The VHF radio horizon calculation is wrong

[Jammer]

Most sailboat antennas are either end-fed half waves or sleeve dipoles that are around 2.1 dBi. There are fiberglass antennas suitable for use on the mast that are 5.1 dBi. These are around 8' long. These are rarely used, in part because the perception is that it doesn't matter because the "radio horizon" limits distance and so people are unwilling to accept the windage and (at least in some cases) the bridge clearance.


Performance over salt water will be somewhat better but 10 dBi is probably a stretch.


I'm not sure how well Longley-Rice does with propagation over water.

[Cadence]

I believe there is a misconception. VHF is a frequency range used by marine radio as FM and can also be used by AM and SSB and TV for that matter. I think marine FM just got somewhat misnamed. I don't believe the people getting skips of thousands of miles are doing it with FM.

[Brian.D]

Actually, the modulation type does not determine if it can skip or not. It is the frequency itself and the atmospheric conditions in which the phenomenon occurs. FM and TV have been known to be heard/viewed up to 2000 miles away.

During the 1st Persian Gulf War, there were reports of hearing NY Taxi drivers on radios in Saudi. I personally never heard one, but I never doubted my communicators. Being a Ham then meant I could believe it was possible.

[Cadence]

Originally Posted by Brian.D

Actually, the modulation type does not determine if it can skip or not. It is the frequency itself and the atmospheric conditions in which the phenomenon occurs. FM and TV have been known to be heard/viewed up to 2000 miles away.

During the 1st Persian Gulf War, there were reports of hearing NY Taxi drivers on radios in Saudi. I personally never heard one, but I never doubted my communicators. Being a Ham then meant I could believe it was possible.

I'd bet the cabbies were Am as was TV. Days of yore.

[Lodesman]

Originally Posted by GrowleyMonster

That sounda about right for two boats with good VHF installations at full power and normal conditions. With shorter masts In real world conditions and typical installations, one learns to expect and get used to about half that.

Normal conditions???
Sure it's possible to get that range, but I would say that it would be atypical. It was rare IME to get better than 30nm with 150 ft antenna heights.

[StuM]

Originally Posted by Cadence

I'd bet the cabbies were Am as was TV. Days of yore.

We used to listen to TV stations on our AN/PRC-77's (military VHF FM) radios back in the '70s. They didn't work with AM signals.

[Brian.D]

It is all good. Soon we will be using Sub-Space Communications. So no worries.

[continuouswave]

Originally Posted by Cadence

I'd bet the cabbies were Am as was TV. Days of yore.

Television audio in c.1990 was transmitted with frequency modulation or FM. Amplitude modulation or AM was not used in television for the aural transmitter.

The NTSC television visual transmitter used amplitude modulation, but it was a vestigial sideband method, not traditional AM modulation as used in AM Broadcast radio. If received on an AM radio it would just sound like a buzz.

Mobile radio in the USA as might be used in a taxi service has been using FM modulation since the 1960's.

The only radio service still using AM is Aviation.

The legend of receiving New York taxi cab radio transmission by military communication radios sounds a bit implausible.

[Cadence]

Originally Posted by continuouswave

Television audio in c.1990 was transmitted with frequency modulation or FM. Amplitude modulation or AM was not used in television for the aural transmitter.

The NTSC television visual transmitter used amplitude modulation, but it was a vestigial sideband method, not traditional AM modulation as used in AM Broadcast radio. If received on an AM radio it would just sound like a buzz.

Mobile radio in the USA as might be used in a taxi service has been using FM modulation since the 1960's.

The only radio service still using AM is Aviation.

The legend of receiving New York taxi cab radio transmission by military communication radios sounds a bit implausible.

Days of yore to me was the early 60s when I was a TV factory troubleshooter. I would call it a tube tapper since the handle of a screw drive was the best tool.

[ka4wja]

For those interested in all the facts and details (and the hows / whys) of VHF radiowave propagation...
Have a look at the thread on that topic:

VHF and AIS Radiowave Propagation and VHF and AIS Radio Range

https://www.cruisersforum.com/forums...ge-149499.html


Fair winds.

John

[Jammer]

John


I personally would find it helpful if you would state exactly what it is in this thread you do and do not agree with rather than posting, as you did on the other thread:

Originally Posted by ka4wja

With much of the recent discussion regarding "VHF radio horizon" / communications range, filled with misunderstandings and some wild misinformation, I thought maybe some here might actually enjoy the facts (and the hows / whys) surrounding VHF radiowave propagation...

