Learn / Guide 7
FT8 and WSPR: weak signals crossing oceans
How the 15-second FT8 cycles work, what each message says, WSPR as a propagation probe and the PSKReporter map.
intermediate 6 min read
Why these modes exist
Voice needs a signal well above the noise to be understood. Telegraphy tolerates less. Weak signal digital modes go further: the computer recovers the message from a signal the human ear cannot even perceive.
FT8 was introduced in 2017 by Joe Taylor, K1JT, and Steve Franke, K9AN, within the WSJT-X program. In a few years it became the most used mode on shortwaves. With just a few watts, an FT8 signal frequently crosses continents. With the same power in voice, it would be much harder.
How FT8 works
Time is divided into 15 second cycles, aligned with the clock: starting at seconds 0, 15, 30, and 45. One station transmits in a cycle and listens in the next.
- Each transmission lasts 12.64 seconds. The rest of the cycle is the time the computer has to decode.
- The signal uses eight tones, separated by 6.25 Hz, and occupies 50 Hz. Dozens of stations fit in a 3 kHz span.
- The message has 77 useful bits, protected by a strong error-correcting code.
- The program decodes down to about 21 dB below the noise, measured across a 2,500 Hz bandwidth.
- The computer clock needs to be accurate to within about one second. Without internet or GPS synchronization, nothing decodes.
Everything happens in USB, on fixed frequencies per band:
| Band | Tuning frequency (MHz, USB) |
|---|---|
| 160 m | 1,840 |
| 80 m | 3,573 |
| 40 m | 7,074 |
| 30 m | 10,136 |
| 20 m | 14,074 |
| 17 m | 18,100 |
| 15 m | 21,074 |
| 12 m | 24,915 |
| 10 m | 28,074 |
| 6 m | 50,313 |
What each message says
A complete QSO in FT8 takes six transmissions, just over a minute. The example below uses the fictional callsigns from the WSJT-X manual.
| Who transmits | Message | What it means |
|---|---|---|
| K1ABC | CQ K1ABC FN42 |
general call from K1ABC, who is in square FN42 |
| G0XYZ | K1ABC G0XYZ IO91 |
G0XYZ replies and reports his locator |
| K1ABC | G0XYZ K1ABC -19 |
K1ABC says he received G0XYZ at -19 dB |
| G0XYZ | K1ABC G0XYZ R-22 |
received, and you arrived here at -22 dB |
| K1ABC | G0XYZ K1ABC RR73 |
all confirmed, best regards |
| G0XYZ | K1ABC G0XYZ 73 |
best regards, out |
Three things to notice:
- The four character square is the grid locator, an approximate address on the map. The ABC region of São Paulo, for example, is in GG66.
- The number is the signal to noise ratio in dB, measured by the program. It ranges from about -24 to positive values. Below -15 you hear nothing in the speaker, and the contact is completed anyway.
- There is no conversation. FT8 exchanges callsign, location and report, and that is it. Anyone who wants to chat uses voice or telegraphy.
You can decode without a license
Receiving is free, and it is a great exercise.
The shortest path is the FT8 extension of KiwiSDR receivers. Open one of them, choose the extension in the menu, select the band and messages start to appear in text, with callsign, grid and signal-to-noise ratio.
You can also install WSJT-X and feed the program with audio from a receiver. In this case, watch out for delay: the sound from a WebSDR reaches the browser with a one or two second lag, and that might be enough to prevent FT8 from decoding. The KiwiSDR extension does not have this issue because it decodes right there on the receiver.
WSPR, the propagation probe
WSPR (pronounced whisper) was created before FT8, also by Joe Taylor, and has a different purpose. It is not used for making contacts. It is used to measure how far a weak signal can reach.
- Each transmission lasts about 110 seconds and starts on an even minute.
- The signal occupies about 6 Hz. On the waterfall display is a wire you can barely see.
- The message carries three pieces of information: callsign, four-character locator and power in dBm.
- The program decodes signals more than 25 dB below the noise.
Powers are small. It is common to see stations with 1 W, with 200 mW or less, received thousands of kilometers away. Some tuning frequencies: 7.0386 MHz, 10.1387 MHz and 14.0956 MHz, always in USB.
Those who receive send logs to a public database. Each line states who heard whom, when, on which band, and with what signal-to-noise ratio. There are millions of propagation measurements per day, made by volunteers. Transmitting WSPR requires a license. Receiving and reporting do not.
PSKReporter: who heard who
PSKReporter is a map fed by the programs themselves. Each station running WSJT-X with the option enabled sends everything it decoded. The site gathers and displays it.
For those who transmit, it directly answers the question: where did my signal reach in the last few minutes. For those who only listen, it is the best real-time propagation map that exists.
- Choose the band (for example, 20 m) and the FT8 mode.
- Request signals received by a country or a square, such as GG66.
- See where the stations arriving there right now are coming from.
- Switch bands and compare.
If 10 meters shows paths from Brazil to Europe, the band is open, no matter what the solar index of the day.
What to observe during listening
- On 7,074 kHz at night and 14,074 kHz by day, the FT8 block is almost always full.
- Compare the signal-to-noise ratio of the same station over an hour. You see the band opening or closing in numbers.