FST4

FST4

Frequencies	137 kHz,474 kHz
Frequency Range	137 kHz - 474 kHz
Mode	USB
Modulation	GFSK
ACF	—
Emission Designator	—
Bandwidth	66.7 Hz,28.6 Hz,12.4 Hz,5.9 Hz,2.2 Hz,0.72 Hz,0.36 Hz
Location	Worldwide
Short Description	FST4 is a 4-GFSK amateur radio communications mode, designed especially for making contacts (QSO's) on LF and MF frequency ranges under extreme weak-signal conditions. It is part of the WSJT-X software.
I/Q Raw Recording	—
Audio Sample

FST4 is a 4-GFSKGaussian Frequency-Shift Keying extreme weak-signal amateur radio communications mode, designed especially for the MFMedium Frequency (300-3000 kHz) and LFLow Frequency (30-300 kHz) bands. It is part of the WSJT-X software. Like other WSJT-X modes such as FT8, it carriers only the minimal amount of information for making contacts (QSO's).

FST4 uses 4-GFSKGaussian Frequency-Shift Keying modulation and can use 15, 30, 60, 120, 300, 900 and 1800-second transmit/receive slots. The slowest and most sensitive submode FST4-1800 is theoretically decodable with signal-to-noise ratio down to -43.2 dBThe decibel (dB) is a logarithmic unit used to express the ratio of two values of a physical quantity, here the strength of a received signal., but it requires high frequency stability for the transmitter and the receiver, and small Doppler spread - all these should stay smaller than 0.089 HzHertz (Hz), unit of frequency, defined as one cycle per second (1 Hz). during one transmission.

Due to the very small tone spacing of the slower FST4 submodes, they can be easily mistaken for interference or spurious emissions when heard or seen on a waterfall display.

FST4 submodes

Submode	Symbol length	Bandwidth	Transmission duration	Minimum SNR for decoding
FST4-15	0.06 s	66.7 HzHertz (Hz), unit of frequency, defined as one cycle per second (1 Hz).	9.6 s	-20.7 dBThe decibel (dB) is a logarithmic unit used to express the ratio of two values of a physical quantity, here the strength of a received signal.
FST4-30	0.14 s	28.6 HzHertz (Hz), unit of frequency, defined as one cycle per second (1 Hz).	22.4 s	-24.2 dBThe decibel (dB) is a logarithmic unit used to express the ratio of two values of a physical quantity, here the strength of a received signal.
FST4-60	0.32 s	12.4 HzHertz (Hz), unit of frequency, defined as one cycle per second (1 Hz).	51.8 s	-28.1 dBThe decibel (dB) is a logarithmic unit used to express the ratio of two values of a physical quantity, here the strength of a received signal.
FST4-120	0.68 s	5.9 HzHertz (Hz), unit of frequency, defined as one cycle per second (1 Hz).	109.3 s	-31.3 dBThe decibel (dB) is a logarithmic unit used to express the ratio of two values of a physical quantity, here the strength of a received signal.
FST4-300	1.79 s	2.2 HzHertz (Hz), unit of frequency, defined as one cycle per second (1 Hz).	286.7 s	-35.3 dBThe decibel (dB) is a logarithmic unit used to express the ratio of two values of a physical quantity, here the strength of a received signal.
FST4-900	5.56 s	0.72 HzHertz (Hz), unit of frequency, defined as one cycle per second (1 Hz).	887.5 s	-40.2 dBThe decibel (dB) is a logarithmic unit used to express the ratio of two values of a physical quantity, here the strength of a received signal.
FST4-1800	11.2 s	0.36 HzHertz (Hz), unit of frequency, defined as one cycle per second (1 Hz).	1792 s	-43.2 dBThe decibel (dB) is a logarithmic unit used to express the ratio of two values of a physical quantity, here the strength of a received signal.

Samples

Showcase sample is FST4-60. These samples were recorded directly from WSJT-X. If you have samples of actual FST4 transmissions, please add them here!

FST4-15	FST4-30	FST4-60

FST4-120	FST4-300

Frequencies

FST4 is designed for LFLow Frequency (30-300 kHz) and MFMedium Frequency (300-3000 kHz) frequency ranges and the most likely bands to find it are the 2200-meter and 630-meter bands.

Decoding Software

• WSJT-X

Additional Links

• WSJT-X 2.6.1 user guide

Additional Images

FST4-120 zoomed in, showing small frequency transitions