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. 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!
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.
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.
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 fairly 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. 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 ten (10) times lower for the frequency range above 30 MHz at an input voltage of 5μV. However, the 6dB step remains the same.
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