FT8 for beginners part 1: background and basic concepts

Over the past couple of years, I've helped a few hams get up to speed on FT8 and related digital modes, so I figured it would be a good topic for my blog. This is a fairly involved topic, so I will be splitting this into multiple blogs. So let's dive in!


FT8 is the most popular HF digital mode, hands down. Over the past few years, it's taken the ham world by storm, to the point where some "old timers" feel that it's the root of all evil, the beginning of the end for ham radio, etc. As an FT8 and digital mode enthusiast, I'm not going to debate the larger ramifications of the mode, but I can say that for me, FT8 was the first mode I was able to successfully use on HF, and I still enjoy using it three years after I first got it working.

Before we jump into the nuts and bolts of FT8, a quick primer and history lesson is in order. FT8 was developed by radio astronomer and physicist Dr Joe Taylor (K1JT), who, among other things, co-won the Nobel prize for physics in 1993 for discovering the first binary star. Being a radio astronomer, Dr Taylor has spent decades working on weak signal theory, and since is also a ham, he has applied many of his discoveries to a suite of weak signal modes, collectively known as WSJT-X.

WSJT-X is completely free, open source software that runs on most modern operating systems, including Windows, MacOS, and various flavors of Linux, including Raspbian, the Raspberry Pi OS. As I mentioned above, WSJT-X is a suite of modes, and FT8 is only one of many. Here's the complete list, along with brief descriptions of each mode:

  • FST4/FST4W - designed for MF and HF bands; FST4 is optimized for two-way QSOs, while FST4W is similar to WSPR-style beaconing (see below for more on WSPR). 
  • FT4/FT8 - the most popular WSJT-X modes, they are designed for quick, two-way QSOs with a very simple message exchange (call sign, signal report, grid square). FT8 uses a 15-second transmission sequence, whereas FT4, designed more for contesting, uses a 7.5-second sequence. More on FT4 and FT8 message sequencing later on.
  • JT4/JT65 - designed for EME (earth-moon-earth), aka "moonbounce" QSOs on VHF/UHF bands; JT9 is similar, except for MF/HF, and Q65 is similar, except it's designed for EME, ionospheric scatter, and other weak signal work on VHF, UHF, and microwave bands. 
  • MSK144 - similar to FT4/FT8, except designed for use with meteor scatter over VHF
  • WSPR - an extreme low-power, weak signal mode, designed for testing propagation paths. I've done a bit of playing with WSPR, even reaching a research station in Antarctica on 1/2 watt from my home in Colorado! WSPR can also do "band hopping," changing bands based on time of day and rotating them so that propagation can be measured across multiple bands. 
  • Echo - designed for detecting and measuring your own signals reflected from the moon
As you can tell, I've barely scratched the surface of what WSJT-X is capable of. That said, using FT8 and FT4, I've completed over 2,300 confirmed QSOs in 117 countries, at the time of this writing. 

FT8/FT4 QSO Basics

Since FT8 and FT4 are the most popular HF digital modes, I will limit the scope of these blogs to those two modes. As I indicated above, these two modes are designed for quick, semi-automated QSOs in which only basic information is exchanged, that being the two call signs (sender / receiver), signal strength report (in decibels), and location (grid square) for sender and receiver. Operators have very little flexibility in changing the message formats; basically only the basic CQ format can change slightly; more on that later. It's also important to note that FT8 and FT4 are not designed to be conversational modes; they're similar to quick CW QSOs like those that take place during contests.

FT4 is designed for faster QSOs, with contesting mind. As indicated above, each FT4 QSO segment takes 7.5 seconds compared with FT8, which takes 15 seconds. In order to accomplish this, FT4 is 3.5 dB less sensitive and requires 1.6 times the bandwidth. Even so, FT4 can work with signals 10 dB weaker what would be required to decode RTTY while using less bandwidth.

Interestingly, others have taken the base FT8/FT4 code and used it to implement conversational modes, similar to RTTY, where operators can conduct conversational QSOs. The most popular of these modes is KN4CRD's JS8Call, usually shortened to JS8. There are other FT8 compatible programs, again based on Joe Taylor's open source code, such as JTDX and MSHV, and some ham radio software suites also embed FT8. 

