R-S-T System of Reports in Amateur Radio

 As soon as radio amateurs establish contact over the radio, they exchange signal reports, typically in the R-S-T format. R-S-T stands for Readability, Strength and Tone, though the last one is meant only for CW signals. The system was developed by Arthur W. Braaten, W2BSR in 1934 and is continuing till date. Readability reports can spread from 1 for an unreadable signal to 5 for a perfectly readable signal. Very often radio amateurs are quite lenient and give readability 5 even if there is great difficulty in copying. Only rarely have I heard people giving a readability report of 4 and hardly do I hear anyone giving reports below that.

Coming to signal strength, the range is from 1 for faint barely perceptible signal to 9 for extremely strong signal. Here also, radio amateurs tend to give liberal reports. Special situations are satellite operations and contests. In contests no one gives a report below 9! Similarly, in LEO satellite operations, every one gives a report as 5,9 regardless of how difficult or easy the copy is. That is because we have only limited time in both situations and do not want to waste time trying to repeat signal reports multiple times. Signal reports have been quantified later on with advancement in radio technology to correspond to actual signal strengths displayed in S meters of the receivers.

For HF radios, S9 corresponds to 50 μV signal strength at the antenna terminal with 50 ohms impedance.  Difference of one S point corresponds to 6 dB of signal strength, which will convert to two times of voltage and 4 times of power. On VHF and UHF bands with usually weaker signals, S9 corresponds to 5 μV at antenna terminal with 50 ohms impedance. In HF radios, S meters display signal strengths over S9 as 20 to 60 dB range. A strong signal on 40m in my region may have a report of 5,9 + 20 dB. I have noted still higher strengths in my S meter only for broadcast stations, though I have heard others giving 5,9+25 dB sometimes.

The last character in the R-S-T system is for tone of CW signals. With CW signals becoming rare on the band, very often we do not hear the tone report on the air. Tone also can be reported from 1 for extremely rough note to 9 for a perfect tone without any ripples. With modern radios, tone is most often 9. While I was on homebrew radio long back, I used to get 8 for tone and a mention that it is 'chirpy'. Chirpy means the frequency is drifting while keying. Very often, 9 for signal strength and tone are replaced by N in CW so that it is much easier to send. So a 5,9,9 report may be sent as 5NN. This avoids the trouble of keying numbers which are more difficult to send.