VHF and AIS Radiowave Propagation and VHF and AIS Radio Range

[ka4wja]

Hello to all,

{Those looking for the basic facts, in a nutshell....

With masthead-mounted antennas, most sailboat-to-sailboat VHF communications is within the typical line-of-sight range of 15nm - 20nm....and once beyond that, the typical "normal communications range" will be approx. 30nm - 35nm (depending on radios, antennas, cable, microphones, operator, etc.) and a typical MAXIMUM range (without any special atmospheric enhancement) is going to be approx. 50nm....

Sailboat-to-sailboat VHF Marine communications beyond 50nm (and typically much beyond the 30nm - 35nm range) ALWAYS uses some form of "enhanced" propagation....almost all times this is "tropo" or "tropo-enhancement" (which can propagate signals hundreds of nautical miles, over quite large geographic areas / regions, typically along the edge of a hi-pressure area, and coastal areas, especially in summertime...)
Except in certain specific geographical areas, actual "tropo-ducting" is rare, and is typically restricted to specific narrow paths, and most times the distant signals (from as much as 1000 - 1500nm away or more) are as stronger as those 200 - 300nm away....}


For those who wish the details and explanations, please read on...

VHF Radiowaves propagate in many different ways....and, while we could get bogged-down in the minutia, that is not my intent here!!

Rather, I will just attempt to inform you all of the main / typical means of VHF communications propagation...."line-of-sight" ("direct-wave") and "tropo-scatter", being the dominant VHF Marine propagation modes) and how those methods work...
As well as describe "tropo-enhancement" (fairly common along the edge of high pressure areas, and in summertime coastal areas, and many times misunderstood and erroneously referred to as "ducting"), and the much more rare "tropo-ducting" (which is actually very rare, except in certain geographic areas, during certain times of the year...)

I will NOT delve into the esoteric / exotic modes of "moonbounce", "meteor-scatter", "aurora", etc., nor the extremely rare Trans-Equtaorial F2, FAI, or even the slightly less rare sporadic-E....none of which is typically possible with wide-bandwidth modes such as FM-Voice or wide-band FM-data, and certainly none are possible with the limited 25-watt transmitter power and simple omni-directional vertical antennas we use on marine VHF....
So, no worries about all of that...
{BTW, I have personally used "moonbounce", "meteor-scatter", "aurora", and Es (sporadic-E), all on VHF (144mhz ham radio)...}


1) Most VHF Marine Communications, whether VHF-FM-Voice, VHF-FM-DSC, or VHF-AIS-data, is accomplished by simple "line-of-sight" mode (technically referred to as a "direct-wave"), which is slightly (approx. 6%) beyond the actual visual line-of-sight....This "line-of-sight" range is very easily calculated and unless there are obstructions between your vessel and another (land masses, etc.) it is very reliable and repeatable....and 100% reliable...

With our 25 watt transmitters and fairly sensitive receivers (typically -117dbm for 12db SINAD, -110 to -113dbm for 20db SINAD, which is a fairly "noise-free" signal....and -107 to -109dbm for full noise-free signals and low BER for data), our line-of-sight ranges are not only 100% reliable but also incorporate significant margins...(typically 50db above the "20db SINAD" spec, at 15nm - 20nm range, with two vessels w/ antennas at 65' above the water...)


For land-based, terrestrial VHF communications, I've gotten used to the VERY simple and easy-to-calculate formula here, which can be easily done in your head in seconds....but this gives results in statute miles, so you'd either need to convert that to nautical miles (divide by 1.15) or simply accept the result in statute miles...
For statute miles.....
VHF Radio line-of-sight distance (in statute miles) = √2 x Af (where Af is the height, in feet, of Antenna)
Line-of-sight communications range between two VHF radio stations (in statute miles) = √ (2x Af1) + √ (2x Af2)

But, for nautical miles, it's only slightly harder to do in your head....
For nautical miles......
VHF Radio line-of-sight distance in nautical miles = 1.23 x √ Af (where Af is the height, in feet, of Antenna)

Line-of-sight communications range between two VHF radio stations (in nautical miles) = 1.23 x √ Af1 + 1.23 x √ Af2




With a typical sailboat-to-sailboat communications path, with mast heights of approx. 50' above the water, you have a "line-of-sight" distance of about 20 statute miles / 17.4nm....If both vessels had mast heights of 65', the line-of-sight range is then 22.8 statute miles / 19.9nm...
If one vessel was a large container ship, with its VHF antennas mounted 190' (max air-draft of a "PANAMAX" ship) above the water, the line-of-sight range to your sailboat would be about 27nm (31 statute miles)...
SO...So, we can now understand that all Marine VHF communications (whether voice, DSC or AIS), that is beyond the "line-of-sight" range must use some other form of propagation....


Regarding reliability and margins above receiver thresholds....
FYI, the Free Space Path Loss at 156.8mhz being 107.6db at 20nm (22.8 statute miles / 36.5km), which is the approx. line-of-sight range between two vessels with VHF antennas at 65' above the water....

With a transmit power of 25 watts (+44dbm), minus the coaxial feedline loss (3-4db typical), plus the antenna gain (3-4dbi typical), each of yours' signal at the other vessel should be very strong, at approx. -63.6dbm.....which is about 50db above the typical marine VHF's receiver sensitivity for "20-db of quieting" (20db SINAD) and approx. 55db above the typical marine VHF's 12db SINAD ("12db of quieting") spec....(and 44db above the typical AIS unit's receive spec for low Bit-Error-Rate BER, but remember a Class A AIS is only 12.5watts / +41dbm....and Class B AIS is only 2watts / +33dbm...so AIS has a bit less margin than the 25-watt Marine-VHF-FM-Voice or Marine-VHF-DSC...)