[betwys1]

Originally Posted by RUSTYNAIL

Brian, That sounds like an incredible distance for a vhf radio. I remember repairing a VHF radio for Station Yerba Buena Island near San Francisco when I was in the Coast Guard many moons ago. The actual VHF radio and antenna was remotely mounted with a telephone line connection on the top of a high mountain called Mt. Diablo. Maybe the same situation in San Diego.

Hot Springs Mountain in San Diego County, rises to an elevation of 6,535 feet (1,992 m) -even so I would be looking for ducting, MeteorScat (??) or relay via satellite or aircraft.

[ka4wja]

Jammer,
I was trying to be polite and just suggest that everyone can find the facts in the other thread....

And, I'm not sure you're going to feel the same after reading it....but, you asked, so here 'ya go...

Originally Posted by Jammer

John

I personally would find it helpful if you would state exactly what it is in this thread you do and do not agree with rather than posting, as you did on the other thread:

Jammer, from what I've read of your postings, you're a good guy!
And, I assume you're sincere in your request of me, to lay out the specifics of what I agree and disagree with here...and, I'd like to do so...

But, I run into a few issues with that.

--- First off, I really don't want to come off as some arrogant know-it-all, that sticks his nose into others discussions to appear superior...(so, I thought a polite suggestion to look at what I spent quite some time putting together, years ago, would be helpful ---- without stirring up controversy?)

Please have a look....I still think it will be helpful to all...

VHF and AIS Radiowave Propagation and VHF and AIS Radio Range
https://www.cruisersforum.com/forums/f13/vhf-and-ais-radiowave-propagation-and-vhf-and-ais-radio-range-149499.html




--- Next, I quite frankly don't have the time to rewrite everything here....(I have elderly family with medical issues, that keeps me quite busy).....the reason I took the few minutes to post the suggestion to look at the other thread is because I received a direct message, referencing your thread, asking me for some info / clarification....and I just sent him a link to my earlier thread, and I thought others here might also desire the same assistance?



--- And, another reason I didn't want to get into a long-running debate here, is due to the fact that I would need to be blunt (and unfortunately might hurt some feelings), and say up front that:
a) I disagree with the premise / title of your thread ("The VHF radio horizon calculation is wrong"),

b) And, that the premise of the very first sentence of your thread is based on misinformation....("Many sources proclaim that VHF is a line-of-sight radio technology limited in distance by the curvature of the earth.")

The VHF "radio horizon" is what it is....and has been known/understood for decades before I was even born....(fyi, Marconi observed and developed tropo-scatter in 1928, and did the first long-range VHF tropo-scatter communications links, in the early 1930's, which were well beyond the radio horizon)....and for > 50 years now, we commonly use the "square-root of twice the height (in feet), is the VHF radio horizon in statute miles", as this is approx. 5% - 6% farther than the optical line-of-sight [fyi, I was taught this rule-of-thumb, by a Motorola 2-way radio engineer, in Ft. Lauderdale, FL, in the 1970's]....and for almost 100 years now, this has been understood to be the approx. limit of direct-wave (line-of-sight) comms on VHF circuits...but, certainly not the limit of VHF signals...

Much VHF communications is of course beyond line-of-sight communications, and hence the "radio horizon" is, of course, not the limit of VHF communications range...


When someone writes that "many sources proclaim that VHF is a line-of-sight radio technology limited in distance by the curvature of the earth.", I actually find that misinformative, somewhat misleading, certainly misunderstanding, etc....I mean, I mean any legit source / authority of radiowave propagation has known/understood otherwise for about 100 years now, so...

Yes, farther down in your post, you do point out the fallacy of this premise, and you provide links to some articles showing this....but, then you go back and write that the "radio horizon" concept is oversimplified and you make the assumption that everyone thinks this is the limit of VHF comms range?

When it is obvious to most this is not the case....So, I'm left wondering why you would post a thread titled that "The VHF radio horizon calculation is wrong", and start the thread discussion with a false premise ("many sources proclaim that VHF is a line-of-sight radio technology limited in distance by the curvature of the earth.")....maybe I'm just not getting the sarcasm??

The facts of VHF Radiowave propagation are out there (including right here on Cruiser's Forum), and I'm not aware of any reputable source that proclaims that VHF is exclusively "a line-of-sight radio technology"....(although, as I have stated for decades now, how most sailors actually use VHF comms, most of their VHF Marine communications is within radio line-of-sight, but that is not the limit....not even close!)....so, why start a thread that stirs up argument / misinformation?



