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Old 17-04-2021, 11:46   #1
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300+ nautical miles VHF range.

Check out the number of targets and the range. RF atmospheric conditions are crazy now
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Old 17-04-2021, 16:28   #2
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Re: 300+ nautical miles VHF range.

guess that is on the AIS ?

sometimes seen a few similar, but never so many...as you say, crazy atmospherics

thanks for sharing

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Old 17-04-2021, 18:33   #3
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Re: 300+ nautical miles VHF range.

Just out of interested what's your location? It would be interesting to see the weather conditions, especially to the SW of you.
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Old 17-04-2021, 18:55   #4
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Re: 300+ nautical miles VHF range.

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Just out of interested what's your location? It would be interesting to see the weather conditions, especially to the SW of you.
We’re in the Abacos, Bahamas
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Old 17-04-2021, 19:23   #5
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Re: 300+ nautical miles VHF range.

I've had that on my AIS before also. I just assumed there was some sort of repeater involved.
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Old 17-04-2021, 19:33   #6
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Re: 300+ nautical miles VHF range.

It’s not uncommon to see this range during conditions known as ducting which, as its name implies, funnels signals through some strange paths. Recently, such an event produced lots of signals from Australia/New Zealand on 6 and 2 meter ham bands. I’m only assuming this is a similar event.
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Old 18-04-2021, 02:09   #7
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Re: 300+ nautical miles VHF range.

Are you sure there is no base station acting as a repeater? We saw this all the time in the Galapagos where each islands mountain peak had a base station, all connected across the island chain, and acting as a repeater. Fishing boats 200 miles to sea would show up.
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Old 18-04-2021, 02:32   #8
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Re: 300+ nautical miles VHF range.

Tropospheric Ducting Forecast
https://www.dxinfocentre.com/tropo.html

About Tropospheric Ducting
https://3fs.net.au/tropospheric-ducting/
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Old 18-04-2021, 02:53   #9
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Re: 300+ nautical miles VHF range.

Had this happen a few times in Queensland. Not really unusual. No, repeaters are not involved, in fact I do not believe they actually exist in relation to AIS.
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Old 18-04-2021, 03:17   #10
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Re: 300+ nautical miles VHF range.

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That makes sense. Level 7-8 ,Intense to Very Intense around Abacos over the last couple of days.
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Old 18-04-2021, 06:52   #11
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Re: 300+ nautical miles VHF range.

Exactly, Gord nailed it again

I remember a time long ago when I was building 3 meter stereo transmitters for radio pirates popular at the time. From my attic in Rotterdam, using only 4W and a directional antenna pointed at The Hague where the “customer” was for a test transmission... I do a CQ and get reply from England where even the stereo sound was bearable! Ducting is wonderful.

My point is that occurrences seem to have become much more common than they used to.

There’s no repeaters for AIS. The AtoN’s I receive from all along Florida can be broadcast by shore stations with very high antennas but the ships are just the ships with antenna at 100’ or so.
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Old 18-04-2021, 09:06   #12
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Re: 300+ nautical miles VHF range.

In a busy shipping area with hundreds of AIS signals anyway, could an event like this overload the AIS channels so that traffic gets dropped? Is the SOTDMA system robust enough to cope? What about class B CS signals, will they get transmitted at all if all timeslots are occupied by ships 100s of miles away?
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Old 18-04-2021, 09:28   #13
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Re: 300+ nautical miles VHF range.

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... My point is that occurrences seem to have become much more common than they used to...
I wouldn’t be at all surprised; one way [more common], or the other [less common]; since climate affects weather, which affects the atmospheric refractive index, which is the primary cause of “ducting” [over-the-horizon radio propagation].

