RST Signal Reports Explained: Readability, Strength and Tone

Almost every amateur radio contact includes an RST signal report. It is the short three-digit code you hear as “five nine” on phone or “five nine nine” in Morse, and it is one of the first things new operators learn. It is also one of the least understood. Many operators send 59 or 599 to everyone, whatever they actually hear, and many receivers show S-meter readings that do not match any standard.

This guide explains what each digit in an RST report means, how the S-meter scale is defined in decibels and microvolts, what the report is used for in everyday operating and in contests, and how to give a report that is useful to the other station. It also covers how digital modes such as FT8 have replaced the traditional report with a signal-to-noise figure.

Where the RST System Comes From

The code was developed in 1934 by amateur radio operator Arthur W. Braaten, W2BSR, as a compact way of describing a received signal. As the overview of the R-S-T system explains, it is a three-digit number with one digit each for readability, strength and tone. It is similar to a code that was written into the ITU Radio Regulations in 1938.

The system has survived nearly a century because it is short and international. A report of “five nine” means the same in Tokyo and in Lima, even when the two operators share no other language. That is the point of a brevity code: you can send it clearly, quickly and with very little ambiguity.

R Is for Readability

Readability is a qualitative judgement of how easy it is to copy what is being sent. On CW it asks whether you can tell each character from the next. On phone it asks whether each word can be understood. The scale runs from 1 to 5:

• 1: unreadable.
• 2: barely readable, with occasional words distinguishable.
• 3: readable with considerable difficulty.
• 4: readable with practically no difficulty.
• 5: perfectly readable.

Readability is the digit that matters most. A signal can be very strong and still be hard to copy because of distortion, splatter, interference or poor audio. The reverse is also true: a weak but clean signal can be copied perfectly. When a report has to be short, readability is the digit that tells the other operator whether you have actually received the information.

S Is for Strength

Strength is rated from 1 to 9, with S1 meaning barely perceptible and S9 meaning extremely strong. Strictly it is an assessment of how powerful the signal is at your receiver, and in everyday use it is usually read from the S-meter on the radio. The table below shows how the S-units map to receiver input power and voltage on HF, using the figures from IARU Region 1 Technical Recommendation R.1.

Two rules summarise the whole scale. First, one S-unit corresponds to 6 dB, which is a voltage ratio of two and a power ratio of four. Second, S9 on HF is defined as -73 dBm, or 50 microvolts into a 50 ohm input. For the VHF and UHF bands the recommendation defines S9 as -93 dBm, about 5 microvolts, which reflects the lower noise levels there. These figures are described in more detail in the overview of the S meter.

Reports Above S9

Signals that exceed S9 are reported in decibels above S9. A signal that is 20 dB stronger than S9 is “S9 plus 20”, spoken “twenty over nine” or simply “twenty over”. On the radio’s meter the scale after S9 is usually marked in steps of 10 dB, such as +10, +20, +40 and +60.

The same logic helps in the other direction. If your meter reads S3 and another operator tells you the signal is S7, that is a difference of four S-units, or 24 dB, which is a very large gap. Thinking in decibels rather than S-units makes it easier to compare antennas, check whether a preamplifier helps, and judge how much power or antenna gain would be needed to move a weak signal up the scale.

T Is for Tone

Tone is rated from 1 to 9 and applies only to CW and other tone-based modes. It is omitted on phone, which is why a voice report is only two digits. In its original form the scale described the sound of a spark or early AC-powered transmitter. In modern use, 9 means a perfect tone with no trace of ripple or modulation, and a lower number means a signal with audible hum, ripple or roughness.

Today almost every transmitter produces a clean tone, so a tone digit of 9 is the normal result. Operators sometimes add a letter after the report to describe a specific fault. Historically, a suffix of K indicated key clicks, C a chirp at the start or end of each element, and D a drifting frequency. Those suffixes are rarely sent now, but they explain the many “599K” and “589C” remarks that appear in old logs and on QSL cards.

What 59 and 599 Really Mean

On phone, the standard report is “five nine”, meaning readability 5 and strength 9. On CW, the equivalent is 599. In practice these numbers are often used as a greeting rather than a measurement. Contest operators usually send 59 or 599 to every station because the exchange has to be fast, and because the receiving station does not use the number for anything. In CW contests the nines are commonly abbreviated to the letter N, which is quicker to send, giving 5NN. Our guide to cut numbers and CW contest exchanges explains this practice in detail.

Outside contests it is polite to give a real report. A true report helps the other operator test an antenna, check a new transmitter or find out whether their audio is clipped. If you give 59 to a station you can barely copy, you have made the report useless. If the signal is genuinely weak, an honest “four three” is a more helpful and more respectful reply than a routine “five nine”.

How to Give an Honest Report

A good report takes a few seconds of attention. Before you answer, listen for readability first, then strength, and then tone if you are on CW.