--- As for more specifics that I disagree with.....
First, I give my kudos to Jim, "continuouswave", for having the patience to try to clarify/correct things here....but, I just don't have that much time....so, here are just some brief comments that are right-off-the-top-of-my-head, that stand out as misunderstandings / misinformation....

In this thread, we have:

- some that say that there are too many variables to ever predict a maximum range, when in reality it is actually an easy thing to calculate (calculate a 100% reliable maximum range), as this is what is done all the time in most tropo-scatter comms (whether VHF links to offshore oil platforms, or VHF / UHF Radio and TV broadcasting, etc., etc. etc...), this actually done all the time....

- and then we have some recounting their own experiences (which is fine, but unfortunately drifts the discussion further afield, and doesn't provide any understanding of the how's and why's)....

- and, we have many that write about "ducting", misunderstanding that what they actually experience is "tropo-enhancement" (or scientifically referred-to as "tropospheric super-refraction"), not "ducting"....

- and another drift in the discussion that mentions using different terms / figures / units, etc....which are valid criticisms of technical papers, but in this discussion/thread they are thread-drift.

- and then the discussion waddles thru the misunderstanding that mode of modulation has anything to do with propagation (it doesn't, of course)....

- and, others suggesting that ham operators know all of this, and/or recommending taking a ham radio class to learn all of this....well, yes this is well-meaning and actually not a bad idea....but....but, unfortunately, I've yet to find more than 1% of my fellow (USA-based) hams that have anything but cursory understanding of the basics of radiowave propagation, and even a smaller number that grasp the intricacies of all of this....(yes, this is an embarrassment, but a sad fact none-the-less)

- and, while some might consider a -130dbm signal usable (and, in a very narrowband system, such as a VHF-CW circuit, it is)....in a VHF-FM circuit, it's not....not to mention that "10dbi" antennas at 20m high, are not just not typical, but unlikely on our boats....and, while I freely admit that I dislike longly-rice modeling, it's not too far off actual measured field-strengths, but if you've got a VHF marine radio system on your mid-size cruising boat with a 100% reliable range of 55nm (with no atmospheric enhancements), please post the specifics, 'cuz I just don't see it possible with off-the-shelf radio/antenna? Of course, typical ranges can be this far and even farther, but I'm asking about 100% always there circuit, not enhanced, nor 50% predictability, etc...

- further, your inclusion of 25mhz and 28mhz into the discussion can cause significant confusion with those trying to figure out HF skywave propagation versus VHF tropospheric propagation....
But, fyi, you are correct that the high end of the HF spectrum can be used for tropo-scatter comms (Marconi started with 30mhz tropo-scatter, in 1928)...and I've actually done so on 28mhz from S. Florida, to central and north Florida....but....but HF antenna physical size necessary to achieve decent range is unpractical for most, and certainly much easier / cheaper to accomplish on VHF and UHF, over similar paths....fyi, remember that while path loss goes up with frequency, so does antenna gain assuming the same physical size antennas, and since receive G/T is based on antenna gain and system noise, you'll find that mid-to-upper VHF (and UHF) tropo-scatter to be easier to accomplish over the same path length, versus that of HF....




To be clear, I disagree with the posting of all of these above, of course some are not out-n-out falsehoods, but some/many are misleading and misinformative, and many are confusing to most sailors.....so these are some reasons to just pass 'em by and point to where some more informative info can be found....(which is what I did)
And, I'm afraid I just don't have the time to argue all of these points....and more importantly, most of those reading all of this (desiring the information) actually need/want a quick understanding of the basics....which I wrote in the first two paragraphs of my original thread, over 5 years ago, and requoted them in my most recent update:

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" [tropo-refraction/super-refraction] (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...

So, what am I to do?
I want to be of assistance to my fellow sailors, and point all of you to some factual information, without being an ass....and, without hurting anyone's feelings, etc...

I want to actually help, but have limited time (and currently no patience for argument)...
What is a fellow sailor to do?


Well, my solution was intended to be polite and simply point those wishing the facts to just read an old discussion here....and, now I'm starting to regret even trying to do even that....

But...
Please have a look anyway....I still think it will be helpful to all...

VHF and AIS Radiowave Propagation and VHF and AIS Radio Range
https://www.cruisersforum.com/forums/f13/vhf-and-ais-radiowave-propagation-and-vhf-and-ais-radio-range-149499.html



So, if you all wish to go on discussing these things....no worries here...perhaps my words / info in the other thread might be helpful, perhaps not....but, this is all I really have the time for...



