1. Ford Amateur Radio League March 8, 2018 David Treharne, N8HKU
FT8 Protocol on WSJT-X
Ford Amateur Radio League
March 8, 2018
David Treharne, N8HKU

2. What is FT8
A digital operating mode, using a nominal 50Hz of bandwidth (if your transmitter is properly configured, of course!)
FT= Franke and Taylor, Steven Franke, K9AN, and Joe Taylor, K1JT
8: 8 Tone Frequency Shift Keyed (FSK) modulation
Each tone is 6.25 Hz apart
Transmit time: 15 seconds turnaround. The message itself takes seconds.
Message length: 72 bits of equivalent message data
Uses Forward Error Correction to fix some of the bit errors to improve reception
12 Bits of cyclical Redundancy Check or CRC
Encodes some data for extra compression (not ASCII)

3. What do you See on a Busy Band?
FT8 has a designed “base” frequency for each band, similar to other WSJT modes.
FT8 is Upper Sideband Only
On the 15 second mark, all signals within the passband are decoded. (Each green line is a 15 second segment)
Each signal is shown below in real time as a spectrum graph

4. Received Signals Decoded
The WSJT program records a .wav file for each 15 second timeslot.
Each one of these signals was decoded after one 15 second interval. 30 signal in all.
Across the top: Time, signal strength, Delta time from the computer time, Frequency offset from the sideband carier, Message content.
Green lines are stations calling CQ, making it easy to pick them out.

5. Received Signals Decoded in 3 Waves!
The decoder starts off with the strong signals, decoding 16 of them from low delta frequency to high frequency
Because the signals are precisely timed with frequency, the strong signals have a known signal envelope. An audio signal is generated, and subtracted from the .wav file, removing this signal
A second pass is made, decoding another 6signals.
even 1 that is exactly 1Hz from the 1st pass!
A 3rd pass is then made, grabbing one more signal! The computer took an extra second or so for this deep decoding

6. What is needed to work FT8?
A computer: Need some memory and a decent processor
I needed about 20% of a Core i6 processor at 2.6GHz to process the signals, so not too bad.
Accurate time clock: Needs to be within 2 seconds or the decoding will be really off. Best to be within a second (more on that in this presentation)
An interface to the radio
CAT is preferred, but not absolutely required. Can use a sound card and an interface such as Rigblaster
Without a CAT interface: Rx and Tx frequencies are the same or hand selected.
A HF or VHF radio with good frequency control and accuracy
A wandering receiver or transmitter will not work with this tight bandwidth
Accuracy is good, but not critical with this mode. (the program does include a very nice frequency calibration routing)

7. Time Accuracy The FT8 receives and transmits on 15 second blocks.
The signal is 12.6 seconds long.
Off in time, decode less and less until nothing is decoded.
No one will hear your signal if your time is off as well.
The Default Windows clock makes a single adjustment once per week.
This program, Meinburg Network Time Protocol program, reads at least 3 time servers and gradually sets your clock on your PC to within seconds!
The program readout:
3 Different time servers.
ST: Stratum: 1= Atomic clock 2= based on one or more 1 clocks. 3=based on one or more 2 clocks. 3 could be more accurate than 2!
Delay: the time it takes to send a request and receive a reply.
offset: The difference between my clock and the server clock. All three are less than 2mS.
Jitter: Delay time variance. The program takes multiple samples and statistically reduces the error.

8. Frequency Accuracy WSJT-X has a frequency calibration tool
It picks known broadcast frequencies from 0.6MHz to 20 MHz with known accurate transmitters
The program runs upper sideband and offsets the radio by 1.5kHz. It then measures the carrier and gives an offset.
Icom 9100:
Synthesized rigs generally have a frequency offset that is directly proportional to the frequency. The Icom 9100 has this case.
Icom has a calibration offset for the readout.
Using the WSJT calibration tool, I was able to dial in the radio to within 2 Hz at 20 MHz. At lower frequencies, it measured well within 1 Hz.

