Field Guide · protocol

Also known as: FT4

FT4 is a fast weak-signal amateur-radio digital mode built for contesting — a quicker sibling of FT8 that trades a little sensitivity for roughly double the rate. It uses 4-tone GFSK in 7.5-second transmit/receive slots and carries the same 77-bit messages protected by the same LDPC code, so a full contact takes under half a minute.1 FT4 shares FT8’s message structure and much of its decoder, but is optimized for the rapid-fire exchanges of a radio contest.

FT8 — 15 s slots FT4 — 7.5 s slots (≈2× faster)
FT4 halves FT8's slot length to 7.5 seconds, roughly doubling contact rate at the cost of a few dB of sensitivity.

Overview

FT4 was designed specifically to answer FT8’s main weakness for contesters: at one QSO per minute, FT8 is too slow to run a busy contest exchange. FT4 keeps the sub-noise philosophy — synchronized time slots, a fixed 77-bit message, and strong forward error correction — but shortens the cycle and widens each signal slightly to move data faster.

Technical characteristics

Property Value
Modulation 4-GFSK, ~23.4 Hz tone spacing
Symbol rate 20.83 baud (105 symbols per transmission)
Occupied bandwidth ~90 Hz per signal
Slot length 7.5 s (4.48 s of tones)
Message payload 77 bits (same as FT8)
FEC LDPC(174,91) with 14-bit CRC
Sync 4×4 Costas-style arrays + ramp symbols
Threshold ≈ −17.5 dB SNR (2.5 kHz reference)

History

FT4 was released in 2019 by Steven Franke (K9AN) and Joe Taylor (K1JT) as an experimental contesting mode in WSJT-X, after FT8’s runaway popularity made clear that a faster variant was wanted for high-rate operating.2 It reuses FT8’s source-encoding and LDPC coding, differing mainly in modulation and timing.

Deployment

FT4 is used on the HF amateur bands during contests, on designated sub-band frequencies (for example 14.080 MHz on 20 m). Outside contest weekends it sees far less traffic than FT8, which remains the everyday weak-signal workhorse.

Decoding it with GopherTrunk

GopherTrunk does not decode FT4; it is a trunked land-mobile scanner, not an HF weak-signal decoder. FT4 is received the same way as FT8 — an SSB receiver or SDR feeding a clean USB audio slice into WSJT-X (or JTDX), with the PC clock synchronized to UTC.

Sources

  1. FT4 — Wikipedia, for FT4’s role as a fast contesting mode, its 4-GFSK modulation, 7.5-second timing, and relationship to FT8. ↩

  2. The FT4 and FT8 Communication Protocols — Franke, Somerville & Taylor, QEX, the primary reference for the FT4 waveform, timing, and message coding. ↩

See also