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Old 14-10-2015, 11:01   #16
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Re: VHF and AIS Radiowave Propagation and VHF and AIS Radio Range

Quote:
Originally Posted by ka4wja View Post
Cadence,
For you sir, I'm try to boil it down a bit...
(but ask the purists to ignore this)

Tropo-scatter is just the way VHF/UHF (and microwave) signals travel (and are useful) beyond line-of-sight, when there are NO special atmospheric conditions nor enhancements... It's not really "bending", but just like it sounds....it's a "scattering"....and again this is just our normal atmosphere, and is entirely calculable (and as long as your transmitter, antenna, etc. are adequate for the length of the path, it is 100% reliable!)

We use tropo-scatter everyday....although most don't realize it...almost all Marine VHF comms beyond line-of-sight (beyond the typical 15nm - 20nm sailboat-to-sailboat, out to about 30-40nm, is via tropo-scatter...) unless some atmospheric enhancements are causing longer ranges...
This sometimes erroneously referred to as "groundwave"...but it is tropo-scatter...


Here is the exact words that I wrote this summer....see my original post for more!




I do hope this explains things???

fair winds..

John
Damn! I thought ground wave was the low frequency subs use under water to communicate. Doubt it is a ham manual.
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Old 14-10-2015, 11:30   #17
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Re: VHF and AIS Radiowave Propagation and VHF and AIS Radio Range

BTW, as an aside....I've been studying these topics now for almost 45 years, since I was a kid....continued thru college, and still try to learn more every year!

And, I've taught much of this for almost 40 years now...gave my first propagation and antenna design seminar for hams, when I was a teenager...


In actual fact, most hams these days (here in the US anyway), unfortunately have little grasp of these subjects...
[heck, even some of good ham buddies are still confused by much of this....and some of them work in RF, everyday!!]


So...
So, please don't worry about the misunderstandings....if it takes a few read-throughs to start to grasp it, that's still better than most!!



Fair winds...

John
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Old 13-12-2015, 10:56   #18
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Re: VHF and AIS Radiowave Propagation and VHF and AIS Radio Range

With all the talk of VHF systems, "AIS splitters", and especially "what coax" and/or "what antenna"....
I thought some may wish to learn how the VHF radio system and the radiowaves work!

Although, I think it would make a good "sticky", it is a bit esoteric...

Enjoy...and fair winds...

John
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Old 13-12-2015, 12:32   #19
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Re: VHF and AIS Radiowave Propagation and VHF and AIS Radio Range

Here are a couple of interesting propagation indicators:

The chart of signals received by my AIS receiver (north of San Francisco, at 1000 ft elevation): AIS station statistics

At the moment the reception is limited to line-of-sight (more or less), but at times I receive signals from ships beyond 1000 miles distance. This long-range reception is definitely weather-related, and I assume it is tropo ducting.

This site has similar charts for thousands of AIS receiving stations all over the world (AIS statistics and vessel counters)

Here is a chart of worldwide troposphere conditions: Tropospheric Ducting Forecast for VHF & UHF Radio & TV
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Old 13-12-2015, 13:08   #20
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Re: VHF and AIS Radiowave Propagation and VHF and AIS Radio Range

Nice ducting forecast.

I've been 42nm off the mouth of the Columbia and hooked up to a telemetry network I (used to - just changed jobs) run in the Columbia estuary.

The network is based on 802.11b at 2.4 GHz with the ship antenna at 60' and the shore side at 200'. Cell phones were working from deck level.

On another occasion I did see the Columbia system connect to the one that OSU runs at Newport. Cape Disappointment to Yaquina Head - Right around 105 nm.... Again at 2.4 GHz.

Radio still has "voodoo" in it which makes it fun
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Old 13-12-2015, 14:27   #21
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Re: VHF and AIS Radiowave Propagation and VHF and AIS Radio Range

Paul,
You are correct that the VHF reception you have at times, that is 1000 miles distant, is tropo ducting!
The California / Pacific duct is a well know one....although not a daily occurrence, it does arise more than most think...
Quote:
Originally Posted by Paul Elliott View Post
The chart of signals received by my AIS receiver (north of San Francisco, at 1000 ft elevation): AIS station statistics

At the moment the reception is limited to line-of-sight (more or less), but at times I receive signals from ships beyond 1000 miles distance. This long-range reception is definitely weather-related, and I assume it is tropo ducting.
This duct, along with the "Spain / Morocco / Canaries" duct, (and to a lesser extent a US E. Coast and sometimes to Caribbean duct), are the main locations of these trope ducts....so, while your particular observations, of 1000 mile distant VHF signals from high above san fran, are ducting....what most refer to as "ducting" isn't...


Unfortunately, the instances that evm1024 wrote about, are not "ducts", but rather are ordinary "tropo" (or tropo-scatter, depending on antenna gains and transmitter powers)....42nm at 2.4ghz, with antennas at 200' and 60', is just a bit beyond line-of-sight, and is quite an easy connection to make 24/7/365 via tropo-scatter....but if power and antenna gains were limited, then it is likely "tropo"....and the 105nm at 2.4ghz, is also just normal "tropo"....no ducting here...



Thanks for reading all of my ramblings!

John
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Old 13-12-2015, 19:20   #22
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Re: VHF and AIS Radiowave Propagation and VHF and AIS Radio Range

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Originally Posted by ka4wja View Post
Paul,
You are correct that the VHF reception you have at times, that is 1000 miles distant, is tropo ducting!
The California / Pacific duct is a well know one....although not a daily occurrence, it does arise more than most think...
This duct, along with the "Spain / Morocco / Canaries" duct, (and to a lesser extent a US E. Coast and sometimes to Caribbean duct), are the main locations of these trope ducts....so, while your particular observations, of 1000 mile distant VHF signals from high above san fran, are ducting....what most refer to as "ducting" isn't...


Unfortunately, the instances that evm1024 wrote about, are not "ducts", but rather are ordinary "tropo" (or tropo-scatter, depending on antenna gains and transmitter powers)....42nm at 2.4ghz, with antennas at 200' and 60', is just a bit beyond line-of-sight, and is quite an easy connection to make 24/7/365 via tropo-scatter....but if power and antenna gains were limited, then it is likely "tropo"....and the 105nm at 2.4ghz, is also just normal "tropo"....no ducting here...



Thanks for reading all of my ramblings!

John
Good to know.
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