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Skip / Tropospheric Ducting What is it?

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VintageJon

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Sure to start a controversey, but I'll give it a wack.

Simply put, if you have a Temperture Inversion or heavy cloud cover the signal can bounce of it and come down at your receiver.
When there are weather systems between you and the transmitter this is most likely.

I have stated this as simply and directly as I can. I'm sure there will be a storm of controversey on this as it is a complex yet little understood area of SWL.

Let the posting begin,
Jon
 

cnmsales

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So what does this matter, do you mean it makes you pick up stuff from further away that you normally wouldnt be able to?
 

KR4BD

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Skip and Tropospheric ducting are not the same thing. Here is the "Readers Digest" explanation:

SKIP is where radio signals bounce off a layer of the ionosphere. Depending on the frequency involved (Usually HF under 30 MHZ), there will be a "dead zone" between the point of transmission and the point of reception. That means, there will be no reception in the area between these two points. A good example of skip can be found on the 10 meter ham band (28 MHZ). A station in California may be working a station in New York, but if you live between these points, there will be no reception of either station. The length of the skip is usually shorther on lower HF frequencies, but you can have multiple "hops" occurring which can really stretch the distance between stations. "The "dead zone" also varies in length depending on many factors including time-of-day, sunspot activity, etc. True SKIP is somewhat rare at VHF and UHF frequencies, although it can happen.

DUCTING, rather than SKIP, does occur FREQUENTLY on VHF and UHF frequencies due to colliding weather fronts (hot/cold, etc.). With DUCTING, signals can travel many hundreds of miles hugging the curvature of the earth. DUCTING is most common on VHF and UHF frequencies (30 MHZ and higher).

You can learn a lot about radio propagation by consulting the ARRL (American Radio Relay Leagues) publications.
 

cnmsales

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Ok great, so here is my next question then. For example and not saying this is possible but lets say someone is transmitting in California and I am in Denver. Do the ducting conditions have to be good in both places for my to be able to hear them does there have to be a good stretch from paoint a to point b or do the conditions only have to be good at my location.
 

VintageJon

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KR4BD is correct- (I overlooked the ship part...)

You can get skip in the HF freqs but it would be unusal to hear them in UHF.

-Jon
 

k9rzz

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cnmsales said:
Could someone please define to me what this is.

Thanks a bunch.
SKIP was coined from the CBers, short for E skip, where the signal bounces off the high atmosphere and comes back to earth some 600 to 1400 miles away. This propagation mode works on HF and up to about 150mhz, although the higher in frequency you go, the rarer it is. There are times of the year when it's more prevalent ... usually in spring and early summer. The foot print of the distant stations you can hear/see/talk to can be really small, perhaps only 100 miles across at times.

Be careful when talking about 'ducting'. That's a narrow path of propagation over a very long distance, like 1000 miles. You might be watching TV channel 18 from 1000 miles away and not see anything else unusual at the same time.

The most common form of VHF propagation is tropospheric 'enhancement' that allows you to hear stations further than line of sight. Sometimes the enhancement is just 100 miles or so, sometimes 300 miles. You can say the band is 'open' somewhat but not REALLY open. When you've got a REAL tropospheric opening, you're going to talk to or hear VHF stations 500 or more miles away. Maybe New York down to Florida along the coast, or Chicago to Dallas. It's a more widespread opening than E skip and caused by weather, rather than upper atmosphere conditions.

You can E skip over mountains, but you won't have any tropo over mountains.

Lots of fun, this VHF stuff!

John K9RZZ
 

nexus

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CNMSALES

Yeah basically. The RF energy from the radios will glide (so to speak) along where the path of the ducting occurs. If a path of ducting goes between Los Angeles CA and Denver CO you're going to hear that signal anywhere in between those two points within the path. There doesn't have to be any specific stretch of distance. IT'S ALL WEATHER DEFINED. It's however and wherever the cold-air and hot-air masses meet up. The ducting may only be 50 or 60 miles long, or it could be 500-600 miles long.

Tropo ducting occurs along temperature inversions, often associated with frontal passage. It often happens over a large, stable high pressure area ahead of a cold front, especially where there is an influx of warm air from the Gulf mixing with colder air from the north. By correlating your tropo DX with weather maps, you should eventually be able to recognize the conditions likely to produce tropo in your area. Pay special attention to areas of the same atmospheric pressure (connected by isobars).

Extremely long distances (up to 1500 miles on UHF) may apply when, as rarely happens, the front is a straight line between you and the station. Tropo is legendary along the Gulf Coast --- where it's known as Gulf tropo. This has been known to blanket the entire coast up to 250 miles inland for a week at a time. This usually happens in non-frigid portions of the winter, and in the fall and spring.

