What Is FT4? A Beginner-Friendly Introduction to the Digital Mode
FT4 and FT8 are weak-signal-condition digital protocols designed for rapid, accurate communication between amateur radio stations. If you have ever watched the waterfall display in WSJT-X and seen dozens of tiny signals being decoded simultaneously at signal levels far below the noise floor, you have witnessed the magic of this family of modes. FT4 is the faster sibling in that family, built not just for weak-signal performance but for high-speed operation that can rival the contact rates of traditional RTTY contesting.
The Origins of FT4 and Who Developed It
FT4 is an amateur radio contesting communication protocol developed by Joe Taylor (K1JT) and Steve Franke (K9AN) that is descended from FT8. WSJT-X developers say serious work on the new FT4 protocol began shortly after the FT8 Roundup. The goal was a mode that could compete with RTTY contesting in terms of contact rates, while preserving many of the benefits of FT8.
Joe Taylor, K1JT, is a Nobel Prize-winning physicist from Princeton University whose passion for weak-signal communication drove the development of the entire WSJT family of software. FT4, a similar but faster protocol designed especially for radio contests, was introduced in 2019. Since its release it has become a staple of digital contesting across the HF bands worldwide.
How FT4 Fits Into the Weak Signal Digital Mode Family
WSJT-X Version 2.7 offers eleven different protocols or modes: FST4, FT4, FT8, JT4, JT9, JT65, Q65, MSK144, WSPR, FST4W, and Echo. The first seven are designed for making reliable QSOs under weak-signal conditions. They use nearly identical message structure and source encoding. Within this ecosystem, FT4 occupies the niche of high-speed contesting: faster than FT8 and vastly more sensitive than traditional modes like RTTY or SSB when signals are marginal.
Key Differences Between FT4 and Other Digital Modes
FT4 is an experimental digital mode designed specifically for radio contesting that — like FT8 — uses fixed-length transmissions, structured messages with formats optimized for minimal contacts, and strong forward-error correction. Unlike PSK31, which is designed for keyboard-to-keyboard conversation, or JS8Call, which supports freeform text messaging, FT4 is highly structured. Information exchanged in a contact typically consists of call signs, four-character Maidenhead locators, signal reports, and acknowledgments.
How FT4 Works: The Technical Breakdown
Understanding the engineering behind FT4 helps you set up your station correctly and troubleshoot problems when they arise. The protocol is elegant in its simplicity, trading some sensitivity for a dramatic gain in speed.
FT4 Transmission Timing and Message Structure
FT4 uses 4-MFSK modulation; transmission takes 4.48s with a 7.5s timing window. Transmit-receive sequences are 6 seconds, making it 2.5 times faster than FT8 and about the same speed as conventional RTTY for radio contesting. Within each 7.5-second window, the actual on-air transmission lasts 4.48 seconds, compared to 12.64 seconds for FT8.
Section 2 of this paper is a summary of how the FT4, FT8, and MSK144 protocols pioneered in WSJT-X compress and convey call signs, Maidenhead locators, signal reports, and certain other information in a very efficient way. FT8 and FT4 use a low density parity check (LDPC) block code designed and optimized for maximum error correction efficiency — which is why both modes can pull intelligible data out of signals buried in noise.
4-GFSK Modulation Explained Simply
FT4 uses a modulation technique known as Gaussian frequency shift keying, or GFSK. The generated audio waveform consists of 105 symbols (tones) sent in sequence at one of four frequencies. Frequency changes are Gaussian smoothed to minimize bandwidth. This smoothing is a key innovation: rather than switching abruptly between tones in a way that would splatter energy across a wide spectrum, FT4 applies a mathematical Gaussian filter so that frequency transitions are gradual and the emitted spectrum is kept very clean and narrow.
The GFSK spectrum has steep skirts, occupying a bandwidth of only 75 Hz at –6 dB, 200 Hz at –60 dB, and 260 Hz at –80 dB. This spectral efficiency is one of the reasons FT4 signals are so courteous to neighboring operators on a crowded band.
Bandwidth, Symbol Rate, and Sensitivity
Modulation uses four-tone frequency-shift keying at approximately 23.4 baud, with tones separated by the baud rate. The occupied bandwidth is 90 Hz. FT4 transmissions can be decoded at S/N down to -17.5 dB in a 2500 Hz noise bandwidth. For comparison, a typical SSB signal requires a signal-to-noise ratio of around +10 dB to be readable, and RTTY requires a comparable or better SNR. FT4 can operate at signal levels roughly 27 dB below what SSB needs — an enormous advantage under poor propagation conditions.
How FT4 Encodes Callsigns, Grid Squares, and Signal Reports
Tables 1 and 2 outline the basic source-encoding framework, with each message payload comprising a sequence of fixed-length bit fields. This Appendix completes the details needed to fully define mappings from human-readable message fragments to relevant fields in the fixed-size 77-bit message payload. Standard callsigns, Maidenhead grid locators, and signal reports are all compressed into this 77-bit structure. Standard amateur call signs can be conveyed in 28 bits, but compound calls such as PJ4/K1ABC and special-event calls like YW18FIFA may require more than twice that number. To accommodate such special calls, message type 4 allows use of one arbitrary call sign with up to 11 alphanumeric characters.
