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EI7GL....A diary of amateur radio activity

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

Posted: 28 Dec 2021 02:28 AM PST https://ei7gl.blogspot.com/2021/12/long-path-echoes-on-24-mhz-band-vo1fog.html



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...
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

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