The refractive index of the troposphere depends on three parameters: air temperature, its humidity, and atmospheric pressure. Temperature inversion prevents vertical movement of air, and contributes to the formation of haze, fog, smog, clouds, and mirages. In such layers of temperature inversion, the refractive index also has an anomalous character, which causes changes in the rectilinear propagation of radio waves. The propagation, in such inversions, is waveguide in nature, where the role of the lower wall is performed by the sea surface, and the upper is the inversion layer. This waveguide is called the duct.
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Old 18-04-2021, 14:40   #14
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Re: 300+ nautical miles VHF range.

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Originally Posted by Kelkara View Post
In a busy shipping area with hundreds of AIS signals anyway, could an event like this overload the AIS channels so that traffic gets dropped? Is the SOTDMA system robust enough to cope? What about class B CS signals, will they get transmitted at all if all timeslots are occupied by ships 100s of miles away?
It sure is slowing my Vesper down significantly, but it copes. I think the two channels is enough for SOTDMA but CS may have some trouble. I still think it’ll be okay here, but busy places like the English Channel etc. might be way worse.
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Old 18-04-2021, 17:26   #15
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Re: 300+ nautical miles VHF range.

Nick, et al,

Just a few brief comments/clarifications....

1) The VHF Tropo-Enhancement that Nick showed a screen shot of, is just that, a local region of "tropo-enhancement" (usually just referred to "tropo" or "local enhancement"), not "ducting"....
And, these "tropo enhancements" can last for days, and days....(some have lasted a week)....contrarily, "ducts" are often short-lived, and last hours, or a day or two, max...

This might seem like I'm "nit-picking", or arguing over semantics, but "ducting" does occur, and is a wonderful cool thing to experience, but is rather rare and as it's name implies, it is a narrow "duct" of long-range VHF/UHF/Microwave communications path, where the signals thru the duct are significantly enhanced.

{Most ducts at VHF freqs are 50 - 100 miles wide at most, some as narrow as 10 - 20 miles, and typically 750 miles to 1500miles long, and while there can be a "duct hole" along the duct, they are rare, more than one or two that vary/open/close sporadically, are very rare!


As a personal example, local/regional "tropo enhancement" happens here all the time (dozens of times / year) !! But ducting is rare...a decade or so ago, we had a great tropo-duct, where I worked stations in Long Island, NY, from my home in Central Florida, on 144mhz....and had many stations in western Long Island and a few in New Jersey, and a dozen or so here in central Florida, but none in the Carolina's, nor Virginia, nor Maryland, nor Delaware, etc...that's a 800 - 900nm path, with strong signals from stations with 25 - 150 watts, on both ends....but, a friend in Georgia, ~ 120nm to my NNW who hears me S-9, heard NO stations at all from NY, NJ....this "duct" lasted all night, and was also there (but, weaker and more sporadic) the next evening...oh, a few years earlier, I've also worked Es (Sporadic-E) from Florida to Colorado, Wyoming, Utah, Nebraska, etc....and, also bounced my signal off the moon, and communicated with others doing the same, but those are WHOLE 'NOTHER discussions....LOL...}

As, you can see from Nick's screen shot, the stations he's receiving are from two different directions, grouped mainly to the SE and to the SW....which is a rather broad area of tropo-enhancement...(and is actually quite common in these area, especially in the springtime...and this time of year, almost a weekly occurrence...)



2) Those that would like the details, please have a look here:

VHF and AIS Radiowave Propagation and VHF and AIS Radio Range
https://www.cruisersforum.com/forums...ge-149499.html


(I've been studying Radiowave Propagation since the early 1970's, professionally and for fun...and been teaching it, almost as long....so, no worries if it gets confusing to some...)



3) Regarding the VK's page that Gord posted links to, he tried to explain things....but, has gotten some things twisted....as in, one sentence he refers to "tropo enhancement" and "ducting" and implies they are the same thing...again, he's trying, and I assume he's read a bit on the subject, but isn't quite explaining things accurately....





I hope this helps, especially this thread here:

VHF and AIS Radiowave Propagation and VHF and AIS Radio Range
https://www.cruisersforum.com/forums...ge-149499.html



Fair winds.

John
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