FT8

FT8 is a popular form of digital weak signal communication used primarily by amateur radio operators to communicate on amateur radio bands with a majority of traffic occurring on the HF amateur bands.^[3] The mode offers operators the ability to communicate despite unfavorable conditions such as during low solar activity, high RF noise, or with low transmit power.^[4] 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, which is significantly lower than conventional CW or SSB transmissions.^[5]

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. The required signal-to-noise ratio in a 2500 Hz bandwidth is −21 dB, so the corresponding E_b/N₀ is 10 log₁₀(2500/6.09) = 26.1 dB greater, or −21 dB + 26.1 = 5.1 dB.^[1]

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 the GPS to achieve and maintain better time accuracy, automatically.

Forward error correction helps achieve reliable communication despite common RF issues such as fading and interference, and weak/noisy signals due to marginal propagation paths, low power operation and inefficient antennas (e.g. in restricted and overcrowded urban locations). If anticipated messages are missed or not acknowledged, the software can re-send them in the next time-slot.

The 77 bits are sufficient for free text messages of up to 13 text characters, reminiscent of SMS (TXT) messages or Tweets, while a clever data compression scheme reduces the number of digital bits required to pass structured messages containing conventional callsigns, reports and locators. Long or unusual callsigns are problematic for the protocol, however, despite using hashing to pass condensed representations. Decoding errors and hash collisions occasionally generate false 'callsigns' creating puzzlement or excitement if they appear to be rare but genuine calls.

 

There are multiple uses for FT8 including contesting,^[6][7] testing antennas,^[8] and for scientific research.^[9]

FT8DMC is a popular club dedicated to this digital mode.

The FT8 Operating Guide gives pragmatic advice on using FT8 for HF communications.