The RST code is used by amateur radio operators to exchange information about the quality of a radio signal being received. Its composition is a 3-digit number. Each digit has an applicability to the quality of a signal that is being received. The code was developed about the time that radio communication was introduced in the early 20th century.
THE FIRST DIGIT
The first digit of an RST code reflects overall readability. This can have incremental values from 1 to 5, with 1 meaning “unreadable” to 5, which means “perfectly readable.”
THE SECOND DIGIT
The second digit reflects a signal’s strength. This digit can have incremental values from 1 to 9. A value of 1 means “barely perceptible” and 9 means a very strong signal. But this second digit can have an absolute value representing S-units. An S-unit is a measure of electrical signal strength appearing at a receiver’s antenna input with no amplification. S9 is defined as representing 5uV at the antenna terminal. Transceivers often have an S-meter that displays this value.
THE THIRD DIGIT
The third digit reflects tone. This third digit is only applicable for CW transmissions and thus, it is only common to hear RST codes having 2 digits. That is because a phone transmission will address phone quality, whereas for CW transmissions, a 3-digit figure will be applicable. If interested in learning the details of this third digit, the reader is invited to refer to a Wikipedia article on it.
RST IN ACTION
Suppose in a QSO that another amateur reported the quality of your signal as a 34 (pronounced three-four). The first digit (3) tells you that the other amateur is able to read your signal with considerable difficulty. The second digit (4) tells you that in his judgment, your signal is fair. What can you take away from this? Maybe you were not speaking clearly since the signal itself was fair.
Suppose the report was a 36 (three-six). Again, the signal is read with “considerable difficulty.” But this is being reported while the signal itself was “very good.” For this, you definitely want to think about what is happening between you and your microphone.