7 ms·

For some context on why this is interesting: since the dawn of radio, we've known that relatively low radio frequencies (up to maybe 30MHz, depending on time of

For some context on why this is interesting: since the dawn of radio, we've known that relatively low radio frequencies (up to maybe 30MHz, depending on time of day, solar activity, and other factors) can propagate for very long distances, by following the earth's surface or bouncing off the ionosphere. This is why a portable shortwave radio can pick up radio stations from the other side of the world. 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.

jiveturkey 7y ago
From TFA: > 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 From you: > this radio contact was made possible by the recently-developed FT8 digital modulation scheme My questions: 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.
- Normal_gaussian 7y ago
  IANA Radio Engineer 1. Both; perhaps the dust 2. No -- 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).
- velosol 7y ago
  1. I don't think you can quantify the two in equal units such that they can be compared - they both played a part. 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.
api 7y ago
Have we started looking for modulations like this with SETI? A signal would have to be quite loud to be ordinarily detectable, but someone might embed e.g. digits of Pi into an ultra-wide spread spectrum transmission that could be detected even though it's far below cosmic microwave background.
- CliffStoll 7y ago
  These modulation methods require knowledge and synchronization of coding by both receiving station and transmitting station. Without knowledge of the sender's modulation schema, the received signal appears as noise.
  - analog31 7y ago
    Indeed, more and more of the Earth's electromagnetic radiation will be designed to be indistinguishable from noise. Or it will propagate in optical fibers. Based on our own experience, I give a civilization a roughly 100 year period during which their radio emanations will be intelligible. A planet interconnected by optical fibers is kind of a poor man's dyson sphere. ;-)
    - Animats 7y ago
      Yes. Some years ago I went to a talk on SETI at Stanford, where they talked about detecting "carriers". Anything they could detect is obsolete technology now. Analog TV was 80% carrier (video was AM, the audio part was FM) with a huge sine wave component. Digital TV looks like narrowband noise. Analog TV was probably Earth's biggest detectable RF export. Lots of transmitters in the megawatt range transmitting 24/7.
      - api 7y ago
        I think technological progress in modulation might be a really good explanation for the "great silence." Advanced aliens could be all over the place, but if they're really advanced their transmissions are low-power, directional or cellular, and look like noise if you don't know the codec.
xelxebar 7y ago
> ... the received signal strength was roughly 100 times weaker than the background noise. Okay, this sniped my curiosity. Where can I read more about the math/physics that makes this possible?
- vvanders 7y ago
  The term you're looking for is Forward Error Correction, send extra bits so you can lose a few along the way.
  - andbberger 7y ago
    See also: information theory and channel capacity
  - devxpy 7y ago
    That takes care of lost information, but how do you reliabiliy filter out signal from noise? Can a simple FIR filter work in this situation?
    - vvanders 7y ago
      FT8 syncs everyone's clock on 15s intervals, you can also send well known sequences at the start of a transmission and search for those.
    - ruslan 7y ago
      FIR filter alone cannot do this, but combined with special coding like Direct Sequence Spread Spectrum (DSSS) does it pretty well. For example, CDMA and GNSS benefit from using DSSS.
  - sizzzzlerz 7y ago
    More accurately, extra bits are added to the transmitted stream so that bit errors in the received signal can be corrected.
- velosol 7y ago
  The general field of signal processing - in particular to pulling data out of noise there are error-correcting codes, algorithms like Viterbi and putting them together let's you pull off pulling data out of noise, really signal from interference. There's an entry for FT8 on wikipedia that I haven't digested but it has some other starting points. 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.
ac29 7y ago
> Higher frequencies don't benefit from these effects, and generally don't propagate beyond direct line-of-sight. 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): https://en.wikipedia.org/wiki/Line-of-sight_propagation#The_actual_service_range https://en.wikipedia.org/wiki/Line-of-sight_propagation#The_...
supernova87a 7y ago
I suppose I get why this is interesting. But I don't get why it's useful. 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?
- frisco 7y ago
  https://en.m.wikipedia.org/wiki/Mile_run_world_record_progression https://en.m.wikipedia.org/wiki/Mile_run_world_record_progre...
- supernova87a 7y ago
  I guess ham radio enthusiasts are easily offended when you ask them practical questions that they don't have a good answer to.
  - QuesnayJr 7y ago
    So, I don't care about ham radio at all, but I can imagine that if you do something for fun that it's grating to have people ask you over and over again "What is it good for?" What is anything good for? What is flying kites good for? What are walks on the beach, or sunsets good for, or any of the other purposeless activities that people engage in? 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.
    - BenjiWiebe 7y ago
      And to add to this, if it's a technical/experimental thing you're doing, you never know; you might discover/learn something useful.
    - supernova87a 7y ago
      Well, that's a legit point. And I won't pursue this topic much longer (tiring to talk meta about threads). 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.
bnegreve 7y ago
I am not sure I understand, it doesn't seem to be the first transatlantic contact made at a frequency above 30Mhz. (e.g.[1]) So what makes this historical? That it was never done at a frequency as high as 432 Mhz? [1] https://ei7gl.blogspot.com/2019/06/historic-trans-atlantic-contact-made-on.html https://ei7gl.blogspot.com/2019/06/historic-trans-atlantic-c...
drmpeg 7y ago
The indicated FT8 signal to noise ratio is very misleading. The calculation is based on a noise bandwidth of 2500 Hz, yet the FT8 signal is only 50 Hz. The actual minimum signal to noise ratio of FT8 is around 1 dB in 50 Hz (-156 dBm in a gaussian channel with a perfect 0 dB noise figure receiver), which is consistent with a 1/2 rate LDPC code like the one FT8 uses.
- madengr 7y ago
  Thank you! Was just about to point that out too. Drives me nuts when the standard SSB bandwidth is used. People transmitting 100W on HF with WJSTX modes is another pet peeve.
  - paulirwin 7y ago
    100W FT8 bothers you? There are people pumping 1500W on FT8. I would get the sentiment on WSPR though. 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
    - SamPatt 7y ago
      I don't get the hate for 100 Watts on FT8 either. Use the power you need to make the contact. Keep an eye on the waterfall to make sure you aren't doubling. 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.
      - ruslan 7y ago
        Isn't 50W that is max allowed power on UHF (432MHz) for amateur radio ? de R9LAJ
        paulirwin 7y ago
        Sorry for the late response. Yes you're correct, 50W max on that band, at least in the US. I was replying to a person referring to FT8 on the HF bands where the max power is 1500W. 73
mabbo 7y ago
> But in rare circumstances, atmospheric refraction can cause high-frequency radio signals to propagate much farther than normal, 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.
- catblast 7y ago
  https://en.wikipedia.org/wiki/Tropospheric_propagation https://en.wikipedia.org/wiki/Tropospheric_propagation It’s episodic and well known phenomenon to any amateur radio operator that prefers the vhf/uhf bands.
  - PaulHoule 7y ago
    One summer afternoon I was walking in the woods with a 2 meter handheld (140-144 MHz) and made a contact on somebody 200+ miles away across Lake Ontario (and land.) He was a Canadian who enjoyed tropospheric propagation events across the lake, particularly tuning into the My Network TV station in Syracuse which has an absurdly powerful transmitter.
- eythian 7y ago
  It is quite possible. I used to use packet radio on 144.45MHz, and when the conditions were right I could reach a BBS ~200km away. That's not terribly exciting, but both stations were pretty low to the ground, not very powerful or directional, and definitely not in line of sight. 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.
fpoling 7y ago
I wonder if reduced air traffic contributed to the success? I.e. could it be that favorite conditions are not that rare if not due to disturbance from all the flights?
- ruslan 7y ago
  I would say the opposite - UHF signal can bounce from plane traces, this effect is known to be used by some UHF radio amateurs to conduct long distance (beyond the line of aight) QSOs.

