Twice around the globe on 21 MHz and 80,000km echoes

Salvadaor, EA5Y near Madrid in Spain recently carried out some interesting tests on the 21 MHz (15m) band at the start of December 2023 and heard his own signal go twice around the world for a total path length of 80,000 kms.

First of all, let's set out the conditions. It's the 5th of December 2023 and the time is 08:30 UTC. Sunrise in Madrid is at 07:22 UTC which was about an hour earlier.

EA5Y is transmitting on 21.022 MHz using 1-kilowatt into a 4-element Yagi beaming at 240 degrees. He is sending a series of single dots and is listening for echoes.

The view from the antenna is shown above and 240 degrees is to the left of 'WEST' and should be close to 'VK2 long path'. There are mountains in the distance but they are some way off and EA5Y probably has a nice low angle of radiation from this antenna at this height.

Echoes... This is the audio plot from Salvador...

Going from the left...

1) He has a backscatter signal at 25-milliseconds which is too short for the human ear to separate from the initial dot.

2) The signal travels around the world and arrives back in Spain 40,000kms later.

3) The signal continues and goes around the globe a second time and arrives back 80,000kms after initially setting off.

Here are some interesting observations....

a) Salvador tried beaming to the north-east in the opposite direction and the echoes were much weaker. He wonders if the path was skewed? i.e. it's not as simple as the map at the top of the post would suggest.

b) He also tried listening to an online receiver based in New Zealand on the 2nd of December and this was 40kms from his antipodal point. You can hear the audio below...

And this is how it sounds... pic.twitter.com/4h62Wc06Kf

— Sal EA5Y (@_EA5Y) December 2, 2023

You can clearly hear the signal reaching New Zealand for the first time at 20,000 kms and then going around the globe again to arrive back in New Zealand again at about 60,000 kms.

No doubt there is some antipodal focusing going on here and multiple paths and options appear for the signal instead of the usual short path / long path options.

As for the propagation mode, it's obvious via the F2 layer but there is probably some chordal hop involved with the signal going between different points of the ionosphere without needing a reflection off the ground.

We often see reports of long path echoes but it's interesting to see a report of a signal going around the world twice.

Further tests??? ... The obvious question is just how many times does the signal travel around the world? Are there times of the day when the signal is ever present going around the world multiple times before conditions change and the path breaks?

I wonder if it would be possible to decode some of the 2nd, 3rd, 4th, etc echoes with a digital signal? Something like the MSK144 meteor scatter mode using fast decode? Some other mode? 

Is there some weak signal mode that is capable of decoding your own weak echoes without relying on looking at ticks on an audio plot?

Maybe there isn't but I'm just putting it out there in case someone wants to investigate.

Previous posts...

1) Long path echoes on the 24 MHz band (VO1FOG) - Dec 2021 

2) Long Path Propagation on the 17m HF band (VO1FOG) - Dec 2022 

Long Path Propagation on the 17m HF band - Dec 2022

Larry, VO1FOG in Newfoundland, Canada has been carrying out a number of long path propagation experiments on the HF bands recently and I found this one especially interesting. For the purpose of clarity, I'll try to give an overview of what happened so people will understand what is in the video clip further down this post.

VO1FOG is located at the eastern side of Newfoundland, Canada and on Wednesday the 21st of December 2022, he was listening to the daily morse code practice transmissions on 18.0975 MHz from W1AW in Newington, Connecticut. 

The distance from VO1FOG to W1AW at the ARRL headquarters is about 1,660kms and the direct short path signal was quite strong. However, VO1FOG was also hearing W1AW via the long path which means it nearly has to go completely around the globe. The long path is around 38,420kms.

VO1FOG was using a 3-element SteppIR antenna at 23-metres above ground level. 

The advantage of this type of antenna is that the lengths of the elements can be lengthened and shortened by motors rather than having to rotate the whole beam. This means that VO1FOG can switch from short path to long path in about six seconds.

The image below shows some morse code signals from W1AW via short path.

The dots and dashes are nice and clean and the word 'SAID' can be clearly seen.

The image below shows some morse received via the long path.

It's not the same word but you can see that it's a lot more messy with the dots and dashes coming from the short path signal mixing in with the dots and dashes coming from the long path.

The audio of the W1AW signal can be heard below. Remember that there is nothing wrong with the signal, you're just listening to the same signal arriving in Newfoundland at different times.

This W1AW code practice...at time 0:31 my antenna is short path to W1AW; code is clean and clear. At time 0:42 my antenna is moved to LP. The code is virtually uncopiable when both sigs merge with a 127 ms delay. At 01:14 I am back at SP and the CW is clean again. pic.twitter.com/KPXsp6Xomj

— VO1FOG (@VO1FOG) December 21, 2022

VO1FOG reports that the test was done at 14:50 UTC. The approximate propagation conditions for W1AW's location on 17m is shown below.

