Or why I believe a 59 report is meaningless!
Receiving an S9 signal doesn't automatically mean
that the signal is clear and intelligible. The S-meter
(or signal strength meter) gives us a snapshot of the signal
strength, but it doesn't tell us anything about the Noise
Level (NL) or the Signal to Noise-Level Ratio
(SNR), which are crucial in understanding the overall quality of the
received signal. A signal might be strong in terms of the S-meter,
but if the noise level is similarly high, then the actual
communication may be difficult or unclear. Most of today's operators will add an Audio quality report of 5, to make the report 59 which in amateur jargon means that the received transmission supposed to be of excellent quality e.g., an excellent signal. However, to often we hear stations giving a report of 59 or even 59+ only to go on to request a retransmission. Audio quality reports have a scale from 1 to 5 and are given by the operator. With 5 being excellent
and 1 being very poor. This relies heavily on the operators HSP, and as we all have experienced, those "Audio reports are ...." well let's say that those reports are not very reliable.
For instance, if the noise level at the receiving station is S8,
and the received signal is S9, the SNR
would only be 6 dB. Which would mean that the signal is just
slightly stronger than the noise. This could make understanding the
transmission challenging, even if it shows up as S9
on the meter. On the other hand, if the noise level is low (say S3) then an S9 signal would be much clearer as the SNR is now 6x better i.e. 36dB vs 6dB.
The Importance of SNR
SNR (Signal to Noise-Level Ratio) gives us a much
better sense of how usable a signal is as it compares the strength
of the desired signal to the current background noise. A high SNR
means that the signal is much stronger than the noise, making it
easier to decode and work with. On the other hand, a low SNR
means the noise is almost as strong or stronger than the signal,
which can lead to poor communication quality or to a completely
unreadable signal.
To calculate the SNR we need two signals, the noise level and the signal strength of the received transmission, both numbers need to be a power level (dB). And since the levels will be rather small we'll use the Decibel in milli Watts e.g, in dBm.
Determining the Noise Level
Determining the Noise Level (NL) of our receiving system is rather easy, all we have to do is tune to a channel where no signal is present and record the S-Value, as shown in the below picture.
Next we tune to a channel with a transmission and note the recived signal, say S9+10 as depicted below.
Converting S-Values to Power
From the IARU Technical Recommendation R.1:
S1 corresponds to -121 dBmS9 corresponds to -73 dBm
And the steps between S-Units are in 6 dB increments. This means:
S1 to S2 = -121 dBm to -115 dBm (6 dB increase)
Above S9 we tend to add a dB value to the S9 value, i.e.
S9 to S9+10 = -73 dBm to -63 dBm (10 dB increase)
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| click to view larger file |
So, to calculate the SNR:
- Convert both, the received signal strength and the noise level into power levels (dBm).
- Calculate the difference between the received signal and the noise level (e.g., SNR = Signal Power - Noise Level).
Using the above values:
Using the above graph we convert the received noise level of S4 to a power level of -103dBm and the received signal level of S9+10 to -63dBm.
Using the below formula:
SNRdB = (Signal-LeveldBm) - (Noise-LeveldBm)
SNR = (-63) - (-103)
SNR = 40 dB
This would mean that the signal is 40 dB
above the noise level, which is a strong signal and would be very
clear.
Conversely, if the received signal would be S9 (-73
dBm) and the noise level would be S8 (-79 dBm), the SNR would only be 6 dB.
While still a reasonable SNR, it would be somewhat noisier than
the 9+10 example, and we might find that we would need to ask for a repeat transmission occasionally.
Potential Benefits of Using SNR instead of S-Meter Values
- More Accurate Representation of Signal Quality:
SNR considers both the signal strength and the noise level, which gives a much clearer idea of the actual quality of the communication link.
- Increased Context:
By reporting SNR, operators would have more information about whether the signal is being received clearly or if noise is impacting communication.
- Noise Awareness:
Instead of just reporting a S9 signal, one could be aware of how much noise is present at the receiving station. This could help with adjusting the transmitting stations output power, e.g. increasing the output from 100W to 400W, which is a power level increase of 6dB and as such should, in principal, add 6dB to the SNR.
Remember bigger SNR = better signal quality.
Challenges
- SNR requires additional measurements:
To report SNR, operators would need to be able to measure or estimate the Noise Level (NL) at their location. This can be challenging if the equipment doesn't provide this information directly. Most modern SDR systems do have the ability to read signal strength as a power or voltage level.
- S-meter and SNR are still subjective:
While SNR is definitely more meaningful than just reporting an S-value, it still depends on the equipment's ability to measure signal strength and noise level accurately. Again, most SDR systems are very accurate.
Conclusion
Reporting 59 without any context of the noise level doesn't provide the full picture. Shifting to SNR as a primary metric would give operators a much better understanding of the quality of the received signal. It would not only account for the signal strength but also how well the signal stands out from the noise in the environment, leading to clearer communication and fewer misunderstandings.
Incorporating SNR reports into amateur radio
communication could definitely improve clarity and signal quality
perception. Perhaps this is something more operators could adopt
in their day-to-day operations.
Most SDR (Software Defined Radio) already have the ability to report the signal strength as a power or voltage level. A few lines of code and the SNR could additionally be displayed.
Appendix
NOTE: The SNR changes with the receiver bandwidth however, for the purpose of a quality/signal report it is not nesseary to include the receiver bandwith.
- HSP = Human Signal Processor (normally found between the ears)
- IARU Technical Recommendation R.1
- Signal Level Strength Meter Calibration and IARU Standards
- Understand SINAD, ENOB, SNR, THD, THD + N, and SFDR so You Don't Get Lost in the Noise Floor
- Signal strength and readability report
- Noise Counter-measures
- What is your S-Meter actually indicating
- S-Values are they useful
- A little history about S-Points
- How to achive receiver bliss
