The RST code is used by amateur radio operators, shortwave listeners, and other radio hobbyists to exchange information about the quality of a radio signal being received. The code is a three digit number, with one digit each for conveying an assessment of the signal’s readability, strength, and tone. The code was developed in the early 20th century and was in widespread use by 1912.
The R stands for “Readability”. Readability is a qualitative assessment of how easy or difficult it is to correctly copy the information being sent during the transmission. In a Morse code telegraphy transmission, readability refers to how easy or difficult it is to distinguish each of the characters in the text of the message being sent; in a voice transmission, readability refers to how easy or difficult it is for each spoken word to be understood correctly. Readability is measured on a scale of 1 to 5.
- Barely readable, occasional words distinguishable
- Readable with considerable difficulty
- Readable with practically no difficulty
- Perfectly readable
The S stands for “Strength”. Strength is an assessment of how powerful the received signal is at the receiving location. Although an accurate signal strength meter can determine a quantitative value for signal strength, in practice this portion of the RST code is a qualitative assessment, often made based on the S meter of the radio receiver at the location of signal reception. “Strength” is measured on a scale of 1 to 9.
- Faint signal, barely perceptible
- Very weak
- Fairly good
- Moderately strong
- Very strong signals
For a quantitative assessment, quality HF receivers are calibrated so that S9 on the S-meter corresponds to a signal of 50 μV at the antenna terminal. On VHF and UHF receivers used for weak signal communications, S9 often corresponds to 5 μV at the antenna terminal.
The T stands for “Tone”. Tone is only used in Morse code and digital transmissions and is therefore omitted during voice operations. With modern transmitter technology, imperfections in the quality of the transmitter modulation that can be detected by humans are rare. Tone is measured on a scale of 1 to 9.
- Sixty cycle a.c or less, very rough and broad
- Very rough a.c., very harsh and broad
- Rough a.c. tone, rectified but not filtered
- Rough note, some trace of filtering
- Filtered rectified a.c. but strongly ripple-modulated
- Filtered tone, definite trace of ripple modulation
- Near pure tone, trace of ripple modulation
- Near perfect tone, slight trace of modulation
- Perfect tone, no trace of ripple or modulation of any kind
An example RST report for a voice transmission is “59”, usually pronounced “five nine” or “five by nine”, a report that indicates a perfectly readable and very strong signal. Exceptionally strong signals are designated by the quantitative number of decibels, in excess of “S9”, displayed on the receiver’s S meter. Example: “Your signal is 30 dB over S9.”
Suffixes were historically added to indicate other signal properties, and might be sent as “599K”:
- X: stable frequency (crystal control)
- C: “chirp” (frequency shift when keying)
- K: key clicks
Because the N character in Morse code requires less time to send than the 9, during amateur radio contests where the competing amateur radio stations are all using Morse code, the nines in the RST are typically abbreviated to N to read 5NN. In general, this practice is referred to as abbreviated or “cut” numbers.