Atmospheric Ducting

I believe that this is also called a tunnel effect, the radio waves are tunneling through a duct formed by temporary ionized layers. Something similar can occur with whales who seem to be able to communicate over distances of thousands of miles also by some sort of an acoustic tunneling effect of sound waves, also called sound channels.


Speaking about tunneling, once on a trip with the Shell tanker Ondina from Iskenderun, Turkey to Rotterdam, while sailing at night under Sicily, the third officer sent in panic the Chinese bridge boy to the captain and mates in the midship to come and hurry to the bridge. The captain almost threw a fit because he expected some sort of disaster was about to happen.


Ondina’s 3 and 10 cm radar scanners with 50 and 60 kW radar transmitters.

As it turned out it was all about the Raytheon’s radar screen which showed to our utmost surprise the map of Italy up to the Alps in the north. We could not believe our eyes. The phenomenon lasted what seemed an eternity, but probably between 8 and 10 minutes. It must also have been some strange, very special tunneling or super propagation effect. We reported it to the Dutch Weather Bureau but they commented that this was impossible. They probably thought that we must have consumed too much of Dutch gin, just drunken talk…


Not impossible obviously. I believe the term is super ducting or similar. The propagation is at such distances that returns are on the next pulse repetition period. The radar returns show up on the scope at the wrong ranges.

Once saw Baja Peninsula this way. When the Baja Peninsula showed up on radar the second mate did call the captain. I had a few years earlier seen Vancouver Island the same way so I was familiar with the phenomena. I tried to explain but all the second mate could say is “If that’s Baja than I"m all fucked up”. Which he was but not the point.


Marconi Radiolocator Mark IV

No special phenomenon but drilling towers on the Lake of Maracaibo.

Ducting was often present in the winter months when approaching the Gulf of Oman on radar. Many years ago in the South Atlantic I was on the VHF with another company ship. We realised that we were in different watches and after exchanging positions we wer more than a 1000 miles from each other.

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That is extraordinary! As far as I am concerned I would say that this is something for the Guinness book of records.

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And here I thought this was going to be about fresh air supply to confined spaces…


As a member of the black gang I usually don’t pay too much attention to the mumbo-jumbo of those who get paid to look out the windows up above but this is some fascinating stuff that I haven’t heard before. Forget Guinness Book, forward this thread to the producers at the History Channel that make the Ancient Aliens series. This is some neat stuff, especially if it can be replicated.

When we came across another Caltex tanker there was always gossip of one kind of another with crew lists exchanged. The tanker that I heard was talking to another Caltex tanker a mile away from him. I only heard the one ship. I was 2nd mate and he was 3rd mate and ships time was about 3 hours different.

When I was on the Seattle to Alaska run we used to monitor 4125 kHz. I recall hearing what sounded like boat pushing on the rivers call in at night and give position/fuel reports. I don’t know where they were but judging from the accents they were south of the Mason-Dixon line.

They’d come in weak at first , get stronger then fade away. You’d hear several boats report before it faded away. Sometimes it’d be a couple nights in a row.

Another tunneling effect was that we could receive the AM broadcast station WSM (We Shield Millions), branded as The Legend, at Nashville Tennessee loud and clear on 650 khz in the middle of the Pacific! They transmitted mainly country music. In five years time WSM will exist 100 years!

The famous 808 feet diamond shaped transmitting antenna,

We received the station especially well during day time hours. At night time the noise level rose due to absence of the ionized D layer. To be honest we used to connect the ship’s main transmitting aerial to the receiver for maximum signal strength but nevertheless. I seemed to be a rather permanent duct situation as this was repeated every time we crossed the big Pacific pond.

RF Skip is a natural phenomena . Back when cars had AM radio everyone was familiar with it. If in NY you would suddenly find a Texas radio station, reception didn’t last long but was kinda neat.

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On one vessels that I worked before the days of vsats & internet the only time I wanted to be in the bridge was at UTC -6 which was 13:00 CST. The Americans master & mates would have the signal sideband on New Orleans wwl 870 or Houston 740 for the news & Paul Harvey. Those channels would come in West Africa down into South America most days on that boat.

Gulf of Alaska, roughly the same era: at 2 or 3 in the morning I would hear the Australian Coast Guard. Either 4125 or 2182, I forget. It was common enough I never thought it odd. Only that time of night.

“Anaprop” (anomalous propagation) is what we used to call it, back in the day. Same principle as sonar ducting i.e. thermal inversions but at varying altitude. Occasionally used to extend (e.g. UHF) radio range relaying between aircraft.
At sea on radar, I have seen coastlines, or more likely high terrain near the coast - at ridiculous range which were actually 2nd-time-around images (e.g. Libya at 150+nm). Also icebergs N. Of NFLD at 60+ nm by eye and radar.

Geez, I used to listen on WOM for Paul Harvey too. Away from home it was like listening to one of my moderately inebriated great uncles on Thanksgiving. Great memories.

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I think many here are confusing ducting with skip.

There are different phenomena associated with low frequency (LF MF HF) vs VHF and UHF transmissions.

Worth looking at are “long path” and polar propogation.


I’ve heard of skip & the waves bouncing off the upper parts of the atmosphere which was what I was referring to with the AM news station. But “ducting” is new to me which is pretty neat IMO. Especially the part about the RADAR freakishly picking up targets & coast lines across the globe.

True, I’m talking about skip. Ducting? Not that familiar with. As a teen I used to take advantage of skip in my amateur radio days to get QSL cards mailed from far away places. [ QSL cards were post cards that confirmed the contact] Those of us that were low budget communicated with CW and were really grateful when we got a QSL card. I looked through my cards a few months ago and it occurred to me the stamps may be worth saving. Some of the countries are not known by the same name now

RADIO - Tropospheric Ducting ( is caused by specific weather conditions that guide VHF-UHF and above signals for long distances. It is not related to the ionosphere at all, radio waves at these frequencies go right through that.
I have talked to USCG Saint Thomas from Annapolis on VHF due to this phenomenon and heard a pilot dispatch operation up around Boston from the Annapolis area as well. The world record for this is around 3,000 miles or so.
WHALES - Are obviously not using VHF radios. I suspect it is related to thermoclines that might duct sound. Hiding under a thermocline is a plot device in about every sub movie/book and you can see them yourself with a fishfinder on occasion. This is a shallow one in a lake and the sounder is strong enough to get the bottom echo through it.