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RST System
by John Shannon, K3WWP

The RST system is a means of quickly telling the other station a wealth of information with just 3 numbers. When you say to someone 'RST 469', they immediately know you are having a little trouble copying them (4), they know you are hearing them at a good level (6), and their signals are in good shape with no hum or buzz (9). These 3 numbers are independent of one another, and simply because a station is very weak, you can still be copying perfectly, and his tone can still be perfect. This would be an RST of 529. I have heard some stations improperly giving out RST's of 555 or 577 when the other station had a perfect tone. They somehow think the T should track with the S, and if the sig is weak, the T number should be lower. This is completely wrong.

Nowadays, virtually every signal you hear is a T9. The meanings of T1-T8 are throwbacks to the early days of radio when you really could hear a T1 signal. Oh, occasionally you will hear a station with some hum, buzz, or ripple, and should give them a T7 or T8.

In addition to the three numbers, once in a while a letter is added to the end of the 3 numbers. These letters are: X meaning the signal is perfectly steady like a crystal controlled signal; C meaning the signal sounds chirpy as the frequency varies slightly with keying; and K meaning the signal has clicks. X is a throwback to the early days of radio when such steady signals were rare. Today most all signals could be given an X and it is hardly ever used. It is helpful if you hear a chirpy or clicky signal to use the C or K, e.g. 579C or 579K.

The following table lists the values and definitions of the R, S, and T.

RReadability
1Unreadable
2Barely readable, occasional words distinguishable
3Readable with considerable difficulty
4Readable with practically no difficulty
5Perfectly readable
SSignal Strength
1Faint signals, barely perceptible
2Very weak signals
3Weak signals
4Fair signals
5Fairly good signals
6Good signals
7Moderately strong signals
8Strong signals
9Extremely strong signals
TTone
1Sixty cycles or less, very rough and broad
2Very rough ac, very harsh and broad
3Rough ac tone, rectified but not filtered
4Rough note, some trace of filtering
5Filtered rectified ac, but strongly ripple-modulated
6Filtered tone, definite trace of ripple modulation
7Near pure tone, trace of ripple modulation
8Near perfect tone, slight trace of modulation
9Perfect tone, no trace of ripple, or modulation of any kind