So, as you see, there is plenty of margin built into the design of the Marine VHF system....which is good, 'cause you need to account for lots of factors other than mother nature, such as vessel heel angle (and subsequent negative gain of the antenna), antenna whipping around from both wind and sea-motion, lossy cable and connectors, poor radio operators that don't speak clearly / loudly enough (as well as those who shout and over-deviate), poor receiver, extraneous noise / RFI on-board, etc. etc....
{But, in a more controlled environ, if you could somehow get your 25-watt Marine VHF signal to actually travel a couple thousand miles, it would still be receivable....this is proven by hams in California and Hawaii when a tropo duct opens, and allows 144mhz ham communications to/from these locales (even some VHF-FM can be heard)....but, I'm getting WAY ahead of myself here!!}




2) Beyond "line-of-sight"?? How is that possible??

Is this something exotic?? Actually, NO....No, it is NOT exotic at all, to the contrary it is normal and common....and is actually VERY reliable and VERY predictable (out to the distance where path loss exceeds the available transmit powers and antenna gains).....And, this is called "tropo-scatter"
"Tropo-scatter" might sound exotic and rare, but it is anything but!!It is simply how almost all VHF and UHF signals (radio, TV, data, etc.) travel beyond line-of-sight..."Tropo-scatter" is simply the spreading of the radiowaves in the troposphere, caused by the natural irregularities in our atmosphere...

The troposphere extends from approx. 1000' - 3000' above sea level up to approx 35,000' above sea level....with most of our "scattering" occurring between 10,000' and 30,000' (although a "tropo-duct" is usually very narrow and defined, "tropo-scatter" uses most of the troposphere, especially when using wide antenna beamwidths, as we do, on our boats)...
The atmosphere above the troposphere is fairly stable, has little humidity, and has a fairly even temperature cline, hence little irregularities to "scatter" VHF signals...


In addition to radio and TV broadcasting, VHF and UHF "tropo-scatter" has been in regular use by both military and commercial users for 60 some years now....and while some of the legacy commercial tropo-scatter communications links have been replaced by newer satellite data links, there are still many in operation.....not to mention the 10's of 1000's of ham radio operators that use "tropo-scatter" daily (some unknowingly, of course)...and if you've read about "over-the-horizon-radar" (whether military surveillance, or for weather), well this is "tropo-scatter-radar" (primarily UHF, but some VHF as well)....
FYI, the first reported use of VHF "tropo-scatter" was way back in 1928, by Guglielmo Marconi, using a 30mhz system, for a link between Sardinia and the Italian mainland!!! (but, the elegance of this mode of propagation wasn't examined much until the early/mid 1930's...)


Let me be perfectly clear here:


VHF (and UHF) "Tropo-scatter" is a 100% / always-there, communications mode/path, and is NOT any special mode, does NOT rely on any special conditions to exist at all....It is simply the way that VHF, UHF, and SHF signals propagate beyond the direct-wave (beyond line-of-sight)....
For these reasons, most laypersons will erroneously call this "ground-wave" (which it is NOT at all like, as VHF and UHF signals have no real "ground-wave" at all!!), but "tropo-scatter" is simply the way VHF, UHF, and SHF signals propagate beyond line-of-sight...and since the phrase "tropo" is in there, some will think that this is relying on some atmospheric enhancement and therefore could not be thought of as reliable, but to the contrary, it does not rely on any special atmospheric conditions, and hence is 100% reliable (up to the distance where the received signal no longer is useful, when the transmit powers and antenna gains, fail to over-come the "tropo-scatter path loss")

Wide-bandwidth modes such as FM Voice are not the usual means of utilizing "tropo-scatter", due to their inherent noise-limited receivers and need to attain threshold in order to have any real useful signal-noise performance....but, if significant transmit power and high-gain antennas are employed, wide-bandwidth "tropo-scatter" can accomplish many wonderful things (such as provide multiple voice circuits and multiple high-speed data links to/from North Sea oil rigs and the mainland, etc.)...

With our limited transmit power (25 watts), and low-gain (dipole) antennas, our wide-bandwidth Marine VHF, is rather limited in its "tropo-scatter" performance...
(typically about 50nm at best...of course slightly longer ranges are possible with antennas mounted up high, such as tall towers of USCG stations or NOAA weather broadcasts, etc.)

According to some old references, VHF "tropo-scatter" path loss at the 55nm (100km) range is approx. 168db....which would not allow most Marine VHF communications, +44dbm - 168db = -124dbm, below the typical -117dbm for 12db SINAD....which means no communications...
But, over sea water, there are times when this path is usable, although it is NOT reliable, which is why the 30nm - 35nm range is typically thought of as the "maximum" normal / reliable range...



3) "Tropo" / "Tropo-enhancement" (also sometimes referred to as "non-ducting-tropo")
This is the most common means of "enhanced" VHF (and UHF/SHF) communications....and is noticed by many mariners along coastal areas, especially in summertimes....
The technical reason for these fairly common, springtime and summertime coastal "tropo-enhancements" are due to daily temperature inversions in the troposphere.....where instead of the air temp descending as altitude increases, there is a slight rise in temperature at a higher altitude in the troposphere...
"Tropo-enhancement" (especially along the edge of hi-pressure areas, as well in coastal areas, or along island chains / archipelagos) typically covers rather broad areas, and can make signals from a few hundred miles away very strong....although there is typically also some fading, and while these signals are fairly strong, they do fall off in strength at typical inverse-square rates, meaning unlike in a "tropo-duct" these "tropo-enhanced" signals from 400 - 500 miles will be weaker than those 200-250 miles distant...
These are common in areas of high-pressure, especially in humid air, along the edge of a high-pressure area, etc.

