Handbook, chapter 15. Digital modes and FT8
FT8 was devised in 2017 by a group led by Joe Taylor, K1JT — the same Joe Taylor who won the Nobel Prize in physics for the binary pulsar. Within a few years the mode had taken over most of the amateur air.
Switch on 20 metres in the daytime and look at the panadapter. The densest place on the band is a narrow strip in which there is nothing to hear by ear: an even burbling with no word, no dot and no dash to be made out in it.
That is FT8, and that is where most of the world's contacts happen today.
A minute of FT8 on the air
An offset of 0,0 s — within the search window. The contact will happen, and the other station will see exactly this offset in the DT column.
Time is divided into fifteen-second slots, the same ones for the whole planet. You transmit in one set, the other station in the alternate set. Inside a slot: half a second of silence, 12.64 seconds of signal, and another 1.86 seconds for the software to decode everything and show you the list.
Drag the clock-error slider and watch exactly where the contact breaks down.
What flies through those 12.64 seconds
Not much. An FT8 message is 77 bits, less than ten bytes. They hold exactly what a contact needs and nothing more: two callsigns, a locator or a report, an acknowledgement.
A 14-bit checksum is added to those 77 bits, and the whole thing is encoded with the error-correcting code LDPC(174, 91) — more than twice the redundancy. That gives 174 bits, which pack into 58 three-bit symbols. Plus 21 synchronisation symbols so that the receiver can find the start of the transmission in the noise. 79 symbols in all.
Each symbol is one of eight frequencies and lasts 0.16 seconds. All the other numbers follow from that: 79 × 0.16 = 12.64 seconds, a rate of 6.25 symbols per second, and a whole transmission occupying 50 hertz — fifty times narrower than a voice channel.
This is where the mode's main property comes from.
Sensitivity: why it works where nothing can be heard
The WSJT-X developers quote the threshold at which the probability of a decode is still fifty per cent. It is referenced to the noise in a 2500 Hz bandwidth — that is, in the bandwidth of an ordinary voice channel:
| Mode | Rate | Bandwidth | Transmission | Threshold |
|---|---|---|---|---|
| FT4 | 20.83 baud | 83 Hz | 5.04 s | −17.5 dB |
| FT8 | 6.25 baud | 50 Hz | 12.6 s | −21 dB |
| JT65 | 2.69 baud | 178 Hz | 46.8 s | −25 dB |
| WSPR | 1.46 baud | 5.9 Hz | 110.6 s | −31 dB |
Minus twenty-one decibels means the signal is a hundred and twenty-six times weaker than the noise it sits in. To the ear there is nothing there, and there cannot be.
The price is visible in the same table, and it is an honest one: the lower the threshold, the longer the transmission and the less it carries. WSPR hears ten decibels better than FT8 but makes no contacts at all — it only reports «I am audible here».
Where decibels come from and why minus twenty-one is a lot is in chapter 6.
Why the clock has to be accurate
The receiver does not search the whole slot for your signal. It knows that a transmission begins half a second after the slot boundary and looks for it nearby — over 62 steps of 40 milliseconds in each direction, that is within roughly ±2.5 seconds.
Everything follows from that:
- An error of up to a second — you will notice nothing. The other station will see your offset in the
DTcolumn, and that is the only consequence. - An error of more than two and a half seconds — you are audible on the air, visible on the waterfall, and no decoder in the world will take you. You will be convinced nobody hears you because of your antenna.
- Both clocks are wrong. Your one second plus their one and a half and you are already at the limit. So the aim is not «to stay within tolerance» but «to have the right time».
There is one cure: synchronisation with a time server. On Linux this already works out of the box; on Windows the built-in mechanism checks far too rarely, and people install a separate client instead. Set it up once and forget it.
Look at the DT column in the messages you receive. If every single station shows roughly the same non-zero figure, it is your clock that has drifted, not theirs. If the numbers are scattered around zero, all is well.
What a contact looks like
Six transmissions, a minute and a half. The callsigns come from chapter 13, the locator from chapter 14:
UR3PKI: CQ UR3PKI KO50 the call and the locator
UT0UI: UR3PKI UT0UI KO50 the reply with its own locator
UR3PKI: UT0UI UR3PKI -12 report: signal 12 dB below the noise
UT0UI: UR3PKI UT0UI R-09 «received», and its own report
UR3PKI: UT0UI UR3PKI RR73 «received, see you»
UT0UI: UR3PKI UT0UI 73
The structure is the same as on voice: call, answer, exchange reports, sign off. The only difference is that all six transmissions are clicked with a mouse, and the report is not a polite «59» but a measured signal-to-noise ratio in decibels. Here it is always honest, because the software works it out.
Two things that burn radios out
Watch the ALC and do not overdrive the chain. FT8 is a single steady frequency at every moment, with no pauses. Push the audio level until limiting kicks in and a clean single-tone transmission turns into a fan of harmonics across the whole band. You will be heard where you should not be, and people will tell you so.
Remember the duty cycle. Twelve and a half seconds at full power with no pause at all is a regime a consumer transceiver was not designed for: on voice the average power is three or four times below the peak, on FT8 it equals the peak. Hence the general practice of setting about thirty per cent of the rated power. All the more so because at that sensitivity the extra watts add almost nothing: look again at the threshold table and recall that doubling the power is three decibels.
What there is besides FT8
- FT4 — the same thing but twice as fast (7.5-second slots). Made for contests.
- WSPR — not for making contacts but for reconnaissance: with two watts you find out how far your antenna reaches and see it on a map. The most useful tool there is for assessing your own station.
- JS8 — FT8 turned into a conversational mode: slower, but you can write free text. It rests on a niche but lively community.
- RTTY and PSK31 — the older digital modes, displaced but still alive in contests.
Software. The main one is WSJT-X from the same team; that is where to start. JTDX and MSHV give more control and are worth having once you know what you are missing. And FT8CN runs straight from a smartphone — handy in the field when you have no computer with you.
Frequencies. They are neither arbitrary nor local: FT8 runs on the same frequencies all over the world, otherwise nobody would hear anybody.
| Band | Frequency | Band | Frequency | |
|---|---|---|---|---|
| 160 m | 1.840 MHz | 17 m | 18.100 MHz | |
| 80 m | 3.573 MHz | 15 m | 21.074 MHz | |
| 40 m | 7.074 MHz | 12 m | 24.915 MHz | |
| 30 m | 10.136 MHz | 10 m | 28.074 MHz | |
| 20 m | 14.074 MHz |
These are dial frequencies on the transceiver, mode USB; the signal itself sits in the audio band above them. They are all built into WSJT-X by default, so you will never have to type them in — the table is here so you know where to tune when you just want to listen.
⚠️ Check against the IARU band plan and against what your licence class permits: the frequency everyone works on and the frequency you are allowed to work on are not always the same.
Is this amateur radio at all
The question comes up constantly, and the honest answer is «both yes and no».
No: there is no operator in FT8. No ear, no fist, no conversation, none of what makes the air the air. Two programs exchange ten bytes while a human presses a button.
Yes: FT8 brought back onto the air everyone who has neither room for an antenna, nor quiet, nor a kilowatt. A contact with the far side of the planet on five watts from a balcony dipole is not a compromise; it is something that simply did not exist before. And it also gave amateurs a propagation-measuring network that no scientific institution has: thousands of receivers reporting around the clock on what is audible from where.
The sensible approach is not to argue but to keep both. FT8 works when the band is nearly closed and there is nobody at all on voice. Voice and CW work when there is somebody to talk to.
Next — chapter 16, awards and contests: what it is all for and where to begin.