S meter

The term S unit refers to the amount of signal strength required to move an S meter indication from one marking to the next.

Some specialized integrated circuits for FM reception like CA3089 and CA3189 provide a DC signal to drive a 100 μA S meter.

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.

[5] IARU Region 1 Technical Recommendation R.1 defines S9 for the HF bands to be a receiver input power of -73 dBm.

Some S meters on traditional analog receivers are calibrated to read S9 for an input of -73 dBm but do not provide the correct 6 dB per S unit correspondence.

Even with a high quality SDR, it is worth keeping in mind that the S meter is measuring the strength of the signal at the 50 ohm input of the receiver, not at the antenna.