VK5HW AR Musings: Reporting of received Signal Strength (or, are S-Points really useful)

The evaluation of signal strength using S units/points in the RST system was developed in 1934 by the Radio Amateur W. Braaten, W2BSR, and adopted in 1938 by the International Telecommunication Union (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 S-1 represented a barely audible signal, while S-9 represented a very strong signal.

In 1981, the International Amateur Radio Union (IARU) assigned shortwave reception up to 30 MHz the following S-points, starting with the upper reference level of 50µV for an S-9 point. Each S-point below S-9 is half the voltage of the previous level, with a level of 0.2µV representing an S-1 point.

Since a voltage ratio of 1/2 i.e., 50% or 0.5, corresponds to a 6dB decrease on a logarithmic scale, the proverbial "6dB per S-Point" was established [1].

According to the IARU Handbook (V 9.0), which has been published in June 2018, Paragraph 4.1.3 states:

In order to make a uniform reporting system on the amateur bands possible, taking into account the widespread use of the 'subjective' S-system, taking into account the large deviations between the characteristics of S-meters on current amateur equipment, the IARU Region 1 recommends the use of the "S"-system for signal strength reporting on the amateur bands, based on the following standards:

“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.”

and for band above 30 MHz the following standard has been created:
“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.”

NOTE: (1) is based on a reference level of 50µV @ 50 Ohm and (2) is based on a reference level of 5µV @ 50 Ohm.

The following Table shows the relationship between S-Point, Voltage – and Power Levels.

Figure 1: S-Points vs Power - and Voltage Levels

The above table is not with respect to an antenna. Signal levels will be different if a signal is fed to an input with different impedance. Many antennas vary in impedance over various frequency ranges, particularly in the case of wideband designs. This is also mostly true with our modern transceivers. However, this should not be seen as a Fieldstrength measurement. It is a baseline to which all Amateur Radio Operator should confirm reception.

For an Amateur Radio Operator knowing how the S-Meter of the receiver is tracking against the IARU Standard will be very helpful in not only profiling its station, e.g., Antennas and Radios etc., but also track propagation changes.

It is not necessary to adjust the S-Meter, which in most cases would be rather difficult. However, the creation of a cheat sheet [2] will help in giving more meaningful reception reports report if required.

If required means that one should not rely to heavily on the so called 59 report or the 20 over S-9 etc. rather, if comparisons are being made those comparisons need to be based an a common reference. The use of a common reference, which is based on a Standard, provides much more meaningful results during station performance evaluation and comparisons which.

Some examples below.

Profiling a couple of VHF Yagi antennas by listening to the background noise. On one antenna a noise level of S-1 (0.02µV) i.e., -141dBm is being observed. Switching to a second antenna the S-Meter indicates an S-1.5 (0.03µV) i.e., -138dBm. The conclusion could be that the second antenna has a 3dB gain over the first antenna.
Profiling a HF Yagi with a gain of 4dBd. A signal received from the back of the Yagi indicates an S-6 (6.3µV) i.e., -91dBm and an S-9 (50.1µV) i.e.,-73dBm from the front. Which would indicate a Front to Rear attenuation of 14dB.
The following can be heard more and more on the Amateur Radio Bands, and that is Noise Floor (NF) comparison. Following a conversation between two fellow Amateur Radio Operators comparing their NF. One is using an S-Meter which has been compared to and does follow the IARU standard i.e., the 6dB per S-Point scale, is reporting an S-3 NF. The other operator brags about an NF of S-2. However, without knowing how his S-Meter really tracks there is no way that a meaningful conclusion can be drawn from that value. Let’s assume that the S-Meter track S-Points in 3dB steps in a linear fashion, it can then be see from the below tables that the second operator really does have a poorer NF then the first operator.

Examples (1) and (2) are simplistic but follow the IARU Standard i.e., the 6dB S-Meter scale. These examples would be rather meaningless if they would have been conducted without following a standard and more importantly if they would have been conducted against an S-Meter with unknown tracking characteristics. Example (3) highlights an issue where people are being told that "Everyones S-Meters are correct".

Figure 2: 6dB (right) steps vs 3dB (left) steps

Looking at the table on the right shows an S-Meter which tracks the S-Meter scale in linear 3dB steps. This table shows that a signal strength of S-2 equates to a -94dBm power level. And a signal strength of S-2 on the right table, which follows the IARU Standard of 6dB per S-Point, shows that an S-2 is a -115dBm power level. This is a 21dB difference. This kind of inconsistency would make it difficult to come to a meaningful conclusion about noise and signal strength. This kind of S-Meter might add to the confusion of the S-0 we do hear more often on the Bands these days. However, it needs to be stressed/taught that an S-0 indicates no signal and as such a report like R-4 S-0 (4 by 0) doesn’t make sense.

One could argue about “Does it really matter”?

It may or may not. It would depend on the content of the QSO. If it is a casual QSO with very little Amateur Radio (AR) related content i.e., no value other then the exchange of pleasantries and or to get the call-sign in to the log, then the answer is probably a “NO”. So for nets, rag-chewing and contests the use of S-Points is really meaningless. During those contact a signal quality report would be more appropriate, e.g., “Your are Q5 with a bit of QSB”

However, if there is a bit more AR substance to the conversation e.g., to better understand band conditions, then it would matter. As AR is proclaimed to be a technical Hobby, and let’s face it we do need to sit a test/exam to acquire the prescribed skills to operate an Amateur Radio Station. As such, would it not be advisable to add this aspect of the hobby to the syllabus?

Now, having said all this, maybe the S-Point system really isn’t suitable any longer. The new breed of Amateur Radio Transceivers are defined by Software, they do not need to pick a Voltage of the AGC to drive a mechanical meter anymore. As such there should be no issue for the standardisation of S-Meters.

Those newer radios can have a multitude of signal strength indicators as the meter is in a digital form. These meters would be able to report the received signal in,

S-Points (old habits die hard),
Voltage Levels (µV),
Power Levels (dBm) and last but not least,
Signal to Noise Ratio (SNR).

There are already Software Applications which have implemented some, if not all of the above. As such isn’t it about time that Clubs and or National Organisations starting to lobby the Industry to add a few more lines of code to their Software Defined Radios (SDR) to give the Amateur Radio Fraternity a better Signal Strength Meter?

NOTE: The Amateur fraternity would need to agree as a whole to how this should be implemented, e.g., that the metering system shall be based on quasi-peak detection with an attack time of X msec ± Y msec and a decay time constant of at least Z msec. This should then be standardised and the industry should be lobbied to implement this agree standard.

Appendix

It needs to be understood that Power and Voltage level reading are dependent on not only a 50 Ohm impedance at the measurement port (antenna port of the radio) but also on the receiver bandwidth. If we do real Field-strength measurements this needs to be taken into account. However, sprucing of that everyone’s S-Meter is correct seems to be rather simplistic and is not very helpful to a Technical Hobby.

The below shows a graphical representations of S-Points vs Power – and Voltage Levels.