FT4
| FT4 | |
|---|---|
| Mode type | Digital mode, 4-tone frequency shift keying |
| Developed by | Joe Taylor K1JT, Steve Franke K9AN and the WSJT-X development team |
| Released | 2019, in WSJT-X version 2.1 |
| Cycle | 7.5-second transmit and receive periods |
| Technical | |
| Signal bandwidth | About 83 Hz |
| Sensitivity | Decodes to about −17.5 dB signal-to-noise in a 2.5 kHz bandwidth |
| Common frequencies | 7.0475 MHz (40 m), 14.080 MHz (20 m), 21.140 MHz (15 m) |
| Designed for | Contesting and other fast-paced operating |
FT4 is a digital mode built for speed. It is a close relative of FT8, written by the same developers and run in the same free WSJT-X program, but its transmit and receive periods last 7.5 seconds instead of 15. A contact therefore takes about half as long. The price is sensitivity: FT4 needs a signal roughly 3.5 to 4 dB stronger than FT8 does. It was introduced in 2019, designed above all for contests, where the number of contacts per hour matters more than squeezing out the weakest possible signal.
How it works
FT4 sends its messages with four audio tones, a scheme called frequency shift keying, in which each tone stands for a different pair of bits. The tones are shaped so the signal switches smoothly from one to the next, which keeps it clean and narrow. A transmission lasts a little over 5 seconds, and the rest of each 7.5-second period gives the receiving software time to decode and queue its answer. The signal is about 83 Hz wide, wider than FT8's 50 Hz, because it carries the same information in less time.
The messages are the same 77-bit messages used by FT8, protected by the same kind of error-correcting code. That means FT4 carries exactly what FT8 does: call signs, grid squares, signal reports and, in contest mode, the exchanges required by events such as the ARRL RTTY Roundup and Field Day. As with FT8, stations transmit in alternate periods, and every station's computer clock must be accurate to within about a second.
FT4 or FT8?
| FT8 | FT4 | |
|---|---|---|
| Period | 15 seconds | 7.5 seconds |
| Tones | 8 | 4 |
| Bandwidth | About 50 Hz | About 83 Hz |
| Decodes down to | About −21 dB | About −17.5 dB |
| Typical use | Everyday operating, weak signals, DX | Contests, strong openings, quick contacts |
For general operating, FT8 remains the more popular choice by a wide margin, and its extra sensitivity makes the difference on a marginal band or with a small antenna. FT4 makes sense when signals are reasonably strong and time is short: during a contest, when a band is wide open, or when working a long line of stations one after another. Both modes appear in WSJT-X, and switching between them is a single menu choice. Some other programs, such as JTDX, also support FT4.
Getting on FT4
The station is the same as for FT8: an HF transceiver, a computer, and a sound-card interface or a radio with a built-in USB audio connection. In WSJT-X, choose FT4 from the Mode menu, and the program offers the usual FT4 dial frequency for each band, such as 14.080 MHz on 20 meters. Set the radio to upper sideband, keep the audio drive below the point where the radio's automatic level control acts, and turn off speech processing.
Operating is the same click-to-call routine as FT8. Double-click a station calling CQ, and the software sends the replies in sequence. Because each step comes around in 7.5 seconds, there is little time to think, which is why most operators let the software sequence the contact automatically.
For contests, WSJT-X has a setting for each supported event that changes the messages to carry the required exchange, and it can pass each contact directly to contest logging software. Many digital contests accept both FT4 and FT8 contacts, and activity during a contest weekend often spreads well beyond the usual dial frequency.