Wednesday, 2 October 2019

An Interesting FT8 Afternoon


Lest my loyal blog-followers think I’ve taken a leap to the digital-darkside, rest assured this is not the case!


The only time that I (somewhat grudgingly) use FT8 is during the summer Es season, since most of the DX seems to have migrated to that mode. On Monday afternoon, during a moment of weakness driven by curiosity, I moved my receiver from 630m JT9 to 40m FT8 ... what might I see at around noon, via this popular weak signal digital QSO mode? To say that the results were surprising is an understatement!

40m at noon
The above screen capture shows the signals decoded between 12 and 1PM local time! Although it was surprising to see all of the east coast signals, it was shocking to see all of the signals (a dozen or more) coming from Japan and South Korea! All of them were working Europeans to their west, none of which were decoding here.

With an all-daylight path between VE7 and Asia, could these signals be coming via the long-path? If so, I would expect to see at least a few signals from other countries along the great circle path to Asia but none were forthcoming. Perhaps it’s a case of there being a sufficiently weak D-layer to allow signals to propagate on the direct path via the F-layer, in spite of the all-daylight path. What do you think?

I then moved the receiver down to 80m, and monitored there for the next two hours.


80m at 1500 local
The 80m screen capture above was made at 3PM local time! Once again, I was surprised to see so many signals coming from the east, in broad daylight! Is this further evidence of a weak D-layer or just a demonstration of the capabilities of FT8?

For both 80 and 40m, the antenna used was my 80m end-fed half-wave configured as an inverted-L ... 70’ straight up and 60’ horizontal. The feedpoint is one-foot above the ground and located beside the ocean, looking towards the east.

I then finished off the afternoon with a look on topband, using my 160m half-sloper. The screen capture of 160m was made at 5PM local time, fully two hours before my local sunset!


160m at 1700 local

Like the previous two bands, 160m was showing signals in broad daylight, from the east coast! Low D-layer absorption? ... salt water horizon gain? ... excellent antennas? ... or is this just the sensitivity of FT8 revealing 'normal propagation' that we can't hear on CW? I suspect that it's a complex combination of all of these factors and maybe others.

In reality, the weak-signal ‘digging power’ of FT8 is not too much greater than the threshold for audible CW ... hearing about 1 S-unit (~6-7db) deeper. Maybe that’s all it takes to peel-back, like an onion skin, another layer of hidden signals.

There are other weak-signal digital QSO modes much more sensitive than FT8. Both JT9 and JT65 can each hear more than an S-unit deeper than FT8 but at the expense of taking longer to do it ... there’s just no free-ride. I believe that the shorter (~15 second) sequencing of FT8 is the main reason for its overwhelming popularity, in spite of its lower sensitivity.

I’ll run this test again soon to see if Monday’s daytime prop was unusual or if it was typical of what to expect with weak-signal digital modes on the lower bands during the daylight hours ... either way, it was indeed, an interesting FT8 afternoon!

4 comments:

Anonymous said...

HAHAHAHAHAHA.... you used interesting and FT8 in the same sentence... GOOD ONE!!!!!!

Steve McDonald said...

Propagation is always interesting!

Anonymous said...

Steve, give 17m FT8 a try. I've seen EU and ZL in at the same time mid-day. It's an amazing band on FT8.

Unknown said...

I am a rookie with only six months of leave and maybe later change my mind; I have been informing myself about the FT8 and my impression is: I am walking on a street in Bombay looking for someone who crosses a glance with me, I look at people, occasionally someone looks at me, and more rarely does anyone cross my gaze with me and we continue walking without stopping nor cross a word; On the other hand, it is to walk around the square of a small town in Andalucia and stop to say hello and talk to each person I meet, suddenly sharing a glass of sherry accompanied by a good ham pata negra ..., I prefer this.
(from mistakes blame Google translator)
Regards from Chiloe
CD7CKU
C├ęsar