Good times to catch ducting is when you begin to notice sudden changes in tempature at your location. If say it's been in the 90's and then as the sun goes down cold air from the north collides with hot air from the south and the temp drops to say the 70's. You will have a good chance at hearing stations SEVERAL HUNDRED MILES AWAY from you. Not only hear them, but if you had a ham license and a radio you could talk that far too just on a few watts of power.

I've personally talked to stations in Texas and North Carolina from my location in South Mississippi on just 10 watts with an antenna up 20 feet in the air. Not going through any repeaters or anything. mode FM. On public safety frequencies its easy to tell, when you start hearing hetrodyning of 2 or 3 repeaters keying up when your local department talks, or hearing other agencies key up your local department's repeater, you know there's a DUCTing going on.

That link that KR4BD gave is AWESOME, it shows you real-time ducting paths all over the US. So you can just pull that map up and see how intense the conditions are at any given time.
 
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k9rzz

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Again, be careful when using the word 'ducting'. A duct goes from point A to point B ... you'll hear nothing in between. Just like a heating 'duct'.

Instead, use 'tropo enhancement', or just 'tropo'. A tropo duct is a specific type of tropo opening.

:^]

John K9RZZ
 

kb2vxa

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Hi guys,

Initially "skip" was adequately and quite accurately explained, then somebody muddied the water. The term originated long before CB existed and leave it to CBers to screw it all up. What the man was talking about is the "skip zone" where the signal bounces off the ionosphere quite litterally skipping overhead. If you're in the skip zone you won't hear it, you're being skipped.

BTW, the IARU doesn't recognize Skipland as a DXCC entity and my cousin Skip doesn't live there. Sometimes CBers get skipped in the brain department. (;->)

Multi-hop is when the signal bounces off the ionosphere AND the Earth often several times which leads directly into tropospheric ducting so keep reading until I get to it. Look at the ionosphere and the Earth forming a really BIG duct much like that square metal tube in your attic that carries the air conditioning. Bounce a rubber ball inside top and bottom until it comes out the other end, at every point it hits the bottom you'll hear the signal, where it hits the top you won't because it skips the bottom. That's ionospheric propagation but there are smaller ducts.

When a temperature inversion caused by colliding air masses happens the boundary layer between cool air near the surface and warm air aloft refracts radio signals just like the ionospheric boundary layer(s). Being at lower altitude it forms a smaller duct but the bouncing back and forth between boundaries (the Earth's surface is a boundary layer) accomplishes the same thing but over shorter distances, well, most of the time. When there is a very extensive weather front that duct can extend for a couple thousand miles and you can't tell tropo from F2 skip.

And now, KB2VXA Radio Bludgeon prrresents, the Radio Reference Follies starring the usual remarks taken out of context! (tadaaa)

"So what does this matter, do you mean it makes you pick up stuff from further away that you normally wouldnt be able to?"
If you have a really BIG roll of duct tape and toss it REALLY far, the sticky stuff will pick up a lot of things.

"Do the ducting conditions have to be good in both places for my to be able to hear them does there have to be a good stretch from paoint a to point b or do the conditions only have to be good at my location."
Uh, could you repeat the question?

"KR4BD is correct- (I overlooked the ship part...)"
Captain Smith of the Titanic overlooked the iceberg part.

"The foot print of the distant stations you can hear/see/talk to can be really small, perhaps only 100 miles across at times."
DAMN! That makes Sasquach seem puny by comparison! "In the valley of the jolly (ho ho ho) Green Cee Bee."

"You can E skip over mountains, but you won't have any tropo over mountains."
Aw, can't skip to the loo my darling?

"Be careful when talking about 'ducting'."
Yeah, you might piss off some ethnic minority.

"The RF energy from the radios will glide (so to speak) along where the path of the ducting occurs."
Yeah, a little antenna grease goes a long way.

"It's however and wherever the cold-air and hot-air masses meet up."
Usually at a bar on Saturday night.

"Tropo is legendary along the Gulf Coast --- where it's known as Gulf tropo."
It was the legendary Gulf Oil but they were bought out by BP so I guess you'll have to call it British Tropo now.

"...when you start hearing hetrodyning of 2 or 3 repeaters keying up...you know there's a DUCTing going on."
Are you sure it isn't ducks overhead?

"It often happens over a large, stable high pressure area ahead of a cold front."
That's another thing that happens in a bar on Saturday night.

Thanks for your contributions to the nonsense that tends to come over me when I'm bored silly at five in the morning with nothing better to do. Loo loo skip to the loo, skip to the loo my darling. Frankly skipping to the loo causes accidents but that's a toilet tale for another time. (;->)
 
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