SNR, a better way to give a signal report

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 dBm

S9 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) 

 

click to view larger file

So, to calculate the SNR:

1. Convert both, the received signal strength and the noise level into power levels (dBm).
2. 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. 

See https://vk5hw.blogspot.com/2022/02/lets-talk-about-receiving-signal-that.html

• 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

ICOM IC-7610 S-Meter Tracking

Having been checking the S-Meter for my prefered software solutions  (SDR) I thought it might be a good time to also (re)check the S-Meter in my ICOM IC-7610. The setup is the same as for the previous two checks. Below is the result of my checks for my IC-7610. A quick graph of how well my ICOM IC-7610 S-Meter is tracking using the IARU Standard of 6dB steps (6dB/S-Unit).

Setup:

• Signal generator: XG3
• Level: -34.8dBm @ 13.8V
• Att: HP-355C & HP-355D (38dB for approx. -73dBm level)
• Frequency: 14.175MHz

Result:

Table 1

Graph 1

The graph shows a pretty linear S-Meter for the IC-7610 down to S4 with a 6dB step per S-unit. The above graph is based on the measurements at 14.175 MHz (20m) with the PreAmp off. However, enabling either preamp we can see from Table 1 that the measurements deviate quite a bit. So for the times that I'd use either of the preamps I need to do a bit mental arithmetic. My measurements show a 6dB increase for PreAmp 1 and a 10dB for PreAmp 2. But please note, even though the data in Table 1 looks spot on, the measured values are rounded up or down to the nearest integer. However, we are talking about an Amateur device not a professional Field Strength Meter. So I'm quite happy with the result.

Here are some older 40m graphs.

Graph 2

Graph 3

Graph 4

Below is my cheatsheet for S-Unit vs dBm vs µV.

Graph 4

CLICK on a graph for a better view.

NOTE: Even though the above graph is displayed with 3dB steps, it is 6dB per S-Unit!

Adjusting the S-Meter in HDSDR

Since ICOM has released Firmware v.1.42 for the IC-7610 the I&Q port is working again. This opened up the possibility to use HDSDR (Sampling rate of 1.92MHz with an effective Bandwidth of 1.66MHz) again. Since I still had the S-Meter check setup "set up" from the "Adjusting the S-Meter in Thetis"  I decided to check and adjust, if need be, the HDSDR/IC-7610 combo.

The setup is basically the same as for Thetis, except the SDR in this case is an IC-7610.

Setup:

• Signal generator: XG3
• Level: -34.8dBm @ 13.8V
• Att: HP-355C & HP-355D (38dB for approx. -73dBm level)

In HDSDR under Options [F7] we find Calibration Setting. This opens the HDSDR Calibration Panel.

Selecting the S-Meter Calibration tab:

The current configuration seems to correspond to an S-Meter reading of S9 +10dB on HDSDR:

and an S9 on the ICOM without the Pre-Amp engaged.

So next we add -73dBm to the Correct Level [dBm] field and press the [Calculate] button.

And the result is:

reducing attenuation by 6dB we get:

and, as expected, adding 6dB we see:

So in a Software Defined Radio (SDR) application written by Amateur's we do get the proverbial 6dB per S-Unit. 

Adjusting the S-Meter in Thetis

After about five (5) years I resurrected my ANAN 100D again. Trying a few versions, including a development version, I settled on Thetis v2.10.3.5 x64 u2. Seems to be running fine on my Windows 11 system. Quite a few improvements over the last five years. Going through the Setup/Configuration of the system I stumbled over a Level Cal inside the [Calibration] tab which can be found under the [General] tab. This allows one the ability to "automagically" set the S-Meter to a user provided level, .... sweet ....

I've decided to use my trusty old Elecraft XG3 RF Signal Source which I have checked against a calibrated RF Powermeter. At 20m the output at the -33dBm level measured -34.8dBm @ 13.8V. So using an attenuator with 38dB attenuation  will give me a -72.8dBm level into the ANAN. A short RG58 cable into a MFJ-1700B switch and another 50cm of RG58 should compensate for the missing 0.2dB to make it -73dBm.

Setup:

• Signal generator: XG3
• Level: -34.8dBm @ 13.8V
• Att: HP-355C & HP-355D (38dB)

And this is how it look in real live.

Here is the Thetis setup:

After pressing the Level Cal [Start] button, the system goes and runs an internal calibration routine. A window pops up to inform us about the progress status of the calibration.