1. Judge readability before looking at the meter. Decide how easy it was to copy, not how loud the signal was.
2. Read the meter on a steady part of the signal. Fading and QSB make the needle swing, so use the typical level rather than the peak.
3. Note the effect of your own noise. If local noise is hiding a weak signal, you may need to add a remark. A report of “four by four, but I have local noise here” tells the other station why the number is low.
4. Be consistent. If you use the same approach every time, your reports become comparable from day to day and from band to band.
5. Say the numbers clearly. On phone, say each digit separately. Our guide to the ham radio phonetic alphabet includes advice on saying digits and call signs so that they survive a noisy band.

Why Your S-Meter May Be Wrong

Most transceivers include an S-meter, but few are calibrated as carefully as the standards suggest. On many radios the meter is driven by the automatic gain control voltage rather than a calibrated measurement, which means the response may not be a clean 6 dB per S-unit. Some radios read S9 at the correct input level but show unequal steps below it. Others read differently on different bands, depending on the preamplifier setting and the filters in use.

Several things change what the meter shows without changing the signal:

• Preamplifier and attenuator settings. Turning a preamp on can lift the reading by 10 dB or more without any change in the received signal.
• AGC settings and receive filters. A narrow filter on CW removes noise and can change the reading.
• Antenna choice. A dipole and a beam at the same station will give different readings for the same signal.
• Band noise. On the lower HF bands, atmospheric and man-made noise often exceeds S5 on its own, so the weakest signals you can copy may be below the noise level shown on the meter. If you are chasing a noise problem, see our guide to finding and killing RF noise in a modern home.

The practical lesson is to treat your own meter as a relative tool. Compare readings on the same radio, antenna and settings, and do not expect them to match the number you receive from a station across the world.

Signal Reports on Digital Modes

The RST report was designed for ears. On FT8, FT4 and similar modes, a program decodes the signal, so the report that is exchanged is a measured value instead. The software reports signal-to-noise ratio in decibels, referenced to a 2500 Hz bandwidth, and it can show values well below zero because the modes can decode signals that are weaker than the noise. A report such as -12 does not mean anything is wrong. It is simply the level of the signal compared with the noise in that reference bandwidth.

Some digital modes, such as PSK31 and RTTY, still use RST by convention. For those, the tone digit is usually left at 9, and strength is based on what the waterfall and decoder show. Several operators have suggested an RSQ report, in which quality replaces tone, for narrow-band digital modes, but it is less widely used. If you are new to digital modes, our WSJT-X and FT8 setup guide shows where the report fields appear in the software.

Common Mistakes With Signal Reports

• Sending 59 to everyone. Acceptable in a contest, but outside one it throws away useful information.
• Confusing strength with readability. A strong but distorted signal deserves a low readability digit.
• Quoting S-meter readings as absolute facts. Without calibration, an S-meter number is only meaningful for the radio that displays it.
• Ignoring tone on CW. If you hear a chirp or key clicks, a friendly remark helps the operator fix the problem.
• Mixing voice and CW formats. A voice report is two digits and a CW report is three. Sending “five nine nine” on phone sounds odd, although it is understood.

The Bottom Line

An RST signal report is a three-digit code for readability, strength and tone. Readability runs from 1 to 5, strength from 1 to 9 with each S-unit equal to 6 dB, and tone from 1 to 9 on CW. S9 is defined as -73 dBm on HF, and signals above it are reported in decibels over S9. In contests the report is a formality, but on ordinary contacts an honest one gives the other station real feedback. Remember that your S-meter is a relative guide, not a calibrated instrument, and that digital modes replace RST with a measured signal-to-noise figure.

Frequently Asked Questions

What does an RST signal report stand for?

RST stands for readability, strength and tone. Readability uses a scale of 1 to 5, strength 1 to 9, and tone 1 to 9. Tone is sent only for CW and other tone-based modes, so a phone report has just two digits.

How many decibels is one S-unit?

One S-unit is 6 dB, equal to a voltage ratio of two and a power ratio of four. S9 on HF is defined as -73 dBm, or 50 microvolts at a 50 ohm receiver input.

Why does everyone give 59 or 599?

In contests the report is a formality and speed matters, so 59 or 599 is sent to everyone. Outside contests, an honest report is more useful and more polite.

What does 5NN mean?

It is 599 with the nine replaced by the letter N, which is faster to send in Morse code. It is one of the cut numbers used in CW contest exchanges.

Is my radio’s S-meter accurate?

Often only roughly. Many meters respond to the AGC voltage and are not calibrated to 6 dB per S-unit, and the reading changes with the preamplifier, filters and antenna. Use it as a relative guide.

Do FT8 and other digital modes use RST?

FT8 and FT4 exchange a signal-to-noise figure in decibels instead, referenced to a 2500 Hz bandwidth. Some older digital modes such as RTTY and PSK31 still use RST by convention.