jiveturkey 7y ago

From TFA: > 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 From you: > this radio contact was made possible by the recently-developed FT8 digital modulation scheme My questions: 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.

Normal_gaussian 7y ago
IANA Radio Engineer 1. Both; perhaps the dust 2. No -- 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).
velosol 7y ago
1. I don't think you can quantify the two in equal units such that they can be compared - they both played a part. 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.

api 7y ago

Have we started looking for modulations like this with SETI? A signal would have to be quite loud to be ordinarily detectable, but someone might embed e.g. digits of Pi into an ultra-wide spread spectrum transmission that could be detected even though it's far below cosmic microwave background.

CliffStoll 7y ago
These modulation methods require knowledge and synchronization of coding by both receiving station and transmitting station. Without knowledge of the sender's modulation schema, the received signal appears as noise.
- analog31 7y ago
  Indeed, more and more of the Earth's electromagnetic radiation will be designed to be indistinguishable from noise. Or it will propagate in optical fibers. Based on our own experience, I give a civilization a roughly 100 year period during which their radio emanations will be intelligible. A planet interconnected by optical fibers is kind of a poor man's dyson sphere. ;-)
  - Animats 7y ago
    Yes. Some years ago I went to a talk on SETI at Stanford, where they talked about detecting "carriers". Anything they could detect is obsolete technology now. Analog TV was 80% carrier (video was AM, the audio part was FM) with a huge sine wave component. Digital TV looks like narrowband noise. Analog TV was probably Earth's biggest detectable RF export. Lots of transmitters in the megawatt range transmitting 24/7.
    - api 7y ago
      I think technological progress in modulation might be a really good explanation for the "great silence." Advanced aliens could be all over the place, but if they're really advanced their transmissions are low-power, directional or cellular, and look like noise if you don't know the codec.

xelxebar 7y ago

> ... the received signal strength was roughly 100 times weaker than the background noise. Okay, this sniped my curiosity. Where can I read more about the math/physics that makes this possible?

