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.

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

This one is on 18 MHz...

Another #radioechoes day on HF. This is #17m #18mHz at 1355UTC Station is in N. Cal. Notice the large delay time yet the station in Texas et al have no hint of echo. pic.twitter.com/k7do8HHMng

— VO1FOG (@VO1FOG) December 25, 2021

This one is on 21 MHz..

Station is in California. Band is #21mHz #15m. Echo present when I beam LP; echo goes away when I turn SP. #radioechoes pic.twitter.com/n8RBH7oolt

— VO1FOG (@VO1FOG) December 24, 2021