Alhough, "tropo-enhancement" can effect signals from great distances and allow for VHF
communications farther than a few hundred miles, it is rare that these conditions extend more than 500 - 750nm, and when it does, it is using the highest part of the troposphere...and, with our 25 watt FM radios, it is unlikely that most will experience "tropo-enhancement" farther than that....
And, for us using Marine VHF, typical "tropo-enhancement" will show good signals from 100nm out to 300-400nm.....and rare instances out to 500nm - 750nm....




4) "Tropo-Ducting":
If you are communicating on VHF Marine radio farther than 500nm - 750nm, then you're likely using a "tropo-duct", but again except for some specific locales (such as Portugal / Morocco to Canaries, S. FL to Long Island, California to Hawaii, etc.) a "tropo-duct" is rare, and is also usually geographically fairly narrow....meaning that the distance covered might be quite long (1000 - 2000 miles), but the "duct" is typically fairly narrow (< 100 miles wide, and many times <50 miles wide...), and almost always over water, in areas of stable hi-pressure, but unlike "tropo-enhancement", a "tropo-duct" always uses the very lowest part of the troposphere (typically only 1000' - 3000' above sea level)....just like its name implies, it acts like a "duct", long and narrow, and close to the ground...

Typical "tropo-ducting" provides very strong distant signals, and many times these distant signals are as strong (or sometimes even stronger) than those in your local area...and a sure sign that you are working stations in a "tropo-duct" is that stations along the duct are all about the same signal strength, rather than the more distant stations being weaker....if you are working a station > 500nm away, and they are as strong (or stronger) than other stations along the same path that are only 100nm away, then you are working in a duct....
BUT...
But, as I write above, except for some specific geographical areas, "tropo-ducting" is rare!!!
So, most "beyond tropo-scatter" range VHF communications is via "tropo-enhancement", not tropo-ducting"...




5) A while back, Jim ("continuouswave") posted some great info about Marine VHF radio communications....lots of math and details....but a great job!!
Those interested in more math, have a look...
http://continuouswave.com/whaler/reference/VHF.html



6) Some links to calculators..

https://www.easycalculation.com/physics/electromagnetism/vhf-uhf-distance.php

http://www.qsl.net/w4sat/horizon.htm

http://www.furunousa.com/LearningCenter/Radar-Horizon-Calculator.aspx


http://www.qsl.net/pa2ohh/jsffield.htm

http://wa8lmf.net/miscinfo/dBm-to-Microvolts.pdf

http://www.pasternack.com/t-calculator-fspl.aspx

http://www.radio-electronics.com/info/propagation/path-loss/free-space-formula-equation.php



Oh, if you're looking for some microwave tropo-scatter systems, General Dynamics will sell you some, if you got the $$$$$$$$$
http://www.gdsatcom.com/troposcatter.php





Well, there is a LOT more to all of this, but as I've been studying this for about 45 years now (and teaching it for almost that long as well), there isn't too much more to post here that most of you will find helpful...
So, fair winds to you all..

John

[MYTraveler]

Originally Posted by ka4wja

2) Beyond "line-of-sight"?? How is that possible??

Is this something exotic?? Actually, NO....No, it is NOT exotic at all, to the contrary it is normal and common....and is actually VERY reliable and VERY predictable (out to the distance where path loss exceeds the available transmit powers and antenna gains).....And, this is called "tropo-scatter"




VHF (and UHF) "Tropo-scatter" is a 100% / always-there, communications mode/path, and is NOT any special mode, does NOT rely on any special conditions to exist at all....It is simply the way that VHF, UHF, and SHF signals propagate beyond the direct-wave (beyond line-of-sight)....
With our limited transmit power (25 watts), and low-gain (dipole) antennas, our wide-bandwidth Marine VHF, is rather limited in its "tropo-scatter" performance...
(typically about 50nm at best...of course slightly longer ranges are possible with antennas mounted up high, such as tall towers of USCG stations or NOAA weather broadcasts, etc.)

According to some old references, VHF "tropo-scatter" path loss at the 55nm (100km) range is approx. 168db....which would not allow most Marine VHF communications, +44dbm - 168db = -124dbm, below the typical -117dbm for 12db SINAD....which means no communications...

Thanks for that clarification -- I feel at least partially vindicated in my claims that I routinely experience good VHF comms way beyond LOS and out to 80 nm. I am wondering whether tropo-scatter could account for those distances, given that my primary antenna is 9db (i?) or maybe 12 dbi, and is 40' or so above the surface?

[ka4wja]

MYTravler,
If you're referring to communications to/from other similarly equipped vessels, or even to/from a sailboat with a masthead antenna, then the answer is...
No, 80nm is not "tropo-scatter", but rather is common "tropo" / "tropo-enhancement"....

Originally Posted by MYTraveler

Thanks for that clarification -- I feel at least partially vindicated in my claims that I routinely experience good VHF comms way beyond LOS and out to 80 nm. I am wondering whether tropo-scatter could account for those distances, given that my primary antenna is 9db (i?) or maybe 12 dbi, and is 40' or so above the surface?

