S meter
An S meter indicates signal strength on communications receivers.
An S meter, or signal strength meter, is a common feature on communications receivers like those used for amateur radio and shortwave broadcasting. Its scale runs from S1 to S9, based on the R-S-T signal reporting system. An S unit is the amount of signal strength needed to move the meter from one marking to the next.
Analog S meters are sensitive microammeters, typically with a full-scale deflection between 50 and 100 μA. In AM receivers, the meter can be connected to the main detector or use a separate detector at the final IF stage, the latter being preferred for CW and SSB receivers. Another older method, from the era of vacuum tubes, involved connecting the meter to the screen grid circuit of the final IF amplifier tube. A third option is to connect it to the AGC line through a level conversion circuit. In FM receivers, the S meter circuit must connect to the IF chain before any limiter stages. Some specialized FM integrated circuits, such as the CA3089 and CA3189, provide a DC signal to drive a 100 μA meter.
The IARU Region 1 Technical Recommendation R.1, agreed upon in 1981, standardizes S meter calibration. For HF bands, S9 corresponds to a receiver input power of -73 dBm, which is 50 microvolts at the antenna input assuming a 50-ohm impedance. For VHF/UHF bands, S9 is defined as -93 dBm, or 5 microvolts in 50 ohms. The recommendation states that one S-unit corresponds to a 6 dB difference, equivalent to a voltage ratio of two or a power ratio of four. Signals stronger than S9 are reported with an additional dB rating, such as "S9 + 20dB".
For example, on HF, an S2 signal corresponds to -115 dBm or 0.40 microvolts RMS in 50 ohms, while an S8 signal corresponds to -79 dBm or 25 microvolts RMS in 50 ohms. Some signal generators are calibrated in dB above 1 μV; to set an HF receiver to S9, the generator output should be set to 34 dB above 1 μV.
Most S meters on traditional analog receivers are not calibrated and provide only a relative measure of signal strength based on the AGC voltage. Some are calibrated to read S9 at -73 dBm but do not maintain the correct 6 dB per S-unit step. The correlation between a listener's qualitative impression of signal strength and the actual signal strength is often poor because the AGC keeps audio output fairly constant despite changes in input.
- S9 hf power
- -73 dBm
- S9 hf voltage 50 ohms
- 50 microvolts
- S9 vhf power
- -93 dBm
- S9 vhf voltage 50 ohms
- 5 microvolts
- S unit difference
- 6 dB
- Analog meter full scale
- 50 to 100 μA
- Iaru region1 agreement year
- 1981
Lore & Background
In the 1930s, it was already agreed that S9 corresponds to 50 μV at the input terminal of the receiver, but this was not a measure of the power received as the input impedance of receivers was not standardized. The International Amateur Radio Union (IARU) Region 1 agreed on a technical recommendation for S Meter calibration for HF and VHF/UHF transceivers in 1981. IARU Region 1 Technical Recommendation R.1 defines S9 for the HF bands to be a receiver input power of -73 dBm, equivalent to 50 microvolts at 50 ohms. For VHF bands, S9 is -93 dBm, or 5 microvolts in 50 ohms. The recommendation defines that one S-unit corresponds to 6 dB, equivalent to a voltage ratio of two or power ratio of four. Signals stronger than S9 are given with an additional dB rating, such as 'S9 + 20dB'.
Reader's Guide
The S meter's significance lies in its role as a standard reference for signal strength reporting in amateur radio, though its accuracy varies widely. Most S meters on traditional analog receivers are not calibrated and can only provide a relative measure of signal strength based on the receiver's AGC voltage. Some are calibrated to read S9 for -73 dBm but do not provide the correct 6 dB per S unit correspondence. The correlation between a listener's qualitative impression and actual signal strength is often poor because AGC holds audio output fairly constant. Software-defined radios (SDRs) with bit depths of 14-bits or more are often accurately calibrated from one end of the S scale to the other out of the box; lower bit depth SDRs suffer precision at the low end. Even with high quality SDRs, the S meter measures signal strength at the 50 ohm input of the receiver, not at the antenna, so impedance mismatches can affect readings. Relative signal strengths at any one frequency remain meaningful, though not necessarily from one frequency to another.
Did You Know?
- S9 for HF bands is defined as -73 dBm, or 50 microvolts at 50 ohms.
- One S-unit corresponds to a difference of 6 dB, equivalent to a voltage ratio of two.
- Software-defined radios with 14-bit or higher bit depths are often accurately calibrated out of the box.
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