An S meter (signal strength meter) is an indicator often provided on communications receivers, such as amateur radio receivers or shortwave broadcast receivers. The scale markings are derived from a subjective system of reporting signal strength from S1 to S9 as part of the RST code. The term S unit can be used to refer to the amount of signal strength required to move an S meter indication from one marking to the next.
IARU Region 1 Technical Recommendation R.1
IARU Region 1 Technical Recommendation R.1 defines S9 for the HF bands to be a receiver input power of -73 dBm. This is a level of 50 microvolts at the receiver's antenna input assuming the input impedance of the receiver is 50 ohms.
For VHF bands the recommendation defines S9 to be a receiver input power of -93 dBm. This is the equivalent of 5 microvolts in 50 ohms.
The recommendation defines a difference of one S-unit corresponds to a difference of 6 decibels (dB), equivalent to a voltage ratio of two, or power ratio of four.
Signals stronger than S9 are given with an additional dB rating, thus "S9 + 20dB", or, verbally, "20 decibels over S9" or simply "20 over 9".
A weak signal with signal strength of S2 corresponds to received power of -115 dBm or 0.40 microvolts in 50 ohms on HF.
A strong signal with signal strength of S8 corresponds to received power of -79 dBm or 25 microvolts in 50 ohms on HF.
Some signal generators are calibrated in dB above 1uV and have an output in emf. For example to set an HF receiver's S-reading to S9 set the signal generator output to 34 dB above 1uV
|S-reading||HF||Signal Generator emf|
|μV (50Ω)||dBm||dB above 1uV|
Most S meters are not calibrated and in practice can only provide a relative measure of signal strength based on the receiver's AGC voltage. Some S meters are calibrated to read S9 for an input of -73 dBm but do not provide the correct 6 dB per S unit correspondence.
Often the correlation between a radio listener's qualitative impression of signal strength and the actual strength of the received signal is poor, because the receiver's AGC holds the audio output fairly constant despite changes in input signal strength.
- International Amateur Radio Union Region I (1981). IARU Region 1 Technical Recommendation R.1. Brighton, England, UK.