====== FT4 ======

**FT4** is a short-message digital mode developed for rapid amateur radio contacts, particularly contest exchanges. Like [[ft8|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.

<code>
Symbol rate = 12000 / 576 ≈ 20.83 baud
On-air duration = 105 × 576 / 12000 = 5.04 s
</code>

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|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.

===== References =====

  * [[https://wsjt.sourceforge.io/FT4_FT8_QEX.pdf|Franke, Somerville and Taylor: the FT4 and FT8 communication protocols]].
  * [[https://wsjt.sourceforge.io/wsjtx.html|WSJT-X project and user documentation]].