FT4
From Wikiham, the ham radio free encyclopedia
FT4 is a short-message digital mode developed for rapid amateur radio contacts, particularly contest exchanges. Like FT8, it is implemented in WSJT-X and carries structured information rather than an unrestricted keyboard conversation.
Timing and waveform
FT4 uses 7.5-second transmit/receive slots. Its four-tone Gaussian frequency-shift keying runs at approximately 20.83 symbols per second. A standard transmission has 105 symbols, taking about 5.04 seconds, within the larger slot.
Symbol rate = 12000 / 576 ≈ 20.83 baud On-air duration = 105 × 576 / 12000 = 5.04 s
The occupied bandwidth is about 90 Hz. A 77-bit message is protected by a 14-bit CRC and encoded using the same (174,91) low-density parity-check code family used by FT8.
Tradeoffs
The shorter interval makes exchanges faster. FT4 is wider and generally less sensitive than FT8 under comparable conditions; it trades some weak-signal performance for speed. These are protocol characteristics, not a promise of a particular contact rate.
Both modes need suitable time synchronisation, frequency stability and audio levels. Their signal reports are referenced to a defined noise bandwidth, so a reported negative SNR should be interpreted with that reference in mind.
Contacts and software
Messages commonly exchange station identifiers, locators, reports and acknowledgements. Operators must watch the actual sequence: decoding one transmission does not establish a completed contact.
WSJT-CB exposes FT4 for its CB-oriented workflow. That is a software adaptation of a mode; it does not make the amateur and CB radio services interchangeable.