BI4OAE

FT8 Explained: How the 15-Second Weak-Signal Mode Actually Works

2026-10-18 · Digital Modes

A contact that does not feel like a contact

Watch someone operate FT8 for the first time and you may not even realize a QSO is happening. Two columns of decoded text scroll down the screen. The operator double-clicks a line, and a few seconds later another line appears. Forty-odd seconds after that, someone sends “73,” and the contact is over. Nobody spoke. Nobody tuned a signal by ear. The whole exchange looks like bookkeeping.

Then you notice the other station is 9,000 km away, and the rig is putting out five watts into a wire draped off a balcony.

That is the strange proposition of FT8: a mode that completes a full contact in about ninety seconds, with signals far too weak to hear, using power lower than a microwave oven. It first appeared in July 2017 as part of WSJT-X version 1.8.0, designed by Steve Franke (K9AN) and Joe Taylor (K1JT), and within a couple of years it had become the busiest digital mode on the HF bands.

The problem it was built to solve

Weak-signal work has always involved an uncomfortable trade-off. The weaker a signal gets, the narrower its bandwidth must be and the longer it must be received for the information to survive. A trained ear can pull Morse code out of noise that would completely swallow speech, which is why CW holds its own at the bottom of the noise floor. But no amount of training lets you understand voice that is 20 dB down.

FT8 takes the trade-off to its logical end. Compress the message until almost nothing is left, then hand the demodulation to software that can do what an ear cannot. The key numbers tell you how aggressive the design is:

That last figure is the whole point. At -20 dB in a 2.5 kHz bandwidth, the signal is nowhere near audible — yet the protocol recovers it reliably, helped by an LDPC(174,87) forward error correction code carried inside a 77-bit message.

Fixed messages, by design

FT8 carries no conversation. A 77-bit block simply is not big enough. What it holds instead is a small set of structured messages: your callsign, the other station’s callsign, a Maidenhead grid locator, a signal report in dB, and a few fixed acknowledgements such as RRR, RR73 and 73. There is also a free-text slot, but it maxes out at 13 characters — enough for “TNX QSO 73,” not much else.

A typical contact runs like this:

  1. Station A calls CQ with callsign and grid square
  2. Station B answers with both callsigns and its own grid
  3. Station A sends a signal report
  4. Station B returns a report plus R or RR73
  5. Station A confirms, and the QSO is logged

Because every station synchronizes to the same clock, WSJT-X decodes dozens of signals across a 3 kHz slice of spectrum in every cycle, and the waterfall fills with short coloured stripes where each one sits. The mode is named for its creators and its modulation — Franke, Taylor, and 8-FSK — and it behaves less like radio conversation and more like a distributed database that happens to use the ionosphere.

Why the hobby argues about it

The criticism is easy to understand. Detractors say FT8 is not really a contact: there is no exchange of information, no operating skill, and the software does the difficult parts. Much of the activity, they argue, is automated DXCC and grid-square collecting.

The defenders have a stronger case than the critics usually admit. Before FT8, working another continent on a modest budget meant a tower, a beam, and an amplifier — hardware most people cannot have. FT8 moved the bottleneck from hardware to software. A five-watt transceiver and a random wire in a constrained urban location, which used to be a receiver, becomes a working DX station. For operators with no possibility of a proper antenna, that is not a dilution of the hobby. It is the only door in.

My own view is simpler: treat FT8 as its own activity rather than a replacement for anything. It is superb for testing propagation, confirming that an antenna actually radiates, and working DX on a tiny budget. If what you enjoy is conversation, SSB and CW are still there, and nobody is taking them away.

Getting on the air

Hardware. Any SSB-capable HF transceiver will do. Five watts is enough to work the world when the band is open; 25 to 50 watts is already generous. There is no prize for running more.

Software. WSJT-X is the reference implementation, free and open source, running on Windows, Linux and macOS. It was created by Joe Taylor, K1JT — who shared the 1993 Nobel Prize in Physics for the discovery of a new type of pulsar — together with K9AN and other contributors. FT8’s faster sibling FT4, with a 7.5-second cycle and roughly 90 Hz bandwidth, arrived in 2019 and is aimed at contesting.

Time sync. This is the requirement new operators forget. Because every transmission starts on a fixed 15-second boundary, your computer clock must be within about a second of UTC. If WSJT-X decodes nothing at all, suspect the clock before you suspect the radio. NTP synchronization, or a GPS-disciplined clock if you want to be thorough, solves it.

Audio. Run the radio in USB (upper sideband). Modern transceivers expose a built-in USB sound card, so a single USB cable carries both audio and CAT frequency control. Older rigs need a sound card interface between the radio’s audio jacks and the computer.

Frequencies. Every HF band has an agreed digital segment — 14.074 MHz on 20 metres and 7.074 MHz on 40 metres are the usual FT8 spots. These are voluntary IARU band-plan conventions, not law, but they are followed closely enough that you will find the crowd immediately.

Practical advice from the air

Listen before you transmit. Watch two or three cycles and read the decoded lines. The format becomes obvious, and you avoid the classic beginner move of calling over somebody.

Keep the drive clean. FT8 is very sensitive to overdrive. Set the audio level so the ALC meter barely moves — splatter from a hot FT8 signal is just as rude as splatter from phone.

Choose a free audio offset. FT8 shares one passband across the whole band, and WSJT-X shows which frequencies are occupied in each cycle. Pick a clear spot rather than piling onto an existing signal.

Log everything. Let the software log automatically and upload to LoTW and PSK Reporter afterwards. The reports you get back are a free check on how well your station is really being heard.

Where FT8 sits in the hobby

FT8’s lasting contribution is not a new modulation. It is the moment weak-signal operation stopped being gated by antenna real estate and started being gated by understanding a piece of software. That shift opened the airwaves to everyone living under a metal roof with a wire out of the window — a group that used to be limited to listening. Whether you love the mode or avoid it, that part is hard to argue with.

References

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