Locked Re: Question about SNR reports

Bob McGraw - K4TAX

I written several posts on proper receiver adjustment for optimum performance based on no signal band noise.     The magic number is to use the ATTN and RF Gain so as to attain the band noise about 10 dB above the receiver noise floor.  


Bob, K4TAX

On 4/12/2020 9:52 AM, Tom V. Segalstad wrote:

You are competely right, Bill, though I didn’t believe that at first.


Listened now to a local station on FT8 with constant signal, and switched in my station’s preamp-1: WSJT-X recorded same dB report.


I switched in my station’s preamp-2: WSJT-X recorded same dB report.


I switched in my external LNA (Low Noise Amplifier) GaAsFET RX preamplifier: WSJT-X recorded same dB report.


So I learned something today, Bill, after this experiment!


73 from Tom (LA4LN / LN1V)



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Fra: Bill Somerville
Sendt: søndag 12. april 2020 kl. 14.05
Til: WSJTX@groups.io
Emne: Re: [WSJTX] Question about SNR reports




it shouldn't so long as the pre-amp has lower noise than the band noise, and if is hasn't there's not much point to it.




On 12/04/2020 12:56, Tom V. Segalstad wrote:

It also depends on if the receiving station is using reception preamplifiers - and the other station not.

73 from Tom (LA4LN / LN1V)

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From: WSJTX@groups.io <WSJTX@groups.io> on behalf of Bill Somerville <g4wjs@...>
Sent: Sunday, April 12, 2020 12:36:53 PM
WSJTX@groups.io <WSJTX@groups.io>
Subject: Re: [WSJTX] Question about SNR reports


On 12/04/2020 01:48, M Lu wrote:

I saw a chat awhile back that if the report you received was *stronger* that the report you sent to the other station, then there was something meaningful about that.  But I can’t recall.  I am seeing that happen occasionally on 80 meters with my inverted L and 100 watts from EU stations.


Mark Lunday, WD4ELG

Hi Mark,

there are many ways to form signal strength reports, each has limitations.

1) S-meter, these are roughly relative to absolute received signal strength but do not account for noise contributions,

2) absolute logarithmic signal power, usually in dBm, which are accurate measurements of received signal strength but are only meaningful alongside details of the aerial system used, or if used for comparative measurements,

3) signal to noise ratio, which accurately measure the power of a received signal relative to the received noise power, so are an accurate measure of signal readability,

4) signal to noise+interference ratio, similar to SNR (3) but accounts for interfering co-channel interference as part of the noise power. Note this is not necessarily relevant with narrow band modes like FT8/FT4 where advanced filtering eliminates the impact of interfering signals.

WSJT-X uses (3) SNR with the noise power estimated across a 2500 Hz bandwidth. It attempts to find the true noise power without contribution from interfering signals, although this is not easy to do accurately.

So to answer your question, SNR reports are often not reciprocal because of various factors. One station may have a more or less radiated power, another station may have a higher or lower noise level, yet another may be using a narrower than 2500 Hz bandwidth receiver that distorts the SNR measurement. None of this is exact science, the numbers are at best estimates. We feel that SNR is the most useful way of recording signal strengths.

BTW the next release of WSJT-X has an improved noise measurement algorithm for FT8/FT4 that measures the noise floor far better in the presence of strong signals. This means that SNR figures with large positive values will be given for the strongest signals, rather than the current situation where they tend to saturate at about the +20 dB range making their value falsely low.


Tom (LA4LN)



Tom (LA4LN)


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