S-meter calculator: S-units, dBm and microvolts
A '5 and 9' report only says something about signal strength if the S-meter is calibrated. IARU Region 1 adopted a recommendation for this in 1981. The Signal-Pirat S-meter calculator converts between S-unit, level in dBm and voltage in microvolts according to this recommendation and shows how far your own S-meter deviates.
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What the page does
- S9 on HF (below 30 MHz): −73 dBm, 50 µV into 50 Ω
- S9 on VHF (from 30 MHz): −93 dBm, 5 µV into 50 Ω
- One S-unit equals 6 dB
- Table S1 to S9+60 dB with dBm and µV
- Deviation of your own S-meter in dB and S-units
Background
A level of 0 dBm is one milliwatt; into 50 ohms that is about 224 millivolts, and one microvolt equals −107 dBm. Six decibels double the voltage. Many radios read too high or too low without calibration, on VHF and with the preamp on often quite differently from HF. A signal generator or a signal of known strength shows the deviation so it can be taken into account in reports.
In practice
How it works: choose the frequency range, enter level in dBm or voltage in microvolts and read the S-unit. To check, enter the displayed S-unit and the actual level; the calculator shows the deviation.
Frequently asked questions
Why is S9 different on VHF?
The IARU recommendation sets a reference 20 dB lower for VHF because signals there are usually weaker.
How many dB is one S-unit?
Six decibels, i.e. double the voltage.