As the tropo-scatter path loss at about 80nm is about 175db, this is beyond the capability of our VHF Marine systems, even with an extra 6db of antenna gain at each end.....
Also, "tropo-scatter" is always there, 100% of the time...not just "routinely", but all the time 24/7/365, rain or shine....

What happens in many of these "enhanced local coverage" situations is, ironically, what we usually refer to as "local enhancement" (technically a "local tropo enhancement"), which is a rather common enhancement (albeit a slight enhancement) in your area, which does not dramatically increase VHF communications range, but rather simply "enhances" your typical range...
Note that while you may see this in your area, the "areas" that are effected can be quite large....meaning that in warm moist coastal areas, this "local enhancement" is quite common...

So, what you are routinely seeing is most likely this "local enhancement" which is a variation / type of the common "tropo-enhancement" (commonly just called "tropo", and sometimes referred to as "non-ducting-tropo", especially by European hams / RF engineers...)....

The bottom line here, that the Marine VHF system is well designed, with plenty of margin, for excellent, reliable communications in the normal line-of-sight range up to approx. 20 miles.....and also allows reliable communications with limited margins, typically out to 30 - 35 miles (depending on antenna heights, etc.), but for the average cruising boat, this is about the maximum practical designed (and reliable) range....even though the true maximum tropo-scatter range can be up to 50nm or so (again, depending on antenna height, etc.), the signal quality at these ranges is poor, and most would not consider this to be to "usable"...

All cruising boat-to-cruising boat VHF Marine comms, beyond the typical 35nm - 50nm, is via some form of special atmospheric conditions (such as the common "tropo-enhancement"/"tropo", or the rarer "tropo-ducting")...



I hope this helps clarify things...
Fair winds...

John

[cptratbag]

Don't forget to consider interference as a range-limiting factor for both voice and digital transmissions.


Art

[ka4wja]

Art,
While the primary purpose of my posting was to emphasize radiowave propagation (so I didn't delve into noise/interference), perhaps I should make some mention of it???

So, good that you mentioned interference....

Originally Posted by cptratbag

Don't forget to consider interference as a range-limiting factor for both voice and digital transmissions.

1) Remember that we are talking VHF FM signals here, whether analog voice or digital data, they are FM....and on VHF freqs...

2) At VHF, this means that there is little natural atmospheric noise to consider, and that these rather wideband FM systems are generally noise-bandwidth-limited / thermal-noise-limited systems....

So, we don't have any natural noise / interference to effect things...


3) And, since they are FM, and most man-made noise is AM, as long as the signals are above the receiver's threshold (where the limiters work effectively), there is virtually no effect on the FM signal from most man-made noise...

Although, once signals are weak (such as at the fringes of tropo-scatter range), man-made noise can be a factor...and having your own boat clear of any man-made RFI that radiates thru the VHF region (which thank goodness is rare), is a factor in determining your typical maximum range...


4) Although rare, unfortunately there can be some wideband RFI / Noise that can raise the receive noise floor, and can effect your max receive range....

A simple way to test for this is:
a) turn on ONLY your VHF radio...
b) set the squelch at precisely the noise threshold (at the point where the squelch engages, and the VHF receiver is quiet), but no higher...
c) then turn on all your other systems / equipment on-board...
d) if the VHF remains quiet (squelch does not open up), you are NOT receiving any extraneous noise/interference, from your own boat...

And, to see if you are receiving extraneous noise/interference from elsewhere, you can do the same tests, with the antenna connected and disconnected...
a) turn on ONLY your VHF radio...
b) unplug the antenna cable from your VHF radio, and set the squelch at precisely the noise threshold (at the point where the squelch engages, and the VHF receiver is quiet), but no higher...
c) then plug the antenna in, if the VHF remains quiet (squelch does not open up), you are NOT receiving any extraneous noise/interference...
{then you can do these tests (antenna connected/disconnected), with all on-board systems on and off....to verify if there is some tiny amount of interference, or not...}


Remember, assuming that you do not have any interference issues, the "noise" that you hear from the VHF radio's speaker when you turn the squelch down, is noise generated INSIDE the radio, and is not atmospheric-noise, nor man-made interference...
And, remember that this opposite from the noise / interference that we hear on the HF freqs (where amplitude modulation, narrower bandwidths, and lower thermal noise receivers, are used)...


5) Of course, there can be "co-channel" interference....which is simply other users of that channel interfering with your communication....

Although sometimes that can be because they are talking to another vessel that is very close to them, and they either do not hear you at all or not very well, or just assume since your signal is weak, that they are not causing you any interference....
In any case, there is little you can do about it....so best to move to another channel...


6) Further, SOME VHF radios can suffer from "adjacent-channel" interference and/or "intermodulation interference"....
These are almost always in crowded harbors, with many radio users on-the-air (as well as other VHF systems on-the-air), and also almost always with the lower-quality / lower-priced VHF radios...


But, luckily these days there is less and less "VHF" systems on-the-air, as pagers are almost all UHF now-adays, and most public service (police, fire, ambulance, border patrol, etc. etc. etc. etc.) are also mostly UHF (and most are also now digital).....so "intermodulation interference" is less seen, and getting rare these days...




So, that's about it for "interference" at VHF and on FM...
I hope this helps...


Fair winds..

John

[cptratbag]

My LED anchor light creates quite a bit of broadband noise on VHF. Of course, proximity to the antenna doesn't help.

We had a big problem with phase distortion on Lake Michigan. The USCG ran simulcast transmitters at times, and in the fringe areas the broadcasts were often unreadable.

