FST4 and FST4W: Weak-Signal Modes for LF and MF

If you have only used FT8 and FT4, FST4 may look like a familiar name with a very different purpose. The WSJT-X User Guide says so directly: do not confuse FST4 with FT4. FST4 is designed primarily for making weak-signal two-way contacts on the LF and MF bands, where signals are slow, stable and extremely weak, and where transmit sequences can last many minutes. Its sibling FST4W does the beacon-like job that WSPR does on other bands. This guide explains how both modes work, what the documentation says about frequency stability and decoder controls, and how to start on the 2200 m and 630 m bands.

What FST4 Is and Where It Fits

The WSJT-X User Guide lists eleven modes in WSJT-X 2.7, and the same set carries on in version 3.0. FT8 uses 15-second sequences for fast HF contacts, FT4 uses 7.5-second sequences and is well suited to contesting, and FST4 is designed especially for LF and MF. Like the other QSO modes it uses the same 77-bit message structure, so a normal exchange of callsigns, grid locator, report and 73 works exactly as in FT8.

The big difference is time. FST4 offers T/R periods from 15 seconds up to 1800 seconds, which is half an hour. On a very quiet, stable channel, the longer you integrate a signal, the weaker it can be and still decode. That is the logic behind the mode, and it is also its main limit, as the next section explains.

Sequence Lengths and Frequency Stability

FST4 sequence lengths are 15, 30, 60, 120, 300, 900 and 1800 seconds, and submodes are named FST4-60, FST4-120 and so on. The guide gives an important warning: longer sequences provide better sensitivity only if the transmit and receive frequency instability and the channel Doppler spread are small enough that signals stay phase coherent over several symbols. In general, frequency changes during a transmission and Doppler spread must each be small compared with the symbol keying rate.

The guide’s own example is FST4-1800, which has a keying rate of 0.089 baud, so successful operation requires frequency stability better than around 0.05 Hz, and Doppler spread below that. A transmission consists of 160 symbols, so the keying rate for any submode is simply 160 divided by the sequence length. The table works that out for each period.

The keying rates in the table come from simple arithmetic, not from a table in the documentation, apart from the FST4-1800 figure. The practical lesson is that the longest sequences are not for everyone. A radio with a free-running oscillator may drift too much, and a stable receiver clocked from a GPS-disciplined reference is a typical aid on these bands.

How the FST4 Signal Is Built

FST4 appends a 24-bit cyclic redundancy check to the 77-bit message payload, creating a 101-bit message-plus-CRC word. Forward error correction uses a (240,101) low-density parity check code. A transmission consists of 160 symbols: 120 information symbols of two bits each, mixed with five groups of eight predefined synchronization symbols. The modulation is 4-tone frequency-shift keying with Gaussian smoothing of the frequency transitions (4-GFSK).

These details matter because they explain how FST4 ends up sensitive. The CRC keeps the false-decode rate low, the strong code corrects many bit errors, and the spread-out sync symbols let the decoder lock onto a signal that is invisible on the waterfall.

FST4W: The WSPR Alternative for LF and MF

FST4W is used in the same way as WSPR, but it has significant advantages on the 2200 m and 630 m bands. According to the guide, WSPR with its two-minute sequences can be decoded at signal-to-noise ratios as low as -31 dB in a 2500 Hz bandwidth, while FST4W offers optional sequence lengths as long as 30 minutes and reaches sensitivity thresholds as low as -45 dB. If you know how WSPR works, you already know the concept: transmit a short message with a callsign, grid and power level, and let receiving stations around the world report it.

FST4W has a scheduling option that WSPR does not. By default the receive frequency is 1500 Hz and the frequency tolerance is 100 Hz, so the active decoding range is 1400 to 1600 Hz, but you can choose other centre frequencies and tolerances. A drop-down control offers a round-robin mode: if three operators agree in advance to select 1/3, 2/3 and 3/3, their transmissions occur in a fixed sequence with no two stations on the air at once. For WSPR-like random scheduling you choose Random. As with WSPR, reception reports can be uploaded automatically to the WSPRnet database, which provides mapping and archives.

Decoder Controls You Should Know

The guide describes several controls that matter more on LF and MF than on HF:

• F Low and F High: these spin boxes set the lower and upper frequency limits of the FST4 decoder, marked by dark green angle brackets on the Wide Graph. Keep the range fairly small, because QRM and transmissions in other modes or sequence lengths slow down decoding.
• Single decode: on the File | Settings | General tab, this limits the range to F Tol either side of Rx Freq.
• Noise blanker (NB): the percentage sets how many of the largest-amplitude samples are zeroed before decoding. Most users find settings between 0% and 10% work best. A setting of -1% makes the decoder try 0, 5, 10, 15 and 20% in turn, and -2% loops through 0, 2, 4 and up to 20%, but only for candidates within 20 Hz of the receive frequency to save time.
• Decode depth: a priori (AP) decoding is always enabled for FST4 and FT4 when the depth is Normal or Deep. The FST4 AP table is the same as FT8’s except that it omits two types to save time.

Noise is the main enemy on these bands, particularly in towns, where switching supplies and lighting produce broadband interference. Our guide to low band receive antennas explains the kinds of antennas that improve the signal-to-noise ratio before the software ever sees the audio.

