Field Guide · protocol

Also known as: JT65

JT65 is a weak-signal amateur-radio digital mode created for the hardest paths in radio — most famously Earth-Moon-Earth (moonbounce), where the round trip to the Moon leaves signals buried deep in noise. It uses a 65-tone MFSK waveform with a powerful Reed-Solomon code, decoding fixed messages at signal-to-noise ratios near −25 dB in one-minute transmit/receive cycles.1 For over a decade JT65 was the standard weak-signal mode until faster designs like FT8 largely displaced it on HF.

tone time sync tone (low, repeated) 64 data tones
JT65 interleaves a fixed sync tone with data drawn from 64 tones across ~47 seconds; a Reed-Solomon code recovers the message deep in noise.

Overview

A JT65 exchange follows the same minimal script as later modes — callsigns, grid locators, signal reports, and 73 — but on a one-minute cadence, so a complete contact takes several minutes. The waveform alternates a constant synchronizing tone with 64 data tones; the receiver first locks onto the sync tone to align timing and frequency, then decodes the data symbols. Three sub-modes (JT65A/B/C) widen the tone spacing for HF, VHF, and microwave use.

Technical characteristics

Property Value
Modulation 65-tone MFSK (1 sync + 64 data)
Symbol rate 2.6917 baud (126 symbols)
Tone spacing 2.69 Hz (A), wider for B/C sub-modes
Slot length 1 min (~46.8 s of tones)
Message payload 72 bits (before RS coding)
FEC Reed-Solomon (63,12) over GF(64)
Threshold ≈ −25 dB SNR (2.5 kHz reference)

History

JT65 was released in 2003 by Joe Taylor (K1JT), initially aimed at amateur EME operators who needed to pull tiny lunar echoes out of the noise. Its strong Reed-Solomon code and matched-filter sync made previously impossible contacts routine, and it soon spread to HF DXing as well. The suite it belongs to grew into JT9, WSPR, and eventually FT8.2

Deployment

JT65 remains in use for VHF/UHF EME and microwave weak-signal work, where its long integration and deep sensitivity still pay off. On HF its everyday role has largely been taken over by the faster FT8, though JT65 sub-bands still see activity.

Decoding it with GopherTrunk

GopherTrunk does not decode JT65; it targets trunked land-mobile systems, not HF/VHF weak-signal modes. JT65 is received with an SSB receiver or SDR delivering clean USB audio to WSJT-X (or the older WSJT), with the PC clock synchronized to UTC and a frequency-stable front end.

Sources

  1. JT65 — Wikipedia, for the 65-tone MFSK waveform, Reed-Solomon coding, one-minute timing, and EME/DX use of the mode. ↩

  2. The JT65 Communications Protocol — K1JT, QEX, the primary technical description of JT65’s modulation, synchronization, and error-correction coding. ↩

See also