The WSJT family is a set of weak-signal modes written by the same group of developers, in one software suite (WSJT-X). They share one idea: send tiny, structured messages, slowly and with powerful error correction, in timed slots, and decode them from signals too weak to hear. Most were designed for the hardest paths in amateur radio, such as bouncing signals off the Moon, and later found use on HF. FT8 and WSPR belong to the same family; this article covers the others.
The family at a glance
Mode
Designed for
Note
JT65
EME and weak VHF/UHF paths
Many tones; decodes very weak signals. Long, one-minute periods.
JT9
HF
Narrower than JT65, similar slow, patient style.
Q65
EME and other weak paths
Successor to JT65; averages several receive periods to dig deeper.
MSK144
Meteor scatter
Very short, fast bursts, to use a brief meteor trail.
FST4
Low-frequency and HF
Variable transmit periods and a choice of tone spacings.
JT65 and JT9 were once the standard HF weak-signal modes; FT8's faster 15-second slots have taken over most everyday HF weak-signal work, while the modes for the hardest VHF-and-up paths remain valuable.
Why different modes for different paths
Many tones, very weak. The decoder finds the stair-step in the noise.
JT65 spreads each message across many tones and adds a strong error-correcting code, so the decoder can reconstruct it even when noise hides most of the signal. Its name does not mean 65 hertz or 65 baud; it is a multi-tone audio FSK mode.
Pick the mode for the reflector: MSK144 for meteors, Q65 for the Moon.
The paths shape the mode:
EME (moonbounce): the signal travels to the Moon and back, roughly 2.5 seconds, and is extremely weak when it returns. Stations take turns in synchronised periods, and Q65 can average several periods so a message too weak for one period appears across a few. See EME (moonbounce).
Meteor scatter: a meteor's ionised trail reflects VHF signals for a fraction of a second. MSK144 sends fast bursts, so the message fits inside one brief reflection. See Meteor scatter.
HF: long periods gave very good sensitivity but a contact took minutes; faster slots won out for everyday use.