Understanding the measurement of signal strength with an S-meter, which is probably on every receiver or transceiver, is a relatively little-known thing. It is a part of the RS(T) report, which radio amateurs report every time they connect.
In 1981, the International Amateur Radio Union (IARU) Region 1 accepted a draft definition where, for the KV bands, the S9 level signal is defined as -73 dBm, which represents a voltage of 50 μV into an impedance of 50 ohms. For the VHF band, the S-meter is calibrated differently. The level of S9 is defined as -93 dBm, which represents a voltage of 5 μV into an impedance of 50 ohms. Each level S then has a distance of 6 dB from the level S9.
S-meter for KV bands
| Signal strength | Relative level to S9 | Tension | Power at Z = 50 Ω | ||
| S1 | -48 dB | 0.20 μV | –14 dBμV | 790 aW | -121 dBm |
| S2 | -42 dB | 0.40 μV | –8 dBμV | 3.2 fW | -115 dBm |
| S3 | -36 dB | 0.79 μV | –2 dBμV | 13 fW | -109 dBm |
| S4 | -30 dB | 1.6 μV | 4 dBμV | 50 fW | -103 dBm |
| S5 | -24 dB | 3.2 μV | 10 dBμV | 200 fW | -97 dBm |
| S6 | -18 dB | 6.3 μV | 16 dBμV | 790 fW | -91 dBm |
| S7 | -12 dB | 13 μV | 22 dBμV | 3.2 pW | -85 dBm |
| S8 | -6 dB | 25 μV | 28 dBμV | 13 pW | -79 dBm |
| S9 | 0 dB | 50 μV | 34 dBμV | 50 pW | -73 dBm |
| S9+10 | 10 dB | 160 μV | 44 dBμV | 500 pW | -63 dBm |
| S9+20 | 20 dB | 500 μV | 54 dBμV | 5.0 nW | -53 dBm |
| S9+30 | 30 dB | 1.6 mV | 64 dBμV | 50 nW | -43 dBm |
| S9+40 | 40 dB | 5.0 mV | 74 dBμV | 500 nW | -33 dBm |
| S9+50 | 50 dB | 16 mV | 84 dBμV | 5.0 μW | -23 dBm |
| S9+60 | 60 dB | 50 mV | 94 dBμV | 50 μW | -13 dBm |
S-meter for VHF bands
| Signal strength | Relative level to S9 | Tension | Power at Z = 50 Ω | ||
| S1 | -48 dB | 20 nV | –34 dBμV | 7.9 aW | -141 dBm |
| S2 | -42 dB | 40 nV | –28 dBμV | 32 aW | -135 dBm |
| S3 | -36 dB | 79 nV | –22 dBμV | 130 aW | -129 dBm |
| S4 | -30 dB | 160 nV | –16 dBμV | 500 aW | -123 dBm |
| S5 | -24 dB | 320 nV | –10 dBμV | 2.0 fW | -117 dBm |
| S6 | -18 dB | 630 nV | –4 dBμV | 7.9 fW | -111 dBm |
| S7 | -12 dB | 1.3 μV | 2 dBμV | 32 fW | -105 dBm |
| S8 | -6 dB | 2.5 μV | 8 dBμV | 130 fW | -99 dBm |
| S9 | 0 dB | 5.0 μV | 14 dBμV | 500 fW | -93 dBm |
| S9+10 | 10 dB | 16 μV | 24 dBμV | 5.0 pW | -83 dBm |
| S9+20 | 20 dB | 50 μV | 34 dBμV | 50 pW | -73 dBm |
| S9+30 | 30 dB | 160 μV | 44 dBμV | 500 pW | -63 dBm |
| S9+40 | 40 dB | 500 μV | 54 dBμV | 5.0 nW | -53 dBm |
| S9+50 | 50 dB | 1.6 mV | 64 dBμV | 50 nW | -43 dBm |
| S9+60 | 60 dB | 5.0 mV | 74 dBμV | 500 nW | -33 dBm |
Additional information: https://en.wikipedia.org/wiki/S_meter