What's the noise floor according to this picture?

Noise Floor on 20m

Is the noise floor S5 as it shows in the Signal meter at the top right or is it S1 as it shows on the panadapter?

The reason why I ask is I'm trying to determine if I'm chasing after general RFI that's bringing up my noise floor or if it's not actually that bad?


1 Answer 1


I always struggle a bit myself interpreting such spectral plots – the y-axis is power, but to assess the noise "intensity", we'd need to convert that into a power density, i.e. "power per bandwidth". Now, what's the bandwidth of every single x-point in that graph? Is it specified somewhere?

What we can do is take the -98dBm that's specified top right, and assume that's the power collected over the 2.9 kHz bandwidth highlighted in white in the spectrum plot (and set on the lower right in the picture.

That yields a spectral density of -98dBm/2.9 kHz ~= -98 dBm - 34 dBHz = 132 dBm/Hz.

That would be 42 dB above thermal noise. For a microwave receiver, a noise figure of 40 dB would be catastrophic, but for low frequencies like 14 MHz, I'm not so sure.

CCIR Report 322 claims noise floor could be 40 dB above thermal noise; but that report is over 50 years old! The sensitivity even of extremely good receivers back in the day as well as the selectivity of extremely good filters of that age would not stand well compared to modern receivers.

The main problem is that the report is based on 1960's models of atmospheric and galactic noise, as well as on 1960's levels of man-made interference. I sincerely doubt the latter point has much to do with what we see today, and I'm not convinced our understanding of the atmosphere hasn't long progressed beyond that.

Anyway, if 1963's data helps you in any way, no, that's at least not significantly above expected noise floor.

  • $\begingroup$ Thank you very much for your response. So based on what you are saying. I should probably stop my obsessive RFI chasing down? I'm not too much above the expected noise floor? $\endgroup$ Oct 29, 2018 at 23:14
  • $\begingroup$ pretty much! But of course, maybe you're 3dB worse than a similar receiver could be; I don't have the experience to tell you that, I'm afraid. $\endgroup$ Oct 30, 2018 at 9:12

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