vvanders 7y ago
The term you're looking for is Forward Error Correction, send extra bits so you can lose a few along the way.
- andbberger 7y ago
  See also: information theory and channel capacity
- devxpy 7y ago
  That takes care of lost information, but how do you reliabiliy filter out signal from noise? Can a simple FIR filter work in this situation?
  - vvanders 7y ago
    FT8 syncs everyone's clock on 15s intervals, you can also send well known sequences at the start of a transmission and search for those.
  - ruslan 7y ago
    FIR filter alone cannot do this, but combined with special coding like Direct Sequence Spread Spectrum (DSSS) does it pretty well. For example, CDMA and GNSS benefit from using DSSS.
- sizzzzlerz 7y ago
  More accurately, extra bits are added to the transmitted stream so that bit errors in the received signal can be corrected.
velosol 7y ago
The general field of signal processing - in particular to pulling data out of noise there are error-correcting codes, algorithms like Viterbi and putting them together let's you pull off pulling data out of noise, really signal from interference. There's an entry for FT8 on wikipedia that I haven't digested but it has some other starting points. 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.

ac29 7y ago

> Higher frequencies don't benefit from these effects, and generally don't propagate beyond direct line-of-sight. 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): https://en.wikipedia.org/wiki/Line-of-sight_propagation#The_actual_service_range https://en.wikipedia.org/wiki/Line-of-sight_propagation#The_...

supernova87a 7y ago

I suppose I get why this is interesting. But I don't get why it's useful. 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?

frisco 7y ago
https://en.m.wikipedia.org/wiki/Mile_run_world_record_progression https://en.m.wikipedia.org/wiki/Mile_run_world_record_progre...
supernova87a 7y ago
I guess ham radio enthusiasts are easily offended when you ask them practical questions that they don't have a good answer to.
- QuesnayJr 7y ago
  So, I don't care about ham radio at all, but I can imagine that if you do something for fun that it's grating to have people ask you over and over again "What is it good for?" What is anything good for? What is flying kites good for? What are walks on the beach, or sunsets good for, or any of the other purposeless activities that people engage in? 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.
  - BenjiWiebe 7y ago
    And to add to this, if it's a technical/experimental thing you're doing, you never know; you might discover/learn something useful.
  - supernova87a 7y ago
    Well, that's a legit point. And I won't pursue this topic much longer (tiring to talk meta about threads). 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.

bnegreve 7y ago

I am not sure I understand, it doesn't seem to be the first transatlantic contact made at a frequency above 30Mhz. (e.g.[1]) So what makes this historical? That it was never done at a frequency as high as 432 Mhz? [1] https://ei7gl.blogspot.com/2019/06/historic-trans-atlantic-contact-made-on.html https://ei7gl.blogspot.com/2019/06/historic-trans-atlantic-c...

drmpeg 7y ago

The indicated FT8 signal to noise ratio is very misleading. The calculation is based on a noise bandwidth of 2500 Hz, yet the FT8 signal is only 50 Hz. The actual minimum signal to noise ratio of FT8 is around 1 dB in 50 Hz (-156 dBm in a gaussian channel with a perfect 0 dB noise figure receiver), which is consistent with a 1/2 rate LDPC code like the one FT8 uses.

madengr 7y ago
Thank you! Was just about to point that out too. Drives me nuts when the standard SSB bandwidth is used. People transmitting 100W on HF with WJSTX modes is another pet peeve.
- paulirwin 7y ago
  100W FT8 bothers you? There are people pumping 1500W on FT8. I would get the sentiment on WSPR though. 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
  - SamPatt 7y ago
    I don't get the hate for 100 Watts on FT8 either. Use the power you need to make the contact. Keep an eye on the waterfall to make sure you aren't doubling. 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.
    - ruslan 7y ago
      Isn't 50W that is max allowed power on UHF (432MHz) for amateur radio ? de R9LAJ
      - paulirwin 7y ago
        Sorry for the late response. Yes you're correct, 50W max on that band, at least in the US. I was replying to a person referring to FT8 on the HF bands where the max power is 1500W. 73

mabbo 7y ago

> But in rare circumstances, atmospheric refraction can cause high-frequency radio signals to propagate much farther than normal, 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.

catblast 7y ago
https://en.wikipedia.org/wiki/Tropospheric_propagation https://en.wikipedia.org/wiki/Tropospheric_propagation It’s episodic and well known phenomenon to any amateur radio operator that prefers the vhf/uhf bands.
- PaulHoule 7y ago
  One summer afternoon I was walking in the woods with a 2 meter handheld (140-144 MHz) and made a contact on somebody 200+ miles away across Lake Ontario (and land.) He was a Canadian who enjoyed tropospheric propagation events across the lake, particularly tuning into the My Network TV station in Syracuse which has an absurdly powerful transmitter.
eythian 7y ago
It is quite possible. I used to use packet radio on 144.45MHz, and when the conditions were right I could reach a BBS ~200km away. That's not terribly exciting, but both stations were pretty low to the ground, not very powerful or directional, and definitely not in line of sight. 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.

fpoling 7y ago

I wonder if reduced air traffic contributed to the success? I.e. could it be that favorite conditions are not that rare if not due to disturbance from all the flights?

ruslan 7y ago
I would say the opposite - UHF signal can bounce from plane traces, this effect is known to be used by some UHF radio amateurs to conduct long distance (beyond the line of aight) QSOs.