Signal guide

FT8

A weak-signal digital mode where stations exchange short, standard messages in 15-second turns. It can be copied even when the signal is too weak to hear.

I've heard this My life list

FT8 sounds like a cluster of soft, quiet musical warbles side by side that all start together, run for about 13 seconds, and stop, repeating every 15 seconds.

What does it sound like?

Plate 22Weak-signal digital

Many soft, quiet musical warbles side by side, starting and stopping together every 15 seconds.

0 Hz1,0002,0003,000 Hz

12-second example. Recorded off the air on the developer's own receiver.

Field marks15-second turns8 tones, 6.25 Hz apartAbout 50 Hz wide each14.074 MHz on 20 m

Hear it, step by step

This FT8 clip, recorded off the air on 10 meters, holds one strong transmission plus a few faint copies of it.

0 Hz1,0002,0003,000 Hz

Tap a step to hear that moment. The box shows where it is on the waterfall.

0.2 sA start-up sync patternThe signal switches on at 0.23 seconds. Its first seven steps follow the fixed pattern 3-1-4-0-6-5-2, which lets a receiver lock onto its exact timing and pitch.1.4 sEight tones, steppingAfter that, every 0.16 seconds the tone jumps to one of eight pitches spaced 6.25 Hz apart, from 1,376 to 1,419.5 Hz. The order of the jumps carries the message.6.0 sThe pattern again, mid-messageAt 6.0 seconds the same seven-step pattern returns. FT8 places it at the start, middle and end of every transmission.0.2 sAbout 50 Hz wideThe whole signal fits in about 53 Hz, from 1,371 to 1,424 Hz. It is still going when the clip ends, because one FT8 transmission lasts about 12.6 seconds.0.2 sFaint copies, not new stationsThe dim stripes 120 Hz above, 120 Hz below and 240 Hz below step through exactly the same tones, 31 to 40 dB weaker. A standard FT8 decoder reads the same message from each, so they are copies of this one signal, not other stations.

Make your own: type a message and hear it as FT8 in the Signal Maker.

Who uses it?

Amateur radio operators; it is the busiest digital mode on the ham bands.

Where and when can I hear it?

For example 7.074, 14.074 and 28.074 MHz. Transmissions start just after :00, :15, :30 and :45 seconds past each minute.

The science

FT8 packs a short, structured message (call signs, a grid locator, a signal report or an acknowledgment) into exactly 77 bits, adds a 14-bit checksum, and protects the result with a low-density parity-check (LDPC) code, an error-correcting code that lets the receiver repair bits lost to noise; the full codeword is 174 bits. Those bits are sent as 79 symbols using eight tones (8-GFSK, frequency-shift keying with smoothed transitions) at 6.25 symbols per second, so one signal occupies only 50 Hz and lasts about 12.6 seconds of each 15-second turn. Three fixed seven-tone patterns called Costas arrays sit at the start, middle and end so the receiver can lock onto the exact time and pitch, and the decoder removes each signal it decodes from the audio and searches again, which can reveal weaker signals hidden underneath.

Why use it?

FT8 trades a little sensitivity for speed: its designers accepted about 4 dB less sensitivity than the older JT65 in exchange for 15-second turns instead of one-minute turns. It still decodes at roughly 21 dB below the noise in a 2500 Hz bandwidth, so stations with a few watts and simple antennas can make contacts across the world, and many 50 Hz-wide signals fit side by side in one ordinary voice channel.

Lore

FT8 was introduced in July 2017 with version 1.8 of the free WSJT-X software and within a short time accounted for a large share of all ham activity on the HF bands. Its lead developer, Joe Taylor (K1JT), shared the 1993 Nobel Prize in Physics with Russell Hulse for discovering a new type of pulsar.

What do the decoded parts mean?

• CQ: A general call: 'anyone want to talk?'
• Call signs: Who is calling whom.
• Grid square: A four-character location code (for example CN85) marking a region about 100 by 70 miles at mid-latitudes.
• Signal report: A number such as -12, in decibels: how strongly one station hears the other.
• RR73 / 73: 'Received, best regards': the contact is complete.

Easily confused with

• FT4: FT4 turns every 7.5 s and is wider
• JS8Call: JS8Call sounds similar but carries free-text chat
• WSPR (propagation beacons): WSPR is far narrower and lasts nearly 2 minutes

How to recognize it

The timing is the giveaway. Every signal in an FT8 segment begins within a second or so of :00, :15, :30 or :45 past the minute, lasts about 12.6 seconds, and then the whole band goes quiet while stations decode and decide what to send next. On a waterfall display, you see neat rows of short stripes, all lined up in time like bricks.

Each individual signal is only about 50 Hz wide. It steps among eight tones spaced 6.25 Hz apart, changing tone about six times a second, which gives it a gentle, burbling, almost melodic sound. Many signals are too weak to hear at all: FT8 was designed to be copied well below the noise.

Easily confused with:

• FT4: the same idea, but turns change every 7.5 seconds and each signal is wider and busier.
• JS8Call: sounds almost identical, but lives on its own frequencies (such as 7.078 and 14.078 MHz) and carries free-form chat.
• WSPR: a single very slow tone lasting almost two minutes, not 15-second bursts.

Where and when to hear it

FT8 has standard spots on nearly every ham band. Common dial frequencies (upper sideband) include 3.573, 7.074, 10.136, 14.074, 18.100, 21.074, 24.915, 28.074 and 50.313 MHz. The signals sit in the first 3 kHz above the dial frequency.

There is almost always activity somewhere: 20 meters (14.074 MHz) during the day, 40 and 80 meters at night, and 10 and 6 meters when those bands open. Because thousands of stations report what they receive to public maps such as PSK Reporter, FT8 has also become a quick way to see which bands are open right now.

A little history

FT8, created by Joe Taylor (K1JT, a Nobel Prize-winning astrophysicist) and Steve Franke (K9AN), was introduced in July 2017 in version 1.8 of the free program WSJT-X. It built on earlier weak-signal modes such as JT65 and JT9, but its 15-second turns made contacts four times faster than the one-minute modes. It spread with unusual speed and within a couple of years accounted for a large share of all activity on the HF ham bands.

Each FT8 transmission carries exactly 77 bits of information, protected by strong error correction and a checksum, so a damaged message is almost always rejected rather than shown wrong.

Common questions

Sources

• Franke, Somerville & Taylor, 'The FT4 and FT8 Communication Protocols', QEX July/August 2020
• WSJT-X home page
• PSK Reporter
• WSJT-X 2.7 User Guide (K1JT and the WSJT Development Team)
• Nobel Prize in Physics 1993 summary
• ARRL News: Nobel Laureate Joe Taylor, K1JT, addresses WRC-12
• WSJT-X source, default working frequencies (models/FrequencyList.cpp, GitHub mirror)