The RST range is used to describe the quality of a radio communication with a three digit number (but often only the two first are used). Intelligibility and strength of the signal are described with this method, and in case of Morse code communications (CW) also the quality of the tone is described.
| R1 | Not intelligible. |
| R2 | Just intelligible: some words only. |
| R3 | Bad intelligibility. |
| R4 | Good intelligibility. |
| R5 | Very good intelligibility. |
| S1 | 0.20 uV | -48 dB | Extremely low signals. |
| S2 | 0.39 uV | -42 dB | Very low signals. |
| S3 | 0.78 uV | -36 dB | Low signals. |
| S4 | 1.6 uV | -30 dB | Poor signals. |
| S5 | 3.1 uV | -24 dB | Medium signals. |
| S6 | 6.3 uV | -18 dB | Good signals. |
| S7 | 13 uV | -12 dB | Very good signals. |
| S8 | 25 uV | -6 dB | Strong signals. |
| S9 | 50 uV | 0 dB | Very strong signals. |
| S9+10 | 0.16 mV | +10 dB | Extremely strong signals. |
| S9+20 | 0.50 mV | +20 dB | Extremely strong signals. |
| S9+30 | 1.6 mV | +30 dB | Extremely strong signals. |
| S9+40 | 5.0 mV | +40 dB | Extremely strong signals. |
| T1 | Only noise. |
| T2 | Very noisy AC tone. |
| T3 | Low AC tone. |
| T4 | AC tone. |
| T5 | Modulated tone, whistle in the background. |
| T6 | Modulated tone, some noise. |
| T7 | DC tone, little noise. |
| T8 | Good DC tone, very little noise. |
| T9 | Pure DC tone. |
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