Tuesday, December 26, 2023

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...

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 


Bill Meara said...

Really interesting post. Thanks. But it looks to me as if he is hearing the "round the world" signals off the back of his 4 element yagi. This may be the case where one of those beams that instantly reverse their patterns would be useful: Transmit the pulse, then instantly shift the beam pattern by 180 degrees to listen for signals making the trip all the way around. In this way you may be able to hear signals from the third trip around (or more).
What do you think?

73 and thanks, Bill N2CQR soldersmoke.blogspot.com

Bill Meara said...

Or, another way to do this would be to have other hams -- perhaps locals -- point their beams 180 degrees from the transmitting station's beam heading. They would then be well positioned to receive the signals circumnavigating signals. 73 Bill N2CQR

John, EI7GL said...

Re N2CQR... "But it looks to me as if he is hearing the "round the world" signals off the back of his 4 element yagi. "

Hi Bill, that certainly looks like the case. Even if the path is skewed, the echo is still coming in off the back of the beam. I'd presume something like a 15-20dB front to back ratio on the Yagi so having the antenna reverse direction would make a big difference.

Then we go down the rabbit hole... how long is the TX/RX delay? Will mechanical relays work or do you need PIN diodes? And if someone is using a high power amp with something like a 1kw, what kind of switching changeover is used? Mechanical or solid state?

As you suggested, there's a lot to be said for a remote RX or have someone else listen for the echoes.

John, EI7GL

Steve said...

A separate receiver nearby, pointing the other direction, can hear both the original and the echos. No need to synchronize. This would allow you to use watts instead of 1kw.

VE7TOA said...

First thing to try would be to average up a few thousand pulses - send one dot per second, record everything and overlay the echoes. Ideally with a separate nearby receiver with its antenna facing the other way.