S Meter Specification

The International Amateur Radio Union (IARU) 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. This is a level of 50 microvolts at the receiver's antenna input assuming the input impedance of the receiver is 50 ohms.
• 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.

• 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".

Some signal signal generators are calibrated in dB above 1uV and have an ouput in emf. For example to set an HF receiver's S-reading to S9 set the signal generator output to 34 dB above 1uV