This has improved somewhat (at least in my area) since Rescue 21 was implemented.

Art

[continuouswave]

Originally Posted by ka4wja

5) A while back, Jim ("continuouswave") posted some great info about Marine VHF radio communications....lots of math and details....but a great job!!
Those interested in more math, have a look...
http://continuouswave.com/whaler/reference/VHF.html

Thank you for the endorsement. That article first appeared in 2005.

The reason there is "lots of math and details": I try to explain the problem in very simple terms and only take one step at a time. Most of the math is just addition or subtraction.

[DefinitelyMe]

Cheers guys. I'm ashamed to say that radio wave propagation is not a subject i've really devoted much time to - i've just taken it as it comes - but i've learned a great deal from your posts. Really well-written. Thanks again!

[ka4wja]

While I'd love it if this was a "sticky", I realize that this is a dry topic for many....so, you are all very welcome!!!

Fair winds...

John

[Cadence]

Originally Posted by ka4wja

Hello to all,

{Those looking for the basic facts, in a nutshell....

With masthead-mounted antennas, most sailboat-to-sailboat VHF communications is within the typical line-of-sight range of 15nm - 20nm....and once beyond that, the typical "normal communications range" will be approx. 30nm - 35nm (depending on radios, antennas, cable, microphones, operator, etc.) and a typical MAXIMUM range (without any special atmospheric enhancement) is going to be approx. 50nm....

Sailboat-to-sailboat VHF Marine communications beyond 50nm (and typically much beyond the 30nm - 35nm range) ALWAYS uses some form of "enhanced" propagation....almost all times this is "tropo" or "tropo-enhancement" (which can propagate signals hundreds of nautical miles, over quite large geographic areas / regions, typically along the edge of a hi-pressure area, and coastal areas, especially in summertime...)
Except in certain specific geographical areas, actual "tropo-ducting" is rare, and is typically restricted to specific narrow paths, and most times the distant signals (from as much as 1000 - 1500nm away or more) are as stronger as those 200 - 300nm away....}


For those who wish the details and explanations, please read on...

VHF Radiowaves propagate in many different ways....and, while we could get bogged-down in the minutia, that is not my intent here!!

Rather, I will just attempt to inform you all of the main / typical means of VHF communications propagation...."line-of-sight" ("direct-wave") and "tropo-scatter", being the dominant VHF Marine propagation modes) and how those methods work...
As well as describe "tropo-enhancement" (fairly common along the edge of high pressure areas, and in summertime coastal areas, and many times misunderstood and erroneously referred to as "ducting"), and the much more rare "tropo-ducting" (which is actually very rare, except in certain geographic areas, during certain times of the year...)

I will NOT delve into the esoteric / exotic modes of "moonbounce", "meteor-scatter", "aurora", etc., nor the extremely rare Trans-Equtaorial F2, FAI, or even the slightly less rare sporadic-E....none of which is typically possible with wide-bandwidth modes such as FM-Voice or wide-band FM-data, and certainly none are possible with the limited 25-watt transmitter power and simple omni-directional vertical antennas we use on marine VHF....
So, no worries about all of that...
{BTW, I have personally used "moonbounce", "meteor-scatter", "aurora", and Es (sporadic-E), all on VHF (144mhz ham radio)...}


1) Most VHF Marine Communications, whether VHF-FM-Voice, VHF-FM-DSC, or VHF-AIS-data, is accomplished by simple "line-of-sight" mode (technically referred to as a "direct-wave"), which is slightly (approx. 6%) beyond the actual visual line-of-sight....This "line-of-sight" range is very easily calculated and unless there are obstructions between your vessel and another (land masses, etc.) it is very reliable and repeatable....and 100% reliable...

With our 25 watt transmitters and fairly sensitive receivers (typically -117dbm for 12db SINAD, -110 to -113dbm for 20db SINAD, which is a fairly "noise-free" signal....and -107 to -109dbm for full noise-free signals and low BER for data), our line-of-sight ranges are not only 100% reliable but also incorporate significant margins...(typically 50db above the "20db SINAD" spec, at 15nm - 20nm range, with two vessels w/ antennas at 65' above the water...)


For land-based, terrestrial VHF communications, I've gotten used to the VERY simple and easy-to-calculate formula here, which can be easily done in your head in seconds....but this gives results in statute miles, so you'd either need to convert that to nautical miles (divide by 1.15) or simply accept the result in statute miles...
For statute miles.....
VHF Radio line-of-sight distance (in statute miles) = √2 x Af (where Af is the height, in feet, of Antenna)
Line-of-sight communications range between two VHF radio stations (in statute miles) = √ (2x Af1) + √ (2x Af2)

But, for nautical miles, it's only slightly harder to do in your head....
For nautical miles......
VHF Radio line-of-sight distance in nautical miles = 1.23 x √ Af (where Af is the height, in feet, of Antenna)

Line-of-sight communications range between two VHF radio stations (in nautical miles) = 1.23 x √ Af1 + 1.23 x √ Af2




With a typical sailboat-to-sailboat communications path, with mast heights of approx. 50' above the water, you have a "line-of-sight" distance of about 20 statute miles / 17.4nm....If both vessels had mast heights of 65', the line-of-sight range is then 22.8 statute miles / 19.9nm...
If one vessel was a large container ship, with its VHF antennas mounted 190' (max air-draft of a "PANAMAX" ship) above the water, the line-of-sight range to your sailboat would be about 27nm (31 statute miles)...
SO...So, we can now understand that all Marine VHF communications (whether voice, DSC or AIS), that is beyond the "line-of-sight" range must use some other form of propagation....


