JT65, JT9 and JT4: The Legacy WSJT-X Modes Explained

Before FT8 took over the HF bands, a family of slower weak-signal modes did the same job. JT65 and JT9, joined by JT4, were the workhorses of early digital weak-signal work, and they are still part of WSJT-X today. This guide explains how those legacy modes work, what separates them, how they compare with FT8 and Q65, and where they are still useful. The details come from the WSJT-X 3.0 User Guide, which describes them in its modes overview, in the VHF features chapter and in the protocol specifications.

The Family in One Paragraph

The WSJT-X User Guide says that JT65 was designed for EME on VHF and higher bands and is mostly used for that purpose today. JT9 was designed for the HF bands and lower, and its submode JT9A is 1 dB more sensitive than JT65 while using less than 10 percent of the bandwidth. JT4 offers a wide variety of tone spacings and has proven highly effective for EME on microwave bands up to 24 GHz. All three use one-minute timed sequences, so a minimal QSO takes four to six minutes: two or three transmissions by each station, one sending in odd UTC minutes and the other in even minutes. For background, the Wikipedia article on JT65 gives the history of the mode.

How They Compare With FT8 and Q65

The table puts the older modes next to the modern ones, using figures from the guide. FT8 is four times faster than the one-minute modes, with 15-second sequences, but it is less sensitive by a few decibels, according to the guide. Q65 offers a range of sequence lengths and tone spacings for fast-fading signals.

The difference between 72 and 77 bits is not just a number. The 72-bit modes use two 28-bit fields for callsigns and a 15-bit field for a grid locator, report or acknowledgment, plus one bit that flags free text of up to 13 characters. The 77-bit modes add five bits to flag special message types, such as nonstandard callsigns, contest exchanges and FT8 DXpedition mode. That is why contest messages exist for FT8 and FT4 but not for JT65 or JT9.

JT65 in Detail

The protocol specification gives the technical profile. JT65 uses a Reed-Solomon (63,12) error-control code to convert 72-bit user messages into sequences of 63 six-bit symbols. These are interleaved with another 63 symbols of synchronization information according to a pseudo-random sequence. Modulation is 65-tone frequency-shift keying at 2.692 baud. The tone spacing equals the keying rate for JT65A and is two and four times larger for JT65B and JT65C. The guide’s EME section says that digital EME has mostly used JT65A on 50 MHz, JT65B on 144 and 432 MHz and JT65C on 1296 MHz.

JT65 includes special messages for EME. The signal report OOO is sometimes used instead of a numerical report, and shorthand messages for RO, RRR and 73 dispense with the sync vector entirely and use pairs of alternating tones for 1.486 seconds. With VHF features enabled, the decoder responds to these formats automatically; you generate them for transmission by checking the shorthand box. The guide recommends checking Single decode for VHF operation, since only one or a few signals normally appear in the receiver passband, and not using the two-pass decoding option. Deep search, averaging and AP decoding can be enabled from the Decode menu.

The signal-report range of JT65 is limited. The guide says reports are constrained to lie between -30 and -1 dB and are significantly compressed above about -10 dB, which is why reports for strong signals do not look very precise. For a wider view of weak-signal reporting, see our guide to WSPR, which uses a different message structure.

JT9 in Detail

JT9 uses a strong convolutional code with constraint length 32 and rate one half, which gives an encoded message of 206 information-carrying bits. Modulation is nine-tone frequency-shift keying at 1.736 baud. Eight tones carry data, so three bits are conveyed by each symbol, and one tone is used for synchronization. A transmission needs 85 channel symbols. For JT9A, the tone spacing equals the keying rate of 1.736 Hz, and the total occupied bandwidth is 9 times 1.736, or 15.6 Hz. This narrow footprint is what the guide means by less than 10 percent of the bandwidth of JT65, and it is why JT9 can fit many signals into a small slice of an HF band.

JT9 supports a wider range of signal reports than JT65, from -50 to +49 dB, and assigns more reliable numbers to relatively strong signals. The guide also mentions faster JT9 submodes, E through H, which are designed for meteor scatter and similar short-lived propagation along with MSK144; these use sequence lengths of 5, 10, 15 or 30 seconds. If you want to see how meteor scatter works in practice, our MSK144 guide covers the operating side.

JT4 in Detail

JT4 is built for EME on the microwave bands, 2.3 GHz and above. It uses the same convolutional code as JT9 and four-tone frequency-shift keying at 4.375 baud. Each symbol carries one information bit and one synchronizing bit. The submode selector sets the tone spacing, and wider spacings are used on the higher bands to cope with larger Doppler spreads; the guide gives JT4F as the usual choice for EME on 5.7 and 10 GHz. As with JT65, some operators use single-tone short messages for EME QSOs, generated by the Sh box, with Tx6 producing a 1000 Hz tone to help the other station find the signal and 1250 Hz to indicate readiness to receive. The Deep option on the Decode menu, averaging and deep search are all recommended for JT4.

Messages in the 72-Bit Modes

The 72-bit message format has some quirks that explain why these modes feel different from FT8. A standard message has two 28-bit fields for callsigns and a 15-bit field for a grid locator, report, acknowledgment or 73. One additional bit flags a message that contains arbitrary free text of up to 13 characters. Special cases allow other information, such as add-on prefixes like ZA/K1ABC or suffixes like K1ABC/P, to be encoded. In the 72-bit modes, compound callsigns come in two types: Type 1 uses one of about 350 common prefixes and suffixes, which you can display from the Help menu, in place of the third word of the message, and Type 2 handles other prefixes (one to four characters) and suffixes (one to three characters) when the message begins with CQ, DE or QRZ. In both cases a message that carries a compound callsign cannot carry a locator or report in the usual place, which is one reason traveling stations were often seen as awkward in the early days of the modes.

