FT8

Lore & Background

FT8 is a popular form of digital weak signal communication used primarily by amateur radio operators on HF bands, with a majority of traffic occurring there. The mode offers operators the ability to communicate despite unfavorable conditions such as low solar activity, high RF noise, or low transmitter power. With advances in signal processing technology, software can decode FT8 signals with a signal-to-noise ratio as low as −20 dB in a 2500 Hz bandwidth, significantly lower than conventional CW or SSB transmissions.

FT8 involves 77-bit message blocks transmitted in regular 15-second periods, consisting of 12.64 seconds of transmission time and 2.36 seconds of decode time, giving a digital data rate of 6.09 bits/sec. Source encoding gives an effective message throughput equivalent to about 5 words per minute. Forward error correction helps achieve reliable communication despite common RF issues such as fading, interference, and weak or noisy signals due to marginal propagation paths, low power operation, and inefficient antennas.

Each 77-bit message can carry up to 13 text characters. A compact encoding method allows standard FT8 exchanges, such as callsigns, signal reports, and grid locators, to fit efficiently within that limit. The protocol uses hashing to handle long or unusual callsigns, but these can still lead to occasional decoding errors or collisions that produce false or corrupted callsigns. Such errors are rare but can be mistaken for genuine or exotic stations, sometimes causing brief confusion or excitement among operators.

Reader's Guide

FT8 has become a mainstream digital modulation mode in amateur radio, operating on a 15-second cycle and usually completing a QSO in about 90 seconds. It is commonly used for short-wave QSOs, DX, and long-distance contacts. Most amateur radio hobbyists monitor FT8 frequencies on the HF bands. During meteor scatter or sporadic-E propagation, VHF or UHF FT8 signals can sometimes be received from over 1000 km away. To avoid interference and maintain reliable operation, hams have designated specific FT8 frequencies in each amateur band, including 7.074 MHz (40 m), 14.074 MHz (20 m), 21.074 MHz (15 m), and 28.074 MHz (10 m). FT8 communication requires strict time accuracy; each transmission must begin exactly at 00, 15, 30, 45, or 60 seconds. If the station clock is not properly synchronized, messages will likely fail to decode. Although FT8 transmissions occur within fixed time windows, the software can cope with discrepancies between sending and receiving systems of up to a second or two. Provided that they are manually set to the correct time every so often (for example, by using WWV or other time standard broadcasters), conventional computer real-time clocks are usually adequate. However, most FT8 users take advantage of online time servers using NTP or time signals from GPS to achieve and maintain better time accuracy automatically. There are multiple uses for FT8 including contesting, testing antennas, and for scientific research.

Did You Know?

• FT8 was released on June 29, 2017, by Joe Taylor, K1JT and Steve Franke, K9AN.
• Each FT8 message block is 77 bits and can carry up to 13 text characters.
• FT8 became the most widely used digital mode reported by automatic spotting networks such as PSK Reporter within two years of its release.