FT8/FT4 QSO Flow

Each FT8/FT4 QSO is comprised of the following basic steps:

  1. Sender calls CQ with grid square: "CQ K0EHR DM79"
  2. Receiver sees CQ in their FT8 console, double-clicks on it to respond, and FT8 sends the  receiver's call sign and grid square to be transmitted back): "K0EHR WB4HZA DM79"
  3. Sender sees CQ, sends the call sign of the receiver and their received SNR: "WB4HZA K0EHR -13"
  4. Receiver sees the response, sends their own received SNR: "K0EHR WB4HZA R-18"
  5. Sender closes out QSO: "WB4HZA K0EHR RRR" (reception report received)
  6. Receiver acknowledges, completing the QSO: "K0EHR WB4HZA 73" 
That's it! Upon completion, WSJT-x can write the QSO to a log file using the ADIF format, which can then be uploaded to QRZ and/or LOTW. Note that the QSO sequence can be optionally shortened a bit, where step #5 is changed to "RR73", combining RRR and 73 into a single step.  One more quick note about how FT8/FT4 QSOs work; there are actually two transmitting "slots", which take place either on even or odd seconds. If you look at the example flow below, you can see that my transmissions (in yellow) were taking place on even seconds, whereas my QSO partner was transmitting on odd seconds. 

On thing that's critical to understand about these modes is that the computer's clock must be very accurate, otherwise it throws off the timing and you may not be able to complete QSOs. If you're working from an Internet-connected PC, it's not usually a big issue since modern computers keep their clocks sync'ed with Internet "nntp" servers. If you're working off the grid, such as during a SOTA activation, make sure your computer's clock is sync'ed before you leave home. You can also connect a small USB GPS receiver to the PC, which can be used to set the clock. There are also apps available that will sync your PC's clock more often. More on all of this later. 

Example FT8 QSO flow

How does FT8/FT4 work?

FT8 and FT4 use a really clever mechanism that allows many different QSOs to take place at the same time, using the same frequency. At the core, these modes use a single SSB frequency (LSB for 160, 80, 40, and 30 meters; USB for 20/17/15/12/10 meters). The frequencies are documented here if you want to take a look; they all fall in the CW/data parts of the bands, in the general class regions. Technician class licenses may operate FT8 on 10/6/2 meters and 70cm.

During a QSO, the sender chooses a transmit frequency, which is different from the actual SSB frequency. FT8/FT4 use a small chunk of the SSB bandwidth, 50 Hz for FT4 and 80 Hz for FT4, and the SSB "channel" ranges from about 300 Hz up to 3000 Hz, which means that theoretically, any single FT8/FT4 SSB channel can support around 54 FT8 and 34 FT4 QSOs, respectively. Note: FT8, FT4, JS8Call all use separate SSB frequencies. In practice, it's not possible to get that many simultaneous QSOs, as improperly configured radios can cause "splatter" that can consume as much as 10x the normal bandwidth. More on that later as well.

Under the covers, FT8/FT4 are transmitting an audio signal over SSB, much the same as other digital modes like RTTY, except they use a much narrower bandwidth, transmitting the same segment information several times to increase the odds of reception. The other WSJT-X modes work similarly.

FT8 band scope showing frequencies in use

What type of rig do I need for FT8/FT4/etc?

Virtually any rig can be adapter to use the WSJT-X digital modes, providing that they can withstand the high duty cycles required. My first HF rig, a Kenwood TS-430S (circa 1988), worked just fine. At a bare minimum, you need a way to convert audio signals from your PC into something that your rig can transmit. For older rigs, I recommend Tigertronics' SignaLink USB, which plugs into your computer's USB port and comes with a wide variety of adapters for just about every rig made. I was able to buy the cable for my Kenwood, which plugged directly into the rig's microphone jack. I had to set the frequency and SSB mode manually, but the Signalink was able to key the transmitter and send the FT8 signals via the microphone jack. 

Newer rigs with CAT control simplify things further by allowing WSJT-X to set the rig's transmit/receive frequencies, and even newer rigs, like the ICOM IC-7300, have a built-in USB interface that can plug directly into your computer. 

It's also possible to use FT8 on 2 meter and 70 cm rigs using FM, but I haven't had a chance to try that. I doubt it's very popular, since FM doesn't typically propagate anywhere nearly as far as HF. I have been successful using 6 meter FM on my IC-7300, and this will hopefully get better as cycle 25 continues to improve propagation conditions.

I'm going to cut this blog post off here, as it's already in "TL;DR" territory ("too long; didn't read"). Much more coming soon!


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