Well, the result is quite pleasing. 

And if we add 6dB attenuation we get:

And not to forget if we do subtract 6dB attenuation the result is:

Struth, 6dB steps who would have thought that is a possibility. 

Oh and this is at every SSB Bandwidth we choose. My default is 2K1, however if I choose 2K9 the S-Meter still shows -73dBm. Yikes, it is possible! It is software defined after all.

It would be nice if my IC-7610 would not change the S-Meter reading with the engagement of the Pre-Amp(s).

References:

• A Little History about S-Points
• IARU Standards
• The beginning of the S-Meter revolution
• IARU Technical Recommendation R.1

Is my NOISE FLOOR (NF) really worse than yours ?

Well, let's answer that question first so you can move on to better things in life. And the answer is:

No it is not, because I use an S-Value indicator (S-Meter) that follows a Standard! The so called 6dB per S-Point (6dB/S) standard!

And that's it, 73 and catch you on the bands (as they say).

Oh, hello you are still here. So, it looks like you would like to know a bit more about this phenomena. Well, then read the below or maybe this "What is your S-Meter Really Displaying".

As far as I know, most Amateur Radios Transceivers from the three main manufactures, ICOM; YAESU and KENWOOD seem to display a 3dB/S scale below S9 (-73dBm). 

So if I read a Noise Floor (NF) of S3 on my S-Meter, which by using the 6dB/S standard would be -109dBm or -2.0dBμV NF.  And my QSO partner, i.e. you, is stating a NF of S2 which, using the applied 3dB/S scale would be a NF of -94dBm or -13dBμV.  

So is my NOISE FLOOR really worse than yours?

Let's have a look at the below two Tables, the left one is scaled in 6dB per S-Value i.e. my S-Meter and the right in 3dB per S-Value like your S-Meter. 

Received Power (Zin = 50 Ω)	Signal Strength S-Value 6dB step		Received Power (Zin = 50 Ω)	Signal Strength S-Value 3dB step
-121 dBm	1		-97 dBm	1
-115 dBm	2		-94 dBm	2
-109 dBm	3		-91 dBm	3
-103 dBm	4		-88 dBm	4
-97 dBm	5		-85 dBm	5
-91 dBm	6		-82 dBm	6
-85 dBm	7		-79 dBm	7
-79 dBm	8		-76 dBm	8
-73 dBm	9		-73 dBm	9

Doesn't that shed some light on this (issue)?

According to the IARU Handbook (Ver 9.0) Paragraph 4.1.3 S-Meter Standards:

1. One S-point corresponds to a level difference of 6dB.

2. On the bands below 30 MHz a meter deviation of S-9 correspond to an available power of a CW signal generator connected to the receiver input terminals, of -73dBm.

3. On the bands above 30 MHz a meter deviation of S-9 correspond to an available power of a CW signal generator connected to the receiver input terminals, of -93dBm.   

It has been a Standard for quite a while, but it seems that it has not been adapted widely.

Signal Level Strength Meter Calibration and IARU Standards

What is your S-Meter actually displaying!

To check the S-Meter on a HF transceiver against the IARU 6 dB standard, you will need to follow a few steps:

1. Connect a known RF signal source, such as an RF signal generator, to the transceivers' antenna input.
   
2. Set the RF signal generator to output a signal at a frequency and power level that is appropriate for the transceivers' band and mode of operation.
   
3. Use an attenuator to reduce the RF signal level in 6 dB steps. You can use a set of switchable attenuators, a variable attenuator or individual attenuators to achieve this.
   
4. Connect a device that can read either power or voltage, such as a digital voltmeter (DVM), digital multimeter (DMM), power meter (PM), or cathode-ray oscilloscope (CRO), to the output of the attenuator.
   
5. Adjust the attenuator to reduce the RF signal level in 6 dB steps and record the corresponding S-Meter readings on the transceiver.
   
6. Compare the S-Meter readings to the IARU 6 dB per S-Unit/point standard to determine if the S-Meter is tracking correctly.
   
7. If necessary, adjust the S-Meter calibration on the transceiver to match the IARU standard.

It's important to note that the accuracy of the check will depend on the accuracy of the RF signal generator, attenuator and the measuring device. Which mean that to get the most accurate results it would be advisable if ones equipment had been checked against calibrated equipment.

