OnAirDecode FST4 with an SDR
FST4: slow weak-signal contacts and FST4W beacons.
- Group
- Amateur: digital
- Start frequency
- 1.8366 MHz
- In the mode list
- WSJT, 1.8366 MHz
What FST4 is
FST4 (and FST4W) is a weak-signal digital mode of the WSJT family for the low bands.
Frequencies and settings
| Start frequency | 1.8366 MHz |
|---|---|
| Tuning range of the mode table | 1 to 30 MHz |
| Sample rate asked | 2 Msps |
| Lowest working rate | 250 ksps |
| Channel width | 3 kHz |
OnAir takes any frequency you type; the numbers above are the mode's starting point and the range its table describes. In the app the shortcut label is WSJT, 1.8366 MHz.
What OnAir shows
FST4 and FST4W (15 s to 30 min periods, 4-GFSK, LDPC (240,101) / (240,74)) for LF, MF and HF weak-signal contacts and beacons.
The mode's own tab has these views: Decodes, Map. The other tabs (Overview, Receiver, Scan, History) are described on the main window page.
Limits and what it was checked with
Checked with generated signals and impairments; recordings of real signals welcome.
Which radio you need
This is a shortwave mode, so the radio has to tune HF. An SDRplay, Airspy HF+ or an RTL-SDR in direct-sampling mode do that directly; with a HackRF an HF upconverter or a direct-sampling receiver is the usual route. A long wire or loop antenna matters more than the radio.
Receiving and the law
Questions about FST4
Can I decode FST4 with an RTL-SDR?
An RTL-SDR can receive HF only in direct-sampling mode, which OnAir offers in its radio settings; results depend on the antenna. SDRplay and Airspy HF+ radios are better suited.
Is it legal to receive FST4?
Amateur radio signals are meant to be received. Receiving needs no licence in most countries; OnAir does not transmit.
Can I try FST4 in OnAir without a radio?
Yes. Most modes include a built-in synthetic test signal, so you can open the screen and try the controls before you own a radio.