FT4 vs FT8: Which Digital Mode Should You Use?
The most common question among operators new to WSJT-X is whether to use FT4 or FT8. The answer depends on what you are trying to accomplish, and understanding the trade-offs will help you make the right choice for any operating situation.
Speed Comparison: 7.5-Second vs 15-Second Cycles
Unlike FT8, which uses a 15-second transmit/receive cycle, FT4 operates on a 7.5-second sequence. This shorter cycle allows more QSOs to occur in the same time period, making FT4 particularly popular during radio contests and high-activity operating events. In practical terms, where FT8 allows you to complete perhaps two QSOs per minute under ideal conditions, FT4 can support up to four — a meaningful difference over a 24-hour contest operating period. Radio QSO rates well above 100/hour are possible using FT4.
Sensitivity and Weak Signal Performance
The trade-off for FT4's speed is a modest reduction in sensitivity. Compared with FT8, FT4 is 3.5 dB less sensitive and requires 1.6 times the bandwidth, but it offers the potential for twice the QSO rate. FT8 is more effective in weak-signal conditions, decoding signals as low as -21 dB, compared to -17 dB for FT4. That 3.5 dB difference translates roughly to needing signals that are about twice as strong at the receive site. Under most normal HF operating conditions this is not a limiting factor, but it becomes important when chasing extremely rare DX with marginal signals or operating QRP under difficult propagation.
While FT4 packs some performance boosts, FT8 still has advantages in specific conditions. FT8's narrower bandwidth can provide better decoding in extreme weak signal situations. And its longer duration helps average out fading issues.
When to Choose FT4 Over FT8
Use FT4 when speed and contact rate are the priority. Contesting is the primary use case for FT4. It allows for rapid contact rates while still providing excellent weak-signal performance compared to legacy modes like RTTY and SSB. When a band is crowded with many stations, the faster QSO rate of FT4 helps to "clear the log" more quickly, reducing interference and allowing for a better flow of contacts.
Choose FT8 when you need maximum sensitivity: if you are working with low power (QRP) or trying to make a contact under very poor propagation conditions, FT8's superior sensitivity gives you the best chance of success.
FT4 for Contesting vs FT8 for DXing
FT8 has a much larger active community and is better for everyday DX operating and marginal band conditions. Most operators use FT8 for general operating and FT4 specifically during contests. This distinction has become a well-established convention in the amateur radio community. For casual DX operating, FT8 is the standard. For contest digital operating, FT4 is increasingly common.
FT4 Frequencies and Band Plans
FT4 uses dedicated dial frequencies separate from FT8, so you need to tune to the correct spot for your chosen band. FT4 uses a different set of frequencies than FT8, so you'll need to update your memories or band plans accordingly.
Standard FT4 Dial Frequencies by Band
The following are the standard WSJT-X default FT4 dial frequencies used in North America and most of the world. All HF digital modes use USB, and values are VFO dial frequencies in MHz. Below is a summary of the most commonly used FT4 frequencies:
- 160m: 1.836 MHz
- 80m: 3.575 MHz
- 40m: 7.047.5 MHz
- 30m: 10.140 MHz
- 20m: 14.080 MHz
- 17m: 18.104 MHz
- 15m: 21.140 MHz
- 12m: 24.919 MHz
- 10m: 28.180 MHz
- 6m: 50.318 MHz
If you have upgraded WSJT-X from an earlier version, you may be missing the predefined FT4 frequency values in the drop-down menu. In this case you may need to reset the frequencies to the default values. Simply go to Preferences → Frequencies, right-click on the frequency table and click on Reset. The new frequency values will now be available.
HF Bands Most Commonly Used for FT4
Because of its speed, FT4 is commonly used during major HF contests where operators want to maximize the number of contacts in a limited time. Typical FT4 operating frequencies include segments on bands such as 80m, 40m, 30m, 20m, 17m, and 15m, though exact frequencies may vary depending on regional band plans. The 20-meter band at 14.080 MHz is the busiest FT4 frequency for non-contest operation, while 40m and 15m are popular contest bands. FT4 is designed for contesting, particularly on the HF bands and 6 meters.
VHF and UHF FT4 Operations
While FT4 is primarily an HF mode, it does see use on 6 meters and even 2 meters, particularly during weak-signal contests where operators are working meteor scatter or tropospheric ducting paths. The 6-meter FT4 frequency of 50.318 MHz is the standard operating spot for VHF weak-signal FT4 work. On 2 meters, FT4 is less common but can be used in conjunction with other WSJT-X modes for weak-signal EME-adjacent operations.
FCC Regulations and Band Privileges for FT4 Use
In the United States, FT4 is treated as a data/digital emission under FCC Part 97. The FCC defines the legal frequency allocations, while band plans organize how those frequencies are used in real-world operation. FT4 is permitted on all amateur bands where data emissions are authorized
Recommended Comments
This article is locked, but your permissions allow you to add new replies.