and can be expressed in dB, just like SNR. In fact, Eb/N0 is sometimes called the "SNR per bit". It is the SNR you'd get if the signal were an unmodulated carrier received in a bandwidth equal to the user data rate (NOT the symbol rate). This will usually be a small positive number; Shannon says it must always be greater than -1.6 dB. The advantage of Eb/N0 is that it puts everything on a level playing field. Measurement bandwidth doesn't matter, nor does data rate. It lets us directly compare different modulation and coding methods.
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