VHF and AIS Radiowave Propagation and VHF and AIS Radio Range

[Cadence]

Originally Posted by ka4wja

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.

[ka4wja]

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

[ka4wja]

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

[Paul Elliott]

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

[evm1024]

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

[ka4wja]

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...

Originally Posted by Paul Elliott

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

[evm1024]

Originally Posted by ka4wja

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.

[ka4wja]

Hello to all,

With all the recent talk of AIS issues, and I suspect many sailors might not be grasping the details of antennas, coax losses, "splitters" (relay/splitter/pre-amp), radiowave propagation, etc., I thought it might be time to push this up the pages a bit??

While there are a lot of details in this thread that can of great help to some, other may find it to be overwhelming...
Sorry about that...
Just read what you can.


And, please take away these simple facts:
a) With most sailboat VHF antenna installations, you do NOT need a "specially tuned AIS antenna"!!
Yes, if you're buying a new one, and/or yur present VHF antenna is like 10+ years old, it should be replaced anyway, so....it is recommended to get one that works well across the entire VHF marine band, including the AIS freqs near the top end of the band....but if you have a "newer" VHF antenna, it is usually not necessary to have one specially tuned for AIS!

{note this is due to two factors:
1- most VHF installs have enough cable loss to mask slight VSWR issues and not recognize any issue...
2- most sailboat VHF antennas are broadbanded enough to cover the entire band with VSWR's of ~ or < 2:1, which should not cause any AIS transponders to reduce power..}


b) most sailboat VHF installation designs assume approx. 3db of coax loss...and while less is better,
please do not be lead down the path of thinking that reducing your loss by 1db or so will make any difference at all!! It won't!
{This means use RG-213, or RG-8x (mini-8), coax...or if you're a fanatic like me, and are using a low-loss AIS splitter (like the SP-160) and cannot fit RG-213 in your mast conduit, you can use LMR-240uf, to compensate for the loss in the splitter...but no need to go crazy here, do NOT spend the money and frustration using larger cables, like LMR-400uf!!}


c) most VHF installs see seriously improved coverage when old cable / connectors (and masthead antenna) are replaced....'cuz old cable can be lossy and old connectors are notorious for causing major issues / losses!
(many sailors that do this with "low-loss" cable, rave at the results, thinking it's due to the "low-loss cable", when most of the time it is due to NEW cable and connectors!


Fair winds.

John

[S/V Illusion]

Originally Posted by ka4wja

..
Just read what you can.



b) most sailboat VHF installation designs assume approx. 3db of coax loss...and while less is better,
please do not be lead down the path of thinking that reducing your loss by 1db or so will make any difference at all!! It won't!




John

To clarify, a3dB loss equates to half power meaning the transmitter rated output would be cut in half at the antenna, a very significant reduction. The way in which the quote is worded can be confusing to some.

[Paul Elliott]

Originally Posted by S/V Illusion

To clarify, a3dB loss equates to half power meaning the transmitter rated output would be cut in half at the antenna, a very significant reduction. The way in which the quote is worded can be confusing to some.

Yes, but a 3db reduction in transmit power does *not* equate to a halving of the effective range of your transmission. In an ideal free-space situation, if, say, your transmitter with a 1dB loss in the antenna feedline can be received at a range of 10 miles, then with a 3dB loss your range has been reduced to 7.9 miles. If you had zero loss (not acheivable) then your range would be 11.2 miles.

At VHF the path loss makes the difference even less significant than my "ideal free-space path-loss" calculation indicates.

So cable loss is to be minimized, but in practice there is definitely a point of diminishing returns that is pretty quickly reached.

[continuouswave]

Regarding how distance affects path loss:

The usual assumption of free-space path loss as a function of distance is that path loss increases at a rate of

20 LOG (d)

So if the distance doubles (d=2) then path loss in dB will increase

20 LOG (d)
20 LOG (2)
20 (0.3)
6 dB

This is a good assumption for FREE SPACE, which means that for both the transmitter and receiver antennas there is nothing between them and nothing around them except a totally empty infinite vacuum of space containing no other conductors or conductive surfaces.

In real world radio signal propagation you cannot have those free space conditions. There is the EARTH to consider, the terrain, and many sources of reflecting surfaces.

For this reason, when estimating path loss over real earth, the effect of distance on signal strength is generally modeled as being much greater than

20 LOG (d)

and a figure of something more like

40 LOG (d)

to perhaps as much as

50 LOG (d)

To give an example, if the distance d doubles, then it is reasonable to expect that the path loss will change much more than the -6 dB suggested in the free-space analysis.

40 LOG (d)
40 LOG (2)
40 (.3)
12 dB

The FCC provides guidance in PART 80--STATIONS IN THE MARITIME SERVICE about the effect of distance on radio signal propagation in Part 80.767.

There you find a chart that shows expected signal strength as a function of distance. There is a line plotted for "Free Space" and then sets of lines for real world with various antenna heights. The real world lines all follow a slope that represents path loss increase at a rate of

-56 LOG (d)

In an article I wrote recently I summarize three different models used for estimating real world VHF signals in marine use:

--FCC Part 80
--Eglis' Model
--ITU-R P.1546-5 (Method for point-to-area predictions for terrestrial services in the frequency range 30 MHz to 3 000 MHz.)

