Field Guide · protocol

Also known as: JT9

JT9 is a narrowband weak-signal amateur-radio digital mode designed to pack sub-noise sensitivity into an extremely small slice of spectrum — an occupied bandwidth of under 16 Hz, so many signals fit in the space of a single SSB channel. It uses a 9-tone MFSK waveform on a one-minute cadence and, like its stablemate JT65, exchanges only minimal callsign, grid, and report messages.1 JT9 was created chiefly for the low bands, where its narrowness and sensitivity suit the quiet, slow paths of LF and MF.

one ~2.5 kHz SSB passband each bar = one JT9 signal (<16 Hz wide)
JT9's sub-16 Hz footprint lets dozens of signals share a single SSB-width passband on the crowded low bands.

Overview

JT9 works like JT65 from the operator’s seat: synchronized one-minute transmit/receive slots, a fixed 72-bit message, and the same station-report script. The difference is under the hood — nine tones spaced only 1.74 Hz apart and a strong convolutional code yield a signal a fraction of JT65’s width, at comparable sensitivity. That narrowness is the whole point on LF/MF, where spectrum is scarce and stability is high.

Technical characteristics

Property Value
Modulation 9-tone MFSK, ~1.74 Hz tone spacing
Symbol rate 1.736 baud (85 symbols)
Occupied bandwidth ~15.6 Hz per signal
Slot length 1 min (~49 s of tones)
Message payload 72 bits
FEC Constraint-length 32, rate-1/2 convolutional code
Threshold ≈ −27 dB SNR (2.5 kHz reference)

History

JT9 was introduced in 2013 by Joe Taylor (K1JT) in WSJT-X, alongside a refreshed JT65 decoder. It replaced JT65’s Reed-Solomon code with the same constraint-length-32 convolutional code used in WSPR, achieving a slightly better threshold in far less bandwidth. WSJT-X could decode JT9 and JT65 simultaneously in adjacent sub-bands, and both later ceded HF popularity to FT8.2

Deployment

JT9 is used mainly on the LF, MF, and lower HF amateur bands, where its narrow footprint and stability are most valuable. Activity is lighter than FT8’s but persists among low-band and QRP operators who prize its efficiency.

Decoding it with GopherTrunk

GopherTrunk does not decode JT9 — it is a weak-signal HF/LF mode outside the scope of a trunked land-mobile scanner. JT9 is received with an SSB receiver or SDR feeding clean USB audio into WSJT-X, with the PC clock synchronized to UTC and a stable, well-calibrated front end.

Sources

  1. WSJT (amateur radio software) — Wikipedia, for the WSJT mode family including JT9’s 9-tone MFSK waveform, narrow bandwidth, and timing. ↩

  2. WSJT-X — the official WSJT-X project page, documenting JT9’s modulation, convolutional coding, and low-band design goals. ↩

See also