Regarding reliability and margins above receiver thresholds....
FYI, the Free Space Path Loss at 156.8mhz being 107.6db at 20nm (22.8 statute miles / 36.5km), which is the approx. line-of-sight range between two vessels with VHF antennas at 65' above the water....

With a transmit power of 25 watts (+44dbm), minus the coaxial feedline loss (3-4db typical), plus the antenna gain (3-4dbi typical), each of yours' signal at the other vessel should be very strong, at approx. -63.6dbm.....which is about 50db above the typical marine VHF's receiver sensitivity for "20-db of quieting" (20db SINAD) and approx. 55db above the typical marine VHF's 12db SINAD ("12db of quieting") spec....(and 44db above the typical AIS unit's receive spec for low Bit-Error-Rate BER, but remember a Class A AIS is only 12.5watts / +41dbm....and Class B AIS is only 2watts / +33dbm...so AIS has a bit less margin than the 25-watt Marine-VHF-FM-Voice or Marine-VHF-DSC...)

So, as you see, there is plenty of margin built into the design of the Marine VHF system....which is good, 'cause you need to account for lots of factors other than mother nature, such as vessel heel angle (and subsequent negative gain of the antenna), antenna whipping around from both wind and sea-motion, lossy cable and connectors, poor radio operators that don't speak clearly / loudly enough (as well as those who shout and over-deviate), poor receiver, extraneous noise / RFI on-board, etc. etc....
{But, in a more controlled environ, if you could somehow get your 25-watt Marine VHF signal to actually travel a couple thousand miles, it would still be receivable....this is proven by hams in California and Hawaii when a tropo duct opens, and allows 144mhz ham communications to/from these locales (even some VHF-FM can be heard)....but, I'm getting WAY ahead of myself here!!}




2) Beyond "line-of-sight"?? How is that possible??

Is this something exotic?? Actually, NO....No, it is NOT exotic at all, to the contrary it is normal and common....and is actually VERY reliable and VERY predictable (out to the distance where path loss exceeds the available transmit powers and antenna gains).....And, this is called "tropo-scatter"
"Tropo-scatter" might sound exotic and rare, but it is anything but!!It is simply how almost all VHF and UHF signals (radio, TV, data, etc.) travel beyond line-of-sight..."Tropo-scatter" is simply the spreading of the radiowaves in the troposphere, caused by the natural irregularities in our atmosphere...

The troposphere extends from approx. 1000' - 3000' above sea level up to approx 35,000' above sea level....with most of our "scattering" occurring between 10,000' and 30,000' (although a "tropo-duct" is usually very narrow and defined, "tropo-scatter" uses most of the troposphere, especially when using wide antenna beamwidths, as we do, on our boats)...
The atmosphere above the troposphere is fairly stable, has little humidity, and has a fairly even temperature cline, hence little irregularities to "scatter" VHF signals...


In addition to radio and TV broadcasting, VHF and UHF "tropo-scatter" has been in regular use by both military and commercial users for 60 some years now....and while some of the legacy commercial tropo-scatter communications links have been replaced by newer satellite data links, there are still many in operation.....not to mention the 10's of 1000's of ham radio operators that use "tropo-scatter" daily (some unknowingly, of course)...and if you've read about "over-the-horizon-radar" (whether military surveillance, or for weather), well this is "tropo-scatter-radar" (primarily UHF, but some VHF as well)....
FYI, the first reported use of VHF "tropo-scatter" was way back in 1928, by Guglielmo Marconi, using a 30mhz system, for a link between Sardinia and the Italian mainland!!! (but, the elegance of this mode of propagation wasn't examined much until the early/mid 1930's...)


Let me be perfectly clear here:


VHF (and UHF) "Tropo-scatter" is a 100% / always-there, communications mode/path, and is NOT any special mode, does NOT rely on any special conditions to exist at all....It is simply the way that VHF, UHF, and SHF signals propagate beyond the direct-wave (beyond line-of-sight)....
For these reasons, most laypersons will erroneously call this "ground-wave" (which it is NOT at all like, as VHF and UHF signals have no real "ground-wave" at all!!), but "tropo-scatter" is simply the way VHF, UHF, and SHF signals propagate beyond line-of-sight...and since the phrase "tropo" is in there, some will think that this is relying on some atmospheric enhancement and therefore could not be thought of as reliable, but to the contrary, it does not rely on any special atmospheric conditions, and hence is 100% reliable (up to the distance where the received signal no longer is useful, when the transmit powers and antenna gains, fail to over-come the "tropo-scatter path loss")

Wide-bandwidth modes such as FM Voice are not the usual means of utilizing "tropo-scatter", due to their inherent noise-limited receivers and need to attain threshold in order to have any real useful signal-noise performance....but, if significant transmit power and high-gain antennas are employed, wide-bandwidth "tropo-scatter" can accomplish many wonderful things (such as provide multiple voice circuits and multiple high-speed data links to/from North Sea oil rigs and the mainland, etc.)...

With our limited transmit power (25 watts), and low-gain (dipole) antennas, our wide-bandwidth Marine VHF, is rather limited in its "tropo-scatter" performance...
(typically about 50nm at best...of course slightly longer ranges are possible with antennas mounted up high, such as tall towers of USCG stations or NOAA weather broadcasts, etc.)

