Higher frequencies don't benefit from these effects, and generally don't propagate beyond direct line-of-sight. But in rare circumstances, atmospheric refraction can cause high-frequency radio signals to propagate much farther than normal, which is what seems to have happened here.
On a technical level, this radio contact was made possible by the recently-developed FT8 digital modulation scheme, which is designed to transmit a minimal amount of data (basically just a pair of callsigns) at only a few bits per second, so as to make even extremely weak signals detectable. The "dB" column in the screenshot illustrates that the received signal strength was roughly 100 times weaker than the background noise.
My uncle, decades ago, was a cop in rural Ontario (based on Kilaloe). Absolute middle of no where, mostly rural, but an area with a bit of elevation here and there.
Somewhere down in the US, Virginia I think, used the same frequency as them for their police radio. They discovered this because every now and then if you were up on a hill under the right weather conditions, they could talk to each other, hundreds of km apart. Very confusing the first few times.
Or so he tells the story. I'm neither a cop nor a person familiar with the magic of radio technology.
For reference, according to wikipedia, if one station was at sea level and the other about 1500m above, you'd expect line of sight to be around 160km.
Similarly, around the same time of year (January or so) but less frequently we used to get VHF TV interference from a station on the same frequency but more like 1000km away (if I'm remembering the source station right, it was a long time ago), strong enough to interfere with the more local transmission.
Radio+atmospheric effects can get weird.
It’s episodic and well known phenomenon to any amateur radio operator that prefers the vhf/uhf bands.
People transmitting 100W on HF with WJSTX modes is another pet peeve.
I run 100W, and I'm rarely picked up above -7dB. Some of us have to given our QTH, antenna, and where we're trying to reach. Not all of us are in it for QRP. Perhaps they should designate a part of the FT8 bandwidth for QRP? 73
If you're getting a +5 SNR report then obviously dial it way back but when I do DX at 100 Watts it's usually -10 or less.
Of course that's for QSOs usually well over 3k miles.
Interestingly, and perhaps you already know this, but the distance radio waves travel is actually a bit farther than line of sight (even at higher frequencies like UHF):
Okay, this sniped my curiosity. Where can I read more about the math/physics that makes this possible?
If you have big enough samples, you can tell a normal distribution with mean 0.01 apart from one with mean 0, even if their standard deviation is both 1000.
Error correcting codes are in some sense a very cool and efficient way to get the equivalent of that bigger sample.
(You could also just send your signal many, many times. But simple repetition is a vastly inferior solution to the ingenious codes people came up with.)
Here's a quick wiki link for that topic:
Basically, error correction can supply “coding gain”. So in the path-loss equation, you get to add gain for error correction.
In the above case, if the signal is 100 times weaker than the noise, you need a code with 20db of coding gain to reach break-even, if that makes sense.
As to the physics there's also the fact that the phenomenon that creates a good transmission line between participants is in flux and you have to get your data through while the transmission line passes enough signal that the receiver can decode it.
This is a method of spreading the power of your transmission over a much wider part of the spectrum. Hence, this doesn't help much if your transmit power is limited. However, it does help if your transmit power per slice of bandwidth is limited.
This is used for a few methods. The first is spectrum-sharing (which requires orthogonal codes). The second is detection avoidance (lower power per bandwidth makes it much harder to recognize the signal unless you already know how to de-spread). The third method is to avoid 'point noise sources' from blocking your signal. E.g. a wifi burst of 1MHz might drown out your signal if you use 1Mhz of bandwidth. But if you use 10 MHz, it only blocks one tenth of your signal.
> The most likely mode of propagation was marine ducting with the signal being trapped close to the ocean.
> There is a whole truckload of Sahara dust getting across the Atlantic this week. I blame it for the modification of the normal atmosphere into a superrefractive one, supporting communications for the first time on this band. Sahara dust dryes the air around it by absorbing moisture and thus modifying the refraction index. Like a G-Line feeder or an optical fiber, the difference in refraction index between different layers allows the propagation path to curve down following the Earth curvature to allow these fantastic QSOs.
> I read about this phenomenon many years ago in an old ITU Bulletin justifying the enhanced superrefraction conditions usual between Cape Verde and the Western African Coast.
> Jose, CO2JA
> this radio contact was made possible by the recently-developed FT8 digital modulation scheme
1. which factor was more important, or were they equal?
2. Is it COVID related? ie, atmospheric conditions are suddenly different around the world due to shutdown of industry.
1. Both; perhaps the dust
I've worked parallel to a whole bunch of radio obsessed electronic engineers; they get really excited about atmospheric conditions, and they regularly use weather maps to win competitions with unusually long transmission ranges (- a whole bunch of mad people across England / Europe / etc. go out on the same day and try and make as many contacts as possible. The team with the most unique contacts wins).
2. Probably not or at least not significantly. Ducting occurs frequently and Saharan dust is blown out to sea every year. Note that 'for the first time on this band [at this distance]' is more accurate.
Analog TV was probably Earth's biggest detectable RF export. Lots of transmitters in the megawatt range transmitting 24/7.
So what makes this historical? That it was never done at a frequency as high as 432 Mhz?
If no one would ever really be using this mode/frequency to communicate long distances given this was such a special circumstance, and HF is already the standard (I understand), what practical has been achieved? What is this for?
Second, practical research into how to transmit information more efficiently has many, many applications. WiFi and mobile-phone technologies would not exist without research like this. 