In the past, I have used an Elecraft XG3, which I checked against a NIST calibrated LP-100A. Which in turn helped to characterise a set of switchable attenuators.

Below, you will see some photos depicting an array of attenuators and the XG3 with a step attenuator in action.

The picture on the left is showing the test setup using the Elecraft XG3 and a switchable attenuator to check S-meter tracking on my radio. As long as the test equipment has been checked against a know standard we can make fairly accurate checks using basic test equipment.

And here to the right, an assortment of attenuators.

Below is a pictorial/sketch of the current test setup I use to check the accuracy of a Radios S-meter.

Staring on the left is the signal-generator, followed by two attenuators, one in steps of 1dB and the next in steps of 10dB. The next device is a 6dB splitter which splits the signal from the signal-generator in two reduced signals. One signal going to a calibrated power-meter and the second signal to the test subject i.e. our receiver. We could replace the power-meter with a Voltmeter.

However, if the signal-generator has an accurate signal output display this can be simplified by removing the splitter and the power-meter. Using this test setup it is very easy to make sure that the check of the S-Meter is accurate. As the above picture shows, to compensate for the losses of the test setup, the signal generator is set to provide a -66 dBm signal. This is about 7 dB higher than the -73 dBm level to compensate for the additional losses, 0.7 dB from the interconnections and attenuators, and about 6.3 dB from the splitter.

Equipment used:

• Signal-generator    :HP 8656
• Attenuators         :HP-355C and HP-355D
• Splitter            :VK5HW
• Powermeter          :VK3AQZ RFPM1

NOTE: Whatever signal source or attenuator you use, make sure that its accuracy has been checked against a known reference.

The equipment I've used has been checked and aligned where necessary against NIST-certified instruments including an LP-100A RF power-meter, a Boonton RF power-meter with a 51011-4B sensor, a Fluke 8842A DMM, and a Brymen BM-869s DMM.

S-Points, are they useful

The S-Meter in an Amateur Radio Receiver/Transceiver is an indicator for the received signal strength (Strength Meter). On HF, signal strength 9 (S9) has been defined to be an input power of -73dBm @ 50Ω (dBm is power expressed as decibels relative to 1mW). This is a level of 50µV (microvolts) measured at the antenna input port. And each step between S-Units corresponds to a difference of 6dB as recommended in the IARU Technical Recommendation R.1. 6dB is equivalent to a power ratio of four and a voltage ratio of two (S-Point History).

The term S-Unit/point is used to refer to the amount of signal strength that move the S-Meter indicator from one marking to the next, i.e. it moves by one S-Point/unit. On Amateur Radio equipment, most S-Meter markings are from S1 to S9, with marking above S9 in 10dB steps.

To be able to add meaning to the S-Meter report, I believe, Amateur Radio Operators should know how the S-Meter of the radio equipment is tracking against the IARU standard. Quite a few Amateur Radio Operators either don't seem to care or don't understand the value of having an instrument that can track precise. Let's consider the below;

• Profiling a couple of antennas by listening to the background noise.  On antenna one we see a noise level of S4 (-103dBm). Switching to the second antenna we see that the S-Meter indicates S6 (-91dBm). We could now conclude that there is a noise difference between ant1 and ant2 of 12dB or that one antenna has a gain of 12dB over the other.

• Checking the front to back ratio of a Beam (Yagi/Uda) Antenna. Receiving the signal from the front of the antenna the meter reads S9. Turning the antenna 180 degree, i.e. pointing the back of the beam to the signal source, the meter reads S5. This would indicate a nice 24dB(4 S-point = 4*6dB = 24dB) front to back ratio of the Antenna, but is it true?

• Comparing receiver prowess, i.e. using a receive splitter to split the receive signal equally to compare receivers. If receiver-1 is displaying S3 on the S-Meter and receiver-2 is displaying S7 on the same signal, is receiver-2 the better receiver?

We can clearly see that if we do not know how our S-Meter is tracking, i.e. are the steps between S-Points are really equal and at the 6dB step size. We can't be sure what is being displayed. Any of the above scenarios become guess work and as such would most likely lead us astray.

 

So to come back to the question at hand, I'd say since Amateur Radio is a Technical Hobby it would be nice to provide and receive a correct S-Value report, one that is trackable to a standard. An even better report would be SNR (one for the future).