None of these models think that path loss on real paths will be similar to free-space conditions. For more about this see

VHF Radio Propagation Over Water
continuousWave: Whaler: Reference: VHF Radio Propagation Over Water

As a corollary, we can also find what effect gain added or subtracted to the system will have on the path distance.

If 3 dB is added to the system, then the path distance will increase, but the amount of the distance increase depends on rate of path loss with distance.

If you use the relationship

dB = 20 LOG (d)

and rearrange to solve for (d), you find

(d) = 10^(dB/20)

Solving this for dB = 3 gives

d = 1.41

That means the path distance can increase by 1.41-times---in free space conditions.

But if you use a more real-world model, even the most generous, you see a different effect.

If you use the relationship

dB = 40 LOG (d)

and rearrange to solve for (d) where dB=3, you find

(d) = 10^(dB/-40)

Solving this gives

d = 1.19

The path distance can only increase by 1.19-times when 3 dB is added to the system. If only 1 dB is added the result is a distance increase of only 1.06.

Of course, at the absolute margin of communication, every single decibel of improved signal is very welcome and could make a difference.

[ka4wja]

Jim, continuouswave,
You're the Man!
Awesome work!
I haven't read it all, but so far looks good! (except I wasn't clear if you converted field intensity, uV/m, to actual power in the antenna, dbm...as the conversion isn't as easy as just uV to dbm?)

BTW, I've found real-world tropo-scatter paths (beyond line-of-sight) to actually be better (lower path loss) than the FCC graphs...
But, whatever the case...at least we know that we've got good margins for 99% of our VHF Marine comms!

Thanks for all the hard work!

John

[continuouswave]

John--thanks for the kind words.

In any of these propagation models involving real world estimates there is usually also a probability factor. If you want a communication circuit that will be available 99.9-percent of the time, you have to plan for more path loss being possible in the worst case conditions. This accounts for the rather discouraging estimates for path loss given in some models. I don't believe the probability is given as an explicit factor in those models, but I would assume it was considered in their estimates.

On the other hand, to figure all path loss will be at free space path loss conditions is very unlikely to result in a circuit that will be reliable or even possible.

ASIDE: I am a big hockey fan. I live less than 20-miles from the border with Canada. I love to watch hockey on CBC television. The CBC station (CBC-9 on 187-MHz) in Windsor, Ontario is about 15-miles inland from the border. My 35 mile path to the transmitter site goes right through the downtown buildings of Detroit and Windsor. There is one high-rise building in Windor which is almost directly in the path.

To get a watchable signal, I have installed a mono-band CH-9 antenna with about 12-elements on an 9-foot boom on a 12-foot mast on a tripod on the roof of the house. Without this dedicated antenna, there is no trace of a signal at my location. I have also some band-pass filters on the antenna so I can cut down on a local signal with a 500-kW ERP on CH-7 (175-MHz) that is just three miles away and almost boresight on the antenna's orientation to the CBC station. Without the bandpass filter the CH-7 signal plus the antenna gain tends to overload the receiver.

On a good day, the CBC signal has about 23-dB SNR, and the digital HDTV picture is rock solid. As the weather has been changing the past month, I have noticed that the signal will be solid all day, but as soon as the sun sets and the temperature starts to drop, the signal begins a slow fade. Once the SNR gets to about 15 dB, the HDTV picture loses lock and becomes unwatchable. But as darkness comes on and the air temperature begins to stabilize, the signal begins to come back, and soon I have a watchable picture again, which will last all night.

There is something going on with the path loss that is related to air temperature; the path likes for the air temperature to be the same across the whole path, as far as I can tell. When the air temperature is rapidly changing, the path loss goes up.

I mentioned this because it demonstrates how path loss can vary, even though nothing changes with the two antenna locations, the transmitter power, or the receiver sensitivity.

This time of year is better than most because there is no foliage on the trees. When the trees have leaves and it rains, all that wet foliage adds a lot of path loss. But hockey season is usually over by mid-June. Then I go boating.

[ka4wja]

With much recent discussion about AIS reception issues, etc. (usually antenna / antenna cable / connector related), I thought some might be interested in having a look at this discussion again...

Enjoy.

Fair winds...

John

[ramblinrod]

Originally Posted by continuouswave

On a good day, the CBC signal has about 23-dB SNR, and the digital HDTV picture is rock solid. As the weather has been changing the past month, I have noticed that the signal will be solid all day, but as soon as the sun sets and the temperature starts to drop, the signal begins a slow fade. Once the SNR gets to about 15 dB, the HDTV picture loses lock and becomes unwatchable. But as darkness comes on and the air temperature begins to stabilize, the signal begins to come back, and soon I have a watchable picture again, which will last all night.

There is something going on with the path loss that is related to air temperature; the path likes for the air temperature to be the same across the whole path, as far as I can tell. When the air temperature is rapidly changing, the path loss goes up.

Could it be RH / dewpoint related?