It's interesting that the long path isn't too far from the Grey Line path which is near where the sun is setting or rising.

Larry has carried out several tests like this recently and he reports that he hears both long path and short path almost daily about an hour either side of his local solar noon.

He has seen this echo effect on the 20m, 17m, 15m and 12 m bands and to a lesser extent on 10m. He did tests with K5WLT in Texas who was using 400w into an 80m dipole and the echo was seen on multiple bands on multiple days.

Larry usually does his tests by getting the other station to send the letter 'E' i.e. one-dot with a space after it. The image below shows the test with G0OVK in England when a second dot can be clearly seen.

Analysis... With the rising solar flux, a lot of people are probably hearing strange echoes on SSB and CW signals and not realise what it is. It obviously helps if the transmitting station is running high power as the long path has to travel almost around the world.

When Larry sent me the information and as I was preparing this post, I couldn't help but wonder if the signal from W1AW went around the globe for a second time? If the path is open and the signal goes around the world, does it keep going round and round the globe getting weaker and weaker each time it passes?

Is it possible to hear the second echo as the signal passes for the second time? Is it strong enough?

What about the time signal WWV on 15.000 MHz and 20.000 MHz? That produces a tick once a second, it should be possible to hear the long path tick as well. Again, is it possible to hear more than one echo in the space between ticks?

It's intriguing to think that there may be signals from high power transmitters going around the globe multiple times before finally dying out when the propagation conditions change. Again, it raises more questions... how long? Up to a second? Several seconds? Minutes? That's leads then to the subject of 'Long Delayed Echoes' which is another topic in itself.

If you have a good beam or antenna for the HF bands then have a listen and see what you can hear.

Thanks to Larry, VO1FOG for all of the background information for this post.

Long path echoes on the 24 MHz band (VO1FOG) - Dec 2021

Larry, VO1FOG in Newfoundland, Canada has noticed a number of strong echoes on SSB signals on the higher HF bands recently and he has been able to confirm that this was due to long path echoes.

There is a nice audio sample below but first, let's set out what is happening.

The map above shows the location of VO1FOG in Newfoundland and VA2AM is 1560kms to the west near Montreal.

VA2AM is on 24.937 MHz and is beaming towards Europe. VO1FOG in Newfoundland is beaming east as well, listening to him for over an hour and the signal has a distinctive echo. The direct short path signal at 1500kms is below the F2 MUF for 24 MHz and is present but weak.

VO1FOG finally manages to catch the attention of VA2AM and VA2AM turns his antenna to the west to check the long path.

FINALLY! I can confirm that these #radioechoes that I have been hearing for the past several days is a TRUE long path/short path echo. Notice how strong VA2AM becomes when he turns LP.. His SP distance is 1500km vs LP distance of 38500km! Delay time is around 135ms. #12m #24mHz pic.twitter.com/VbVZx1XU2d

— VO1FOG (@VO1FOG) December 27, 2021

The signal with the echo is when VO1FOG is hearing both the short path and long path signals. You'll notice that when VA2AM turns his beam, the long path signal is much stronger and the echo disappears.

As VO1FOG points out, the long path signal has to travel almost right around the globe with a distance of about 38,500kms.

Analysis... The solar flux is currently around the 120-130 mark and the higher HF bands are now beginning to open. It's likely that a large part of the long path was probably due to chordal hop propagation with the signal going between different parts of the ionosphere and not touching the ground.

It's also worth pointing out that as far as I know, both VO1FOG and VA2AM are using directional beams up on towers and as such, both would have a low angle of radiation. These type of signals may not be so obvious on simple antennas like dipoles at a low height.

Questions... As is often the case, this example of long path echoes raises more questions..

1) Is this happening on the 28 MHz band at the moment? Is anyone hearing strange echoes?

2) What about FT8 signals? Does the echo destroy their ability to decode? With FT8 signals, it's all or nothing. It either decodes or it doesn't. With CW or SSB, the human brain can piece together what was being sent but what happens with a digital mode? Will the long path echo mess up the signal?

3) Is the 'long path' really a simple long path? For example, let's say someone with a transmitter sent a series of pulses like say a train of CW pulses with maybe a second or two in between each one.

The receiving station could then look at the received audio with an oscilloscope. In theory, the scope would trigger on the first short path signal and the long path signal would be seen as an individual peak in between the short path peaks.

But would it? Would there be more than one 'long path' peak? Would there be a chance of seeing secondary peaks if the signal went twice around the world? How would the peaks change as the antenna is turned?

The beauty of an oscilloscope it that it would show things that you can't hear and it would also show the delay of the signal in milliseconds allowing you to calculate the distance traveled. 

Wouldn't it be nice if there were CW beacons like this to allow people to carry out experiments?

There are probably many more questions and experiments but these are the ones that I thought of while I was listening to the recordings.

***

Here are a few more examples from Larry, VO1FOG...