According to some old references, VHF "tropo-scatter" path loss at the 55nm (100km) range is approx. 168db....which would not allow most Marine VHF communications, +44dbm - 168db = -124dbm, below the typical -117dbm for 12db SINAD....which means no communications...
But, over sea water, there are times when this path is usable, although it is NOT reliable, which is why the 30nm - 35nm range is typically thought of as the "maximum" normal / reliable range...



3) "Tropo" / "Tropo-enhancement" (also sometimes referred to as "non-ducting-tropo")
This is the most common means of "enhanced" VHF (and UHF/SHF) communications....and is noticed by many mariners along coastal areas, especially in summertimes....
The technical reason for these fairly common, springtime and summertime coastal "tropo-enhancements" are due to daily temperature inversions in the troposphere.....where instead of the air temp descending as altitude increases, there is a slight rise in temperature at a higher altitude in the troposphere...
"Tropo-enhancement" (especially along the edge of hi-pressure areas, as well in coastal areas, or along island chains / archipelagos) typically covers rather broad areas, and can make signals from a few hundred miles away very strong....although there is typically also some fading, and while these signals are fairly strong, they do fall off in strength at typical inverse-square rates, meaning unlike in a "tropo-duct" these "tropo-enhanced" signals from 400 - 500 miles will be weaker than those 200-250 miles distant...
These are common in areas of high-pressure, especially in humid air, along the edge of a high-pressure area, etc.

Alhough, "tropo-enhancement" can effect signals from great distances and allow for VHF
communications farther than a few hundred miles, it is rare that these conditions extend more than 500 - 750nm, and when it does, it is using the highest part of the troposphere...and, with our 25 watt FM radios, it is unlikely that most will experience "tropo-enhancement" farther than that....
And, for us using Marine VHF, typical "tropo-enhancement" will show good signals from 100nm out to 300-400nm.....and rare instances out to 500nm - 750nm....




4) "Tropo-Ducting":
If you are communicating on VHF Marine radio farther than 500nm - 750nm, then you're likely using a "tropo-duct", but again except for some specific locales (such as Portugal / Morocco to Canaries, S. FL to Long Island, California to Hawaii, etc.) a "tropo-duct" is rare, and is also usually geographically fairly narrow....meaning that the distance covered might be quite long (1000 - 2000 miles), but the "duct" is typically fairly narrow (< 100 miles wide, and many times <50 miles wide...), and almost always over water, in areas of stable hi-pressure, but unlike "tropo-enhancement", a "tropo-duct" always uses the very lowest part of the troposphere (typically only 1000' - 3000' above sea level)....just like its name implies, it acts like a "duct", long and narrow, and close to the ground...

Typical "tropo-ducting" provides very strong distant signals, and many times these distant signals are as strong (or sometimes even stronger) than those in your local area...and a sure sign that you are working stations in a "tropo-duct" is that stations along the duct are all about the same signal strength, rather than the more distant stations being weaker....if you are working a station > 500nm away, and they are as strong (or stronger) than other stations along the same path that are only 100nm away, then you are working in a duct....
BUT...
But, as I write above, except for some specific geographical areas, "tropo-ducting" is rare!!!
So, most "beyond tropo-scatter" range VHF communications is via "tropo-enhancement", not tropo-ducting"...




5) A while back, Jim ("continuouswave") posted some great info about Marine VHF radio communications....lots of math and details....but a great job!!
Those interested in more math, have a look...
http://continuouswave.com/whaler/reference/VHF.html



6) Some links to calculators..

https://www.easycalculation.com/physics/electromagnetism/vhf-uhf-distance.php

http://www.qsl.net/w4sat/horizon.htm

http://www.furunousa.com/LearningCenter/Radar-Horizon-Calculator.aspx


http://www.qsl.net/pa2ohh/jsffield.htm

http://wa8lmf.net/miscinfo/dBm-to-Microvolts.pdf

http://www.pasternack.com/t-calculator-fspl.aspx

http://www.radio-electronics.com/info/propagation/path-loss/free-space-formula-equation.php



Oh, if you're looking for some microwave tropo-scatter systems, General Dynamics will sell you some, if you got the $$$$$$$$$
http://www.gdsatcom.com/troposcatter.php





Well, there is a LOT more to all of this, but as I've been studying this for about 45 years now (and teaching it for almost that long as well), there isn't too much more to post here that most of you will find helpful...
So, fair winds to you all..

John

You lost me past line of sight and skip. Skip is nice for DXing not worth a damn for reliability. JMHO

[ka4wja]

Cadence,
You lost me, when you mention "skip"...
'Cause, I don't remember mentioning / discussing this at all!

Originally Posted by Cadence

You lost me past line of sight and skip. Skip is nice for DXing not worth a damn for reliability. JMHO

I do know that most sailors will never care about this topic and few will ever need to understand this....so no worries!!

But, if you do wish to grasp it...
The facts are quite clear (and have been for > 60 years), that "tropo-scatter" is a regular / reliable / repeatable mode of beyond-line-of-sight vhf comms...(see details in my original post, above)


"Skip" is a slang/generic term that refers to ionospheric paths...and yes, you are correct that on VHF, this is not reliable...
But again, this is not what I'm talking about, not at all!



I hope this helps clarify things?

fair winds...

John

[Cadence]

Originally Posted by ka4wja

Cadence,
You lost me, when you mention "skip"...
'Cause, I don't remember mentioning / discussing this at all!
I do know that most sailors will never care about this topic and few will ever need to understand this....so no worries!!

But, if you do wish to grasp it...
The facts are quite clear (and have been for > 60 years), that "tropo-scatter" is a regular / reliable / repeatable mode of beyond-line-of-sight vhf comms...(see details in my original post, above)


"Skip" is a slang/generic term that refers to ionospheric paths...and yes, you are correct that on VHF, this is not reliable...
But again, this is not what I'm talking about, not at all!



I hope this helps clarify things?

fair winds...

John

My error! I guess I mistook tropo-scatter for ionospheric paths. I never heard tropo-scatter if I am reading between the line correctly it is the lower atmosphere holding or bending the signal such that follows the curvature of the earth. I need it in simple terms.

[Eyeback]

So how do you explain my vhf conversation with the USCG 150 miles east of Cape Mays back in July? Ducting?


Sent from my iPad using Cruisers Sailing Forum

[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...

Originally Posted by Cadence

My error! I guess I mistook tropo-scatter for ionospheric paths. I never heard tropo-scatter if I am reading between the line correctly it is the lower atmosphere holding or bending the signal such that follows the curvature of the earth. I need it in simple terms.

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!

NO....No, it is NOT exotic at all, to the contrary it is normal and common....and is actually VERY reliable and VERY predictable (out to the distance where path loss exceeds the available transmit powers and antenna gains).....And, this is called "tropo-scatter"
"Tropo-scatter" might sound exotic and rare, but it is anything but!!It is simply how almost all VHF and UHF signals (radio, TV, data, etc.) travel beyond line-of-sight..."Tropo-scatter" is simply the spreading of the radiowaves in the troposphere, caused by the natural irregularities in our atmosphere...

The troposphere extends from approx. 1000' - 3000' above sea level up to approx 35,000' above sea level....with most of our "scattering" occurring between 10,000' and 30,000' (although a "tropo-duct" is usually very narrow and defined, "tropo-scatter" uses most of the troposphere, especially when using wide antenna beamwidths, as we do, on our boats)...
The atmosphere above the troposphere is fairly stable, has little humidity, and has a fairly even temperature cline, hence little irregularities to "scatter" VHF signals...


In addition to radio and TV broadcasting, VHF and UHF "tropo-scatter" has been in regular use by both military and commercial users for 60 some years now....and while some of the legacy commercial tropo-scatter communications links have been replaced by newer satellite data links, there are still many in operation.....not to mention the 10's of 1000's of ham radio operators that use "tropo-scatter" daily (some unknowingly, of course)...and if you've read about "over-the-horizon-radar" (whether military surveillance, or for weather), well this is "tropo-scatter-radar" (primarily UHF, but some VHF as well)....
FYI, the first reported use of VHF "tropo-scatter" was way back in 1928, by Guglielmo Marconi, using a 30mhz system, for a link between Sardinia and the Italian mainland!!! (but, the elegance of this mode of propagation wasn't examined much until the early/mid 1930's...)


Let me be perfectly clear here:


VHF (and UHF) "Tropo-scatter" is a 100% / always-there, communications mode/path, and is NOT any special mode, does NOT rely on any special conditions to exist at all....It is simply the way that VHF, UHF, and SHF signals propagate beyond the direct-wave (beyond line-of-sight)....

For these reasons, most laypersons will erroneously call this "ground-wave" (which it is NOT at all like, as VHF and UHF signals have no real "ground-wave" at all!!), but "tropo-scatter" is simply the way VHF, UHF, and SHF signals propagate beyond line-of-sight...and since the phrase "tropo" is in there, some will think that this is relying on some atmospheric enhancement and therefore could not be thought of as reliable, but to the contrary, it does not rely on any special atmospheric conditions, and hence is 100% reliable (up to the distance where the received signal no longer is useful, when the transmit powers and antenna gains, fail to over-come the "tropo-scatter path loss")

Wide-bandwidth modes such as FM Voice are not the usual means of utilizing "tropo-scatter", due to their inherent noise-limited receivers and need to attain threshold in order to have any real useful signal-noise performance....but, if significant transmit power and high-gain antennas are employed, wide-bandwidth "tropo-scatter" can accomplish many wonderful things (such as provide multiple voice circuits and multiple high-speed data links to/from North Sea oil rigs and the mainland, etc.)...

With our limited transmit power (25 watts), and low-gain (dipole) antennas, our wide-bandwidth Marine VHF, is rather limited in its "tropo-scatter" performance...
(typically about 50nm at best...of course slightly longer ranges are possible with antennas mounted up high, such as tall towers of USCG stations or NOAA weather broadcasts, etc.)

According to some old references, VHF "tropo-scatter" path loss at the 55nm (100km) range is approx. 168db....which would not allow most Marine VHF communications, +44dbm - 168db = -124dbm, below the typical -117dbm for 12db SINAD....which means no communications...
But, over sea water, there are times when this path is usable, although it is NOT reliable, which is why the 30nm - 35nm range is typically thought of as the "maximum" normal / reliable range...

I do hope this explains things???

fair winds..

John

[ka4wja]

Eyeback,
The complete answer is right there in my original post!
Please have a look!

Originally Posted by Eyeback

So how do you explain my vhf conversation with the USCG 150 miles east of Cape Mays back in July? Ducting?

But, in brief....the answer is No...
No, not ducting....but simply "tropo-enhancement" (sometimes just referred to as "tropo")...

I'm afraid the words "duct" and "ducting" are woefully misunderstood....except for certain well known areas / regions, the actual occurrences of "ducts" are rare


I never intended to write a treatise on radiowave propagation, but rather just explain how VHF signals travel and how we sailors typically use VHF...
But, if you wish more info, it is all right there in my original post...

Fair winds..

John