Radio: RST Codes

As a ham radio operator, you might hear something from another operator like “I have you three eight in Omaha,” “your signal report is five seven in southern Ohio,” or “copy you five by nine.” What would that mean?

Amateur radio operators, also known as ham radio operators, don’t hear the signal they send out, but to get a message out clearly, it is important to know how they sound on the air. In 1934 Arthur Braaten developed the RST code as a systematic way to give feedback. The RST codes provide a nuanced report in just two or three numbers. 

A complete RST report describes three characteristics: Readability, Strength, and Tone. Readability is rated on a scale of one to five, while Strength and Tone is rated on scales of one to nine, as follows:

R = Readability:

• R1 – Unreadable
• R2 – Barely readable, some words occasionally distinguishable
• R3 – Readable, but with considerable difficulty
• R4 – Readable with practically no difficulty
• R5 – Perfectly readable

S = Signal Strength

• S1 – Faint signal, barely perceptible
• S2 – Very weak signal
• S3 – Weak signal
• S4 – Fair signal
• S5 – Fairly good signal
• S6 – Good signal
• S7 – Moderately strong signal
• S8 – Strong signal
• S9 – Extremely strong signal

T = CW Tone

• T1 – 50/60 hertz a.c., very rough & broad
• T2 – Very rough a.c., harsh & broad
• T3 – Rough a.c. tone, rectified but not filtered
• T4 – Rough note, some trace of filtering
• T5 – Filtered rectified a.c. but strongly ripple-modulated
• T6 – Filtered tone, definite trace of ripple modulation
• T7 – Near pure tone, a trace of ripple modulation
• T8 – Near perfect tone, slight trace of modulation
• T9 – Pure Tone, no trace of ripple or modulation of any kind

A perfect score is “599,” the highest rating in each category. However, the third category, Tone, is a quality that only applies to continuous wave (CW) transmissions – that is, Morse code. So if you’re operating in voice mode (i.e., AM, SSB or FM), that third number is meaningless. It gets dropped, and you have a two-digit RS report. In that case, “59” is the perfect score. 

So “three eight in Omaha” would indicate “readable with some difficulty, with a fairly strong signal.” “Copy you five by nine” means “perfectly readable with full signal strength.” 

How to Understand RST Codes

Readability

“Readability” assesses how easy it is to copy the information being sent during the transmission, whether it’s a series of characters in Morse code or spoken words. Things that affect readability include atmospheric noise (static), interference, or fading (signal fades up and down). 

Strength

Most transceivers have an S meter, so you could just read your S meter to get your Strength number. However, S meters are not calibrated to a specific standard, so the same meter reading means different things on different sets. A perfectly audible signal might barely move your S meter. Therefore, you might adjust the number you report based on how loud a signal you actually hear. 

Exceptionally strong signals can pass the S9 reading on the S meter. “You’re 20 over 9” means “your signal’s 20 decibels over S9.”

Tone

The Tone report goes back to the early days of radio when most hams were building their own transmitters for Morse code, from spare parts – with varying results. The science of radio was still poorly understood, and RST reports helped radio operators significantly. 

Old radios (and some modern ones) suffered from “ripple” (from ineffective capacitors in the power supply, which are used to filter the rectified AC sine wave into a DC voltage). This was heard in the transmitted CW tone. Ripple might cause a “brrrrring” instead of a clean “beep.”

“Key click,” “hum,” and “chirp” also describe problems caused by power supply. Occasionally, suffixes indicate these issues. For example, 599K indicates a clear, strong signal, but with bothersome key clicks. 599C indicates chirp. 

Nowadays, most radios are well-built commercial equipment producing a clean tone, always worth nine on the scale, so Tone simply gets dropped from RST reports. 

Competitions

Ham radio operators use the RST code whenever they communicate with each other. However, a competition’s pressure for speed influences its use. 

Modern ham radio contestants using Morse code often abbreviate the “nines” in the RST to “Ns,” like “5NN.” Typing “N” in Morse code is faster than typing “9.” Using letters to stand for numbers is called “cut numbers.” 

During contests, hams are trying to reach as many contacts as possible in a given time. They often don’t bother to really assess the actual signal quality and instead send hasty, meaningless reports of 59 (phone) or 599 (CW) just to move on. 

This practice has become a gripe of ham operators who truly want an accurate picture of their signal. Some feel the rush of a contest makes accurate reports so unlikely that signal reports shouldn’t even be part of the exchange.

Things to keep in mind

In general, give accurate reports. Don’t write what you think the ham wants to hear. It’s helpful to tell someone if their signal isn’t coming through clearly. In turn, if you receive a three for readability, you might decide to repeat important things, like your location, or spell out your name to help folks understand you. 

If you receive a low S number, you might note the time of day, what frequency you’re using, and the space weather. Space weather, like solar flares and radiation storms, changes the density and structure of the Earth’s ionosphere and can disrupt (even block) the transmission of radio signals in the High-Frequency range, which includes the range hams use.

Along with learning about the conditions affecting your signal quality, you can use an accurate report to decide whether to linger. If either party has poor signal quality, contact (or QSO) is best kept brief. But if both hams have good signals, you can converse comfortably and confidently. 

Related Articles

Morse Code
Military Radio Protocol
Best HAM radio

Other Popular Articles

Military Alphabet
Military Watches
Zulu Time