However, if one uses the Radio for NET chats and contesting, then who cares if the S-value is S9 or S9+20dB or S5 for that matter.

Would it then not be more appropriate to say the quality of your signal at my station is Q5 or Q4, i.e. going back to the old Q(SA) system. One can always ask for an S-value, but what would be the usefulness in a value that does not track to a standard.

A little history about S-Points

The evaluation of signal strength using S units in the RST system was developed in 1934 by the Radio Amateur W. Braaten, W2BSR, and adopted in 1938 by the ITU. As receivers in the past often lacked a reception level indicator (S-meter), the RST system was based on a subjective hearing assessment of the received signals. RST stands for Readability, Strength and Tone Quality. And S1 represented a barely audible signal, while S9 represented a very strong signal. 

• In 1981, the International Amateur Radio Union (IARU) assigned shortwave reception up to 30 MHz the following S-levels, starting with the upper reference value of 50 microvolts (-73dBm) for an S9 level. 
• Each S-level below is half the voltage of the previous level, with a level of 0.2 microvolts (-121dBm) assigned to an S1 level. 
• Since a voltage ratio of 1/2 (50% or 0.5) corresponds to a 6dB decrease on a logarithmic scale, the proverbial "6dB per S value" was established.

NOTE: One S-Point really equates to 6dB!

Received Voltage		Received Power (Zin = 50 Ω)	Signal Strength (S-Value)
-14.0 dBμV	0.2 μV	-121 dBm	1
-8.0 dBμV	0.4 μV	-115 dBm	2
-2.0 dBμV	0.8 μV	-109 dBm	3
4.0 dBμV	1.6 μV	-103 dBm	4
10.0 dBμV	3.2 μV	-97 dBm	5
16.0 dBμV	6.3 μV	-91 dBm	6
22.0 dBμV	12.6 μV	-85 dBm	7
28.0 dBμV	25.1 μV	-79 dBm	8
34.0 dBμV	50.1 μV	-73 dBm	9
40.0 dBμV	99.9 μV	-67 dBm	9 +6
44.0 dBμV	158.3 μV	-63 dBm	9 +10
46.0 dBμV	199.3 μV	-61 dBm	9 +12
52.0 dBμV	397.6 μV	-55 dBm	9 +18
54.0 dBμV	500.6 μV	-53 dBm	9 +20
58.0 dBμV	793.4 μV	-49 dBm	9 +24
64.0 dBμV	1.6 mV	-43 dBm	9 +30
74.0 dBμV	5.0 mV	-33 dBm	9 +40
84.0 dBμV	15.8 mV	-23 dBm	9 +50
94.0 dBμV	50.1 mV	-13 dBm	9 +60

As such the S-Meter/indicator should display the receiver input voltage divided into S levels from S1 to S9 based on 6dB per step.

Oh no, there is no S0

Yes, to the surprise of some people, there is no S0. There is no S0 because it represents the absence of a signal, which is not measurable. (Albeit lots of HAMatuers think there is)

An observed on-air signal report: "I have noise on the frequency, so your signal report is R5 by S0 to S1" this can be confusing, as S0 really means I can't hear anything. (see S-Point are they useful) 

On the other end of the scale, input voltages greater than S9 are displayed as "x dB over S9". I have found that most newer HF-Radios are tracking very good above S9 and that S9 also seems to be fairly accurate to the -73dBm/50μV.

The definition of S9 = 50μV (-73dBm) as the reference level for frequencies up to 30 MHz was based on the sensitivity of conventional receivers at the time and the atmospheric background noise typically present at shortwave (a different reference level applies above 30 MHz).

Most S-meters are not exact level indicators but simply display the AGC control voltage. The result is often no more than an estimate of the actual input voltage. However, accurate S-Meters (low-power level meters like a field-strength-meter) are quite possible by tapping the IF and using log-amps as can be seen here (page 21), and of course in software as in Software-Defined Radio (SDR).

Remember that S levels are not a physical quantity, they are but a practical tool for the simple specification of received field strength levels. 

I believe a better choice would be a meter with a scale in dBm or μV, but only if the meter tracks properly.

NOTE: Due to the lower external noise above 30 MHz, a higher receiver sensitivity is required at VHF.

For this reason, the reference level for S9 was set 10 times lower for the frequency range above 30 MHz at an input voltage of 5μV. However, the 6dB step remains the same.