It's fun to do something that nobody has done before. Even though I don't care about the actual goal, it's interesting to read about what the obstacles are, and how people overcame them.
But equally, if someone is sensitive about being asked what's it good for, then maybe don't seek worldwide attention for it on the front page and get offended when the world asks you such questions.
Also I don’t see any reply that I thought demonstrated “offense”. Perhaps it was deleted.
It's boring to meta-debate the quality of discussion / people's voting on a story or comment, so I won't pursue it much more here. But if something doesn't have a known practical value then let's talk about what its value is, or could be. Don't just vote someone down because you're offended that your pursuit / hobby was questioned.
This feels like its a "street car mods", "CPU overclocking", or even "distance running" kind of hobby. Part ego, part technical, part seeing how far you can push something. Sometimes these things have use (to you, or others), sometimes not.
The second is the use of FT8, a new digital mode invented by Joe Taylor, W1JT, a nobel laureate. See https://en.wikipedia.org/wiki/Joseph_Hooton_Taylor_Jr. Of particular interest is how his research confirmed Einstein's theory about gravitaitional radiation From the web site,
On VHF bands and higher, QSOs are possible (by EME and other propagation types) at signal levels 10 to 15 dB below those required for CW.
The mode and other related modes are described here: https://physics.princeton.edu/pulsar/K1JT/wsjtx.html.
(Side note--Joe spoke at the Dayton Hamvention Contest Dinner a few years back, and brought some recordings of pulsars.)
Of course, a call sign can only be held by one licensee at a time, and most (or all) of the 1x2 and 2x1 are assigned (I'm not sure on the current state of things with 1x3 and 2x2). So one only becomes available when the previous holder loses it (by license expiration or death). There are far more Advanced/Extra licensees than short call signs.
There are also 1x1 call signs, but they are only issued on a temporary basis to special event stations (e.g., I have operated W8C, with a club whose normal call sign is W8YY).
Additionally, some have two digits, for example a Slovenia call might be S52.
So the above rule would be modified to be zero or one digit, one or two letters, and one or two numbers, followed by suffix.
--w8lvn, ex-YJ0VN. --
The whole QSO took just a couple of minutes. It only needed to re-transmit one frame. Each frame takes 15 seconds.
What's great is that since the QSO (conversation) is so fast, the conditions can be really variable and you can still make the contact. I'm not surprised it was first done with FT8.
73 de W7RLF
The other thing is the PA. The layout is very poor. The higher the frequency, the lower the output. It's great at 30 meters and below, but 20 meters and up suffers. The BITX40 suffers from this too, but it's a single band radio and at 40m it works fine. I replaced the IRF510 with an RD15HVF1 and it worked great on 20M too. Mind you, this isn't an issue with the IRF510- it's an issue with board layout. The QRP Labs 10W PA uses two IRF510's in push/pull and can even work on 10m at full output.
TL;DR: Over complicated, poor PCB layout, feature creep.
That's not to say that Farhan isn't a nice fellow and a great ham- he definitely is! They just let this one out too soon before all the bugs were worked out, and allowed compromises that shouldn't have made it into production.
Keep an eye out for the QRP Labx QSX. It's been in development for a long time but will probably blow everything else out of the water, even things costing 10-15x as much.
More details here:
But in fact this 432 MHz contact is over a greater distance, 3867 km, than Alcock and Brown's flight of 3040 km (https://en.wikipedia.org/wiki/Transatlantic_flight_of_Alcock...)
So calling it trans-Atlantic actually undersells it in comparison with other well known trans-Atlantic feats while at the same time setting people up to think the opposite.
What are possible applications?
Possible applications from a practical point of view not that I can see, though I'm sure that now that it has been shown to be possible there will be attempts to replicate it and from harder locations.
But HAMs are known for doing things the hard way, after all, you could just pick up the phone and talk, that's not what this is about. It's about being able to do it, just like mountain climbing.
FT8 is ~10 bits/second (think of it as CW/morse code + better modulation + forward error correction).
I guess you could say the SDR based ICOM IC-9700  transceiver FG8OJ used is enabling this by being reasonably priced  and off-the-shelf. It was released in January 2019 and has made experimentation of VHF/UHF more accessible.
- The FT-8 mode makes barely-possible communication possible
At higher frequencies (such as the 432 MHz used in this trans-Atlantic contact), the radio waves will not be reflected by the ionosphere and will disperse.
Page 9 of IR2028 if you’re interested:
For local communication, e.g., state to state, a common mode of attempts involve bouncing signals off of airplanes. But transatlantic using this method would be quite unlikely.
WSJT-X is the software by Joe Taylor, who is a Nobel Laureate. The signal processing part of the program is written in Fortran.
Related, I'm amazed that my little $70 WSPR transmitter can push a 20m/14MHz signal from San Francisco to Georgia using only 0.2W, transmitting on a pretty lousy backyard dipole only 10 feet off the ground. True, it's only a beacon ("call sign Kxxxxx transmitting from SF at 200mW"), but still...
WSPR being another Joe Taylor invention https://en.wikipedia.org/wiki/WSPR_(amateur_radio_software)
Stars are many lightyears away, and radiate insane amounts of power the vast majority of which is lost. By the time it reaches here you may need a very long exposure to be able to see them at all. But it is line of sight. These waves were of the line-of-sight variety and somehow they were bent around the planet enough to register. That's a very hard to achieve thing.