Directed CQ messages are another feature. The compression algorithm supports messages that begin with CQ AA through CQ ZZ, which are encoded internally as if they were the callsigns E9AA through E9ZZ and converted back for display. This lets a station call CQ for a particular region or group without needing a separate field in the message. The same structure was carried forward into the 77-bit modes, which is why directed CQ calls appear in FT8 as well.

Averaging, Deep Search and Depth

JT4, JT65 and Q65 offer message averaging, which sums successive transmissions that convey the same message and allows decodes at signal-to-noise ratios several decibels below the threshold for a single transmission. JT4 and JT65 also allow Deep Search decoding, in which the decoder hypothesizes messages containing known or previously decoded callsigns and tests them for reliability using a correlation algorithm. Decoded lines show a letter d when Deep Search was used and a confidence indicator from 0 to 9. These options are on the Decode menu, and the guide recommends Deep for most weak-signal work. The cost is processing time, but modern computers handle one-minute sequences easily, so there is rarely a reason to leave these options off when you are working at the edge of the noise.

For an EME operator these options are practical tools. A first echo of your own signal or a weak partner’s message may not decode in a single minute, but after a few repeats the averaged signal rises above the threshold. That is why EME schedules often involve repeating the same message several times, and why consistent timing and frequency matter: averaging only works if the successive transmissions line up.

Why Q65 and FT8 Replaced Them

The guide says that digital EME was done mostly with JT65A on 50 MHz, JT65B on 144 and 432 MHz and JT65C on 1296 MHz until the advent of Q65, and that on the higher microwave bands the typical choices were JT65C, wider JT4 submodes or QRA64. It now recommends a suitable Q65 submode for EME on any VHF or higher band, and says Q65 has replaced QRA64. For HF, FT8 made the one-minute modes less attractive by offering four times the speed. Many operators prefer faster QSOs and a bigger audience on FT8, while JT65 and JT9 activity on HF has dwindled.

Even so, the legacy modes are still in WSJT-X, and the 3.0 release extends the modern conveniences to them. According to the guide, auto-sequencing, filters, reply-to-CQ and the wait features are now available for all WSJT-X QSO modes, including the 72-bit modes JT4, JT9 and JT65. That means you can run JT65 on 6 meters EME with the same filters and wait options as FT8, which helps if you work moonbounce on schedule.

Where the Legacy Modes Still Make Sense

• EME on VHF and UHF: JT65 is still used where the activity group has not moved to Q65.
• Microwave EME: JT4 remains a proven choice on bands up to 24 GHz.
• Narrow HF segments: JT9 uses very little bandwidth, so it suits quiet corners of the band.
• Experimentation and learning: the slower pace makes the structure of weak-signal QSOs easy to follow, and the waterfall for each mode has a distinctive look.
• Rare regional activity: some local nets and beacons use the older modes, so a decoder that includes them lets you listen in.

If you are planning to try the legacy modes, check which mode the active stations use before you transmit. Our guide to digital modes beyond FT8 helps you compare, and the EME beginners guide shows what stations use today.

Setting Up JT65 or JT9 in WSJT-X

1. Choose the mode on the Mode menu. For JT65 pick the submode (A, B or C) that matches the band and the local convention.
2. Check your clock. One-minute sequences are timed to UTC, and an error of more than a second or so hurts decoding; our guide to FT8 time sync applies equally.
3. For VHF operation, enable the VHF and submode features on the General settings tab, check Single decode and avoid two-pass decoding.
4. Select Deep on the Decode menu if your computer can handle it, and add averaging, deep search or AP if you are working at the limit.
5. Remember that transmit periods alternate: one station sends in odd minutes and the other in even minutes.

The Bottom Line

JT65, JT9 and JT4 are the one-minute weak-signal modes that came before FT8. JT65 serves EME on VHF and above, JT9 was built for HF with a very narrow signal, and JT4 is a microwave EME mode with a choice of tone spacings. They are slower than FT8 and use a 72-bit message format with no contest messages, but they are still decoded by WSJT-X, and in version 3.0 they gain the same auto-sequencing, filters and wait features as the newer modes. Use them where activity exists, and use Q65 or FT8 where it does not.

Frequently Asked Questions

Which is more sensitive, JT9 or JT65?

According to the WSJT-X guide, JT9A is 1 dB more sensitive than JT65 while using less than 10 percent of the bandwidth.

How long does a JT65 QSO take?

The one-minute modes need four to six minutes for a minimal QSO, with two or three transmissions by each station.

What is JT4 used for?

JT4 is designed especially for EME on the microwave bands, 2.3 GHz and above, and has been effective up to 24 GHz.

Are JT65 and JT9 still in WSJT-X?

Yes. WSJT-X 3.0 still includes JT4, JT9 and JT65, and extends auto-sequencing, filters and the wait features to them.

Why do FT8 and Q65 have more message types?

They use a 77-bit payload, with five extra bits that flag special message types such as nonstandard callsigns and contest exchanges, whereas JT4, JT9 and JT65 use 72 bits.

Should I use JT65 or Q65 for EME?

The guide recommends a suitable Q65 submode for EME on any VHF or higher band, but check which mode the stations you want to work are using.