9. The communication Sequence
The standard communication sequence is basic, and automated
You can run manually, but likely to slow process and miss some stations, possibly not completing the QSO.
A station sends a CQ, with their grid location.
A receiver double clicks on the CQ message, highlighted in Green.
The computer generates standard messages to use.
Enable Tx to send.
Program automatically sends when the CQ station is listening, every other 15 seconds.
My computer sends their call, my call, and my location.
They send my call, my signal strength.
My computer sends R (Received) and their signal strength
Their computer ses their call, my call, and 73.
My computer Sends their call, my call, and 73.
My computer asks if I may log it, and Enable Tx ends.
Their computer asks to log, and disables Tx. They can Re-enable Tx and send CQ again.

10. A Sample Contact: N8HKU with DF2SD at distance of 6,464km (4,040mi) in Germany.
Time 01:25:15- CQ from DF2SD in grid JO31
My Tx1 response starting at 01:26:37-- Not time aligned, but I double clicked and forced it. Oops!
01:27:00:My next response now aligned with his I am on the same frequency he is transmitting on.
Why is his CQ at below my CQ? Answer: It takes time to decode, but the transmit is commanded right at the 00 second mark!
01:29:15: DF2SD returns my call. My signal strength is -24dB, very low.
01:29:30: The program was going to send message Tx1, but decoding the reply, sends Tx 3 now, which says I Received the call and your signal strengh is -5dB. the signal report is sent 3 times, as there was not a response from the other station that was decoded. Note: 9K2HM and K5CD were on the same, exact frequency at the same time!
01:30:15: Message from DF2SD, RRR, all received.
01:30:30: My reply, 73. End of contact.

11. How Much Power to use? Nominally, I will try to run 10-20 watts.
Signals can be received below the noise floor, so no need for a big signal.
Need to keep the radio transmiting the tones linearly, so cranking up the power may distort them, and no one will hear
They will notice my signal splattered about the band, though!
I look at my received signal strength from the other station
If my signal is less than -20dB, they may have trouble hearing me.
I will then raise my power up, maybe 30 watts.

12. Saving a QSO The QSO logging screen is filled out.
Signal reports are listed in dB.
I can add Tx power before saving it.
If the operator sends his name in an additional message, it can be loaded here. The name can be sent in a special message, 13 characters long.
Hit OK to save the log.
The log can be exported to ADIF. The operating mode is saved as the standard RTTY, so the comments give the actual operating mode.

13. Operating Tips:
This is a busy band. The green cursor is my targeted receive station I am calling, and the Red cursor is my transmit frequency.
Note that the frequency is completely busy, with possibly multiple conversations going on at once.
If I were to move the operating frequency to here or here, I have a better chance to get heard!
My radio is set with Automatic Gain Control off.
Attenuator on, RF gain turned low so that the signal meter on WSJT is about 80dB.
Band filters set wide, 3.6kHz. Actual signal seems to end at 3.35kHz.
Signals can be received that we cannot hear!

14. Final Comments
A good computer interface is best, but you can use the built-in sound card if you need to.
Sound card use would transmit on receive frequency, or us manually dial a Tx split.
My Icom 9100 is having USB problems, probably damage I accidentally caused while working on a rotor control project, injecting 30V AC into the ground.
I had the laptop with WSJT-x running after the USB failed, and the the program was listening on the computer microphone.
The screen began filling up with decoded signals, although I could barely hear the audio coming out from the speaker on the radio!
The program is really sensitive! If your radio does not drift, and the computer time is set, it will pick out signals with very low strengths!
The Automation is required, but makes the QSO automatic, more so than RTTY or PSK. But, it is efficient and more effective with its strong error correction.
The authors of the program made you turn on the Tx for both CQ and for responding, so it is not a robot. They may give limited additional automation to a DXPedition, so the DX station can put up this station and not have to pay much attention to it, concentrating on CW and SSB.

15. Bibliography
WSJT-X Home page:
WSJT-X Operating Guide: file:///C:/WSJT/wsjtx/share/doc/WSJT-X/wsjtx- main html#NEW_FEATURES
ARRL.org Introducttion to WSJT-X:
FT8 Operating Tips:
Meinberg Network Time Protocol: