WSJT, Weak Signal communication by K1JT is a form of digital communication mode optimised for weak signal amateur radio communication.
WSJT was originally developed by K1JT around 2000 / 2001 but now the software is open source and its development is continuing under the management of a small team of radio amateurs.
WSJT was developed for 2 metre and 6 metre meteor scatter but now its use has spread far more widely and its low signal properties have been used to good advantage.
The mode is very versatile in terms of its performance and it can decode steady state signals as well as short bursts from short lived meteor trails.
In view of the take up of WSJT, a number of variants optimised for particular bands and applications have been developed.
- FSK441: FSK441 was the first version of WSJT to be developed and it was introduced in 2001. The aim of FSK441 was to replace the very high speed Morse that was a feature of meteor scatter operation. The high speed Morse was needed because of the short duration of the meteor trails that would support communication. Accordingly FSK441 used high speed data transfers as well. Obviously high speed is comparable to that of the Morse data, but not comparable to data transfers using other wideband data transfers like those used in cellular telecommunications.
FSK441 uses frequency shift keying of four tones at a rate equivalent to 441 baud.
- JT6M: JT6M is a form of WSJT that is optimised for 6 metre operation - hence the 6M part of the name.
- JT65: The JT65 form of WSJT is aimed at very low signal communications for applications like EME or long distance troposcatter.
The modulation format used is MSK and there is a total of 65 tones (hence the name) and it uses forward error correction, FEC. This provides a particularly resilient form of communication which can be provide reliable communication even when it is well below the noise level.
- JT4: JT4 is a WSJT mode that was designed specifically for EME or “moon-bounce’ transmission. JT4 uses 4-tone FSK, and this enables it to accommodate both a sync bit and a data bit in each symbol. The keying rate is 4.375 baud, and a number of different levels of tone spacing are available.
- FT8: Although many of the variants of WSJT, one variant that has been very rapidly taken up for HF communications is FT8. It gains its name from the fact that it was developed by Steven Franke, K9AN,
and Joe Taylor, K1JT and the fact that uses 8FSK modulation. FT8 is described as an excellent digital mode for HF DXing and for situations like multi-hop Es on 6 meters, where deep fading may make fast and reliable completion of contacts a necessity. Some of the key features of FT8 include: T/R sequence length: 15 s,
Message length: 75 bits + 12-bit CRC,
FEC code: LDPC(174,87),
Modulation: 8-FSK, keying rate = tone spacing = 6.25 Hz,
Waveform: Continuous phase, constant envelope,
Occupied bandwidth: 50 Hz,
Synchronization: three 7x7 Costas arrays (start, middle, end of Tx),
Transmission duration: 79 x 1920/12000 = 12.64 s,
Decoding threshold: -20 dB or possibly -24dB,
Operational behaviour: similar to HF usage of JT9, JT65,
Multi-decoder: finds and decodes all FT8 signals in passband,
Auto-sequencing after manual start of contact.
The take-up of FT8 has been exceedingly rapid, with many stations finding it is particularly useful for HF band operation.
These are some of the major variants of WSJT. There are more, but they tend to be used less frequently.
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