Trying FST4 With the Sample File

The documentation includes a sample exercise that is worth doing before you transmit. Select FST4 on the Mode menu, set T/R to 60 s and Decode to Deep, and set NB to 0%. In the Wide Graph, try Bins/Pixel 2 and N Avg 4, set the start frequency below 1000 Hz and the width to include 1400 Hz, set F Low to 1000 and F High to 1400, then open the sample wave file from the save\samples\FST4+FST4W folder. For FST4W the guide’s example uses T/R 1800 s, Bins/Pixel 1, N Avg 150 and Start 1200 Hz. After decoding you will see a single decode, even though the signal is essentially invisible on the Wide Graph spectrogram, which is the best demonstration of what the mode can do.

Getting on the Air on 2200 m and 630 m

The two bands are the home of FST4. The 630-meter band and 2200 m band are allocated to amateurs in many countries, but power limits, permitted segments and licence conditions differ, so check your own regulator before building a station. Our 630 and 2200 meter band guide covers the practical side of antennas and transmitters.

A typical beginner path is to start by receiving. Set up WSJT-X in FST4W mode with an audio connection from a receiver, decode what other stations transmit, and watch the reports appear. Receiving costs nothing in terms of licence or antenna, teaches you the noise environment, and shows you whether your location is workable. When you are ready to transmit, an FST4W schedule is a polite way to start, because it behaves like a beacon and does not tie up the band while stations work out a QSO.

For QSOs, agree on a sequence length and a calling frequency before you start. Long sequences make a normal exchange slow: a complete contact with six messages needs a few periods of 120 or 300 seconds, and much longer with 1800 s. That is part of the charm of these bands, but it means you must plan your time, and it means any clock error shows up quickly, as discussed in our 160 m guide for a related low-band mindset.

Station Requirements Beyond the Software

The software side of FST4 is simple compared with the station side. WSJT-X needs the usual things: a computer clock synchronized to UTC within one second, audio input and output at 48000 Hz and 16 bits, and a way to key the transmitter. On LF and MF the real challenges are different. Antennas are electrically tiny at these wavelengths, which means low efficiency and high losses in loading coils and ground systems. Many stations use a vertical or inverted-L wire with a large ground screen, and an inverted-L is also a favourite on the neighbouring bands; our inverted L antenna guide explains the geometry.

Because efficiency is low, power limits and legal conditions matter. Some countries allow only modest power on these bands, and the allowed segments are narrow. This is where a long, very sensitive mode helps: you can be heard at levels that would be useless in SSB or CW. Even so, an hour spent reducing local noise or improving the ground system will often be worth more than any amount of fiddling with decoder settings.

Choosing Between FST4W, WSPR and FT8

The three modes overlap in purpose, so it helps to compare them by what you want to find out. If you want a quick two-way contact on a busy HF band, FT8 is the right choice. If you want to probe propagation on HF or VHF with a low-power beacon, WSPR is the traditional tool and has the largest network of receivers. If you want the same on 2200 m or 630 m, FST4W is designed for that job, and its longer sequences let you work down to lower signal levels than WSPR allows. For actual QSOs on those bands, FST4 is the matching two-way mode.

A sensible routine for a new LF station is to run FST4W for a few days, study the reports to see which stations hear you and at what signal-to-noise ratios, adjust the antenna or the transmitter power, and then arrange schedules with a regular partner for FST4 contacts. This feedback loop is slow, but every step teaches you something concrete about your own station.

Common Mistakes

• Picking the longest sequence on the first day. If your oscillator is not stable enough, a long sequence performs worse, not better.
• Setting the decode range too wide, which slows the decoder and invites decodes of other modes.
• Treating FST4 as a faster FT4. They are different modes with different purposes.
• Forgetting that FST4W is a beacon mode, not a QSO mode; there is no exchange of reports.
• Ignoring noise at home. A good receive antenna usually does more than another decibel of transmit power.

The Bottom Line

FST4 and FST4W are WSJT-X modes built for the slow, stable world of LF and MF: sequences from 15 to 1800 seconds, strong error correction, and sensitivity to match the long integration times. Learn the stability requirements, keep the decode range tight, use the noise blanker sensibly, and begin by receiving FST4W reports before you transmit. Once you understand your own noise floor and oscillator, the longer sequences become a tool instead of a risk.

Frequently Asked Questions

Is FST4 the same as FT4?

No. The WSJT-X User Guide warns against confusing them. FT4 is a fast mode with 7.5-second sequences, well suited to contesting, while FST4 is designed primarily for weak-signal QSOs on LF and MF.

Which sequence lengths does FST4 support?

15, 30, 60, 120, 300, 900 and 1800 seconds, named FST4-15 through FST4-1800.

What is FST4W?

FST4W is a beacon-style mode used in the same way as WSPR, but with advantages on 2200 m and 630 m, including sequence lengths up to 30 minutes and sensitivity as low as -45 dB according to the guide.

How stable must my radio be?

The guide’s example is FST4-1800: frequency stability better than around 0.05 Hz is required, along with Doppler spread below that. Shorter sequences are more forgiving.

Can I upload FST4W spots?

Yes. Users with internet access can automatically upload WSPR and FST4W reception reports to WSPRnet, which provides mapping and archival storage.

Does FST4 use AP decoding?

Yes, AP decoding is always enabled for FST4 when the decode depth is Normal or Deep.