FT8 has been around for some time now. It was adopted fast by the amateur radio community and although I do not know exact numbers, it is clear that FT8 is overtaking JT65.
Let me first say that Franke and Taylor (FT) have done a tremendous job with the development of all weak signal modes and FT8 is certainly a valuable addition to the ‘line’ of JT modes.
I have been observing FT8 for a considerable amount of time. For me, everything has a sort of behaviour, a signature or character perhaps.
With JT modes, this is not only about signals and white noise, but about real world situations with fading, interference, static, overlapping stations and so on. One could also say that you are trying to establish the reliability of a transmission circuit.
For me, JT65 with JTDX has been the ultimate combination. Decoding up to -30 dB and very reliable under difficult conditions, like overlapping stations (JT65, FT8 and even PSK), multiple stations calling on the same frequency as well as other interference from various sources.
Thanks to JT65 it was possible to work stations like 5W1SA and 3D2AG on 60 metres with a modest setup and a challenging man made noise environment.
My experience with FT8 is that signals below about -18 become unreliable. Lower values are seen, but it seems that FT8 is more sensitive to QRM than JT65 which does not surprise, given the shorter transmissions.
I tried to establish the decoding margin, using real world background noise on the 60 metre band. I fed the signal from my antenna to a combiner. The other input was connected to the transmitter via attenuators. The transverter output of my IC7600 was used, thus less attenuation was needed, avoiding errors from unintended coupling between transmitter and receiver. The receiver was connected to a second computer with JTDX v. 18.0 and WSJT-X v. 1.8.0 rc2.
With a spectrum analyser, the noise level was measured in 3 kHz bandwidth and a signal was injected at the same level. This resulted in (near) -1 reports for JT65 and FT8. After adding attenuation, the reports nicely tracked the attenuation within about 2 dB, confirming that the software reports the signal to noise ratio well.
In the real world, fading and multipath effects, for example, affect the signal. In my tests, only the noise was somewhat realistic, whilst the transmission path was only a bunch of attenuators:-)
I conducted two tests. One during evening hours with some static and some noise from a TV set. The second test was during daytime with little QRM thus effectively white noise. JT65 decodes to nearly -30 were seen, but below -25, the evening tests saw lost decodes. The daytime results were better and stable up to -28 (with hinted decoding).
The evening results with FT8 showed that decodes were lost below -18. Good decodes below -20 were rare. During the day, FT8 was stable up to -22. With one dB attenuation added, equivalent to -23, decodes were lost completely. The results confirm that FT8 is more sensitive to QRM.
Roughly 6 dB difference in sensitivity
My conclusion is that the practical difference in sensitivity between FT8 and JT65 is roughly 6 dB even though the theoretical difference should be less. As said, with JT65, good contacts were made with weaker signals, even close to -30. On 60 metres, I have seen stable series of CQ’s from ZS stations with about -27 dB and when taking the 6 dB into account, it would require FT8 to be stable at around -21 dB, which is not the case. So I would say that the 6 dB estimate is not far from the truth or even a bit optimistic.
The picture shows CQs from ZS2ACP with QRM from over the horizon radar (OTHR). The green and orange markers indicate the sync tone of the transmissions. Incredible, how the software decodes so well, you can hardly see the traces!
Having made numerous QSOs with JT65, it seems that fading affects JT65 less. Again, the longer transmissions are expected to be responsible for the better reliability.
When talking about “user experience”, FT8 is a bit nervous and does not leave much time to decide who to call on a busy band. The auto sequence feature is nice, but is also a drawback in the sense that working FT8 in auto mode does not require much from the operator and gets boring after some time. You want to operate your station and FT8 does not feel that way.
With JT65, one could easily grab a coffee and such, because after the start of your transmission, you can leave the operating position alone for almost 2 minutes. When FT8 runs smoothly, you can lean back until the final 73s, but if things go wrong, you can mess up an unattended QSO.
I found that the FT8 auto sequence mode with TX=RX on, sets the TX frequency to the remote station, but this is not always clever, because in busy situations, it can be beneficial to call/answer “split” on a clear frequency. Most stations decode the whole audio passband and will see your messages anyway.
Another aspect to mention is “cross mode” QRM. I have noticed FT8 stations jumping on a JT65 QSO and vice versa. One would assume that the band is monitored on the waterfall, but it seems that this is not always the case. It could be that there are those who only watch the FT8 decodes and consider other signals as QRM. For me, ham spirit is also about being considerate…
But the most important drawback is the fact that stations who switch to FT8 are unable to work JT65 at that time. Because of the lesser sensitivity, the difference in threshold means that the weakest FT8 signals are lost. In case a contact would be just possible with JT65, it will be missed with FT8.
When looking at the path loss, 6 dB difference is a distance ratio of about 2. This implies that the range of FT8 is about half the range of JT65 (assuming equal power and noise).