Fair winds.

John

P.S. As to your query regarding what real-world experiences we've had.....gosh, I really don't have the time for all of it....and, of course none of it actually adds much useful info to the discussion of VHF Radiowave Propagation (rather just adds more confusion)....but, you did ask, so.....

I typically have reliable (100% always there) VHF marine comms to other pleasure boats / sailboats of about 35nm to 45nm, or so (depending on the other boat's system), that's 24/7/365, rain or shine....

I also have reception of VHF shore stations / NOAA weather radio broadcasts on 162mhz, on 5 (or 6, if you accept some noise) stations, right here at my dock, which gives me signals from a bit over 80nm, again 100% of the time, always there....(tropo-scatter comms).....and of course there are times when these signals at these ranges are much stronger than at other times, so as you see this is when "tropo-enhancement" / tropo-super-refraction is increasing signal strengths....

BTW, New-ish, and/or clean, and properly installed coaxial cable connectors (and decent, but not "exotic" / low-loss, coaxial cable), and a new-ish antenna at the masthead, are all important things here, in order to optimize your VHF marine radio system....I recommend replacing the antenna every 5 - 10 years, whether you think it needs it or not, and if not also replacing the coax and connectors at that time, at least inspect / clean the coaxial connectors at that time, and if there is much noticeable corrosion on them, replace the coax and connectors as well....doing these things will usually have much more positive effect than whatever minor changes (if any) due to using a higher-gain antenna or lower-loss coax....now, of course, I have no scientific proof, just my personal observations over the past 40 some years....btw, I don't do this at home / on-shore, where most of my antenna feedlines are Heliax (at least all for VHF/UHF) and well-weatherproofed, and I can measure loss variations, etc...but, on-board I'm a bit more intense about reliability...LOL


fyi, I've worked tropo-scatter on VHF out to ~ 850 miles (2m ham, not marine VHF), and used to have a station set-up to do that 24/7/365, but hurricanes damaged my antennas.....fyi, same antennas I used to bounce my VHF signals off the moon....yep, I've done that too, as well as meteor-scatter, aurora, etc....but, none of this has any bearing on the discussion here...(oh, and no, I don't have the room nor the $$$$ to build antennas big enough to hear the Voyager spacecraft....but, aside from the Deep Space Network, nobody else can receive them either, so I don't feel too inadequate...LOL)


To be clear, the confusion that comes from posting all of the "I can talk xx miles" is why I haven't posted much about it, but rather simply tried to provide the facts of how / why VHF signals propagate...why not just read the other thread? Is there a reason that referring some to the other thread is bad? I'm actually confused by that possibility, but oh well, I've already spent more time on this than I can....so, do hope it helps?

[Jammer]

John


Thanks for all that



I think we mostly agree on the facts. I am not sure that we agree on how best to summarize the facts for individuals who are not experts in propagation.


For what it's worth I had read through your lengthy pinned post before writing the OP and read it again when you posted the link.

Originally Posted by ka4wja

a) I disagree with the premise / title of your thread ("The VHF radio horizon calculation is wrong"),

b) And, that the premise of the very first sentence of your thread is based on misinformation....("Many sources proclaim that VHF is a line-of-sight radio technology limited in distance by the curvature of the earth.")

Here's the thing.


Pick up any article about Marine VHF written in a sailing or boating magazine or similar source. I guarantee you that VHF will be presented as a technology that is strictly line-of-sight and that the subject calculation will be presented, along with some arithmetic showing that the practical maximum distance between two sailboats is around 20 miles.


Then they stop. The nuanced discussion that follows in your pinned post, does not exist in most other summaries of marine VHF operations.


This treatment of the subject is so pervasive that mariners widely believe that the "radio horizon" is an absolute limit of reliable coverage. I believed it myself because my experience with VHF has been limited to land where edge diffraction is the usual explanation for coverage beyond LOS and troposcatter usually isn't a factor except in systems specifically designed to exploit it -- and because nearly every article in the sailing/boating press about VHF presents the "radio horizon" as the limit.


So, I was fooled. (Not the first time, won't be the last)


I spent a day reading through all kinds of sources trying to understand why everything I had experienced with VHF on land didn't apply on the water. The absence of texture and ridges in terrain, I thought. I reread a bunch of stuff about propagation and now we're here.

Much VHF communications is of course beyond line-of-sight communications, and hence the "radio horizon" is, of course, not the limit of VHF communications range...

Right. We're in vehement agreement about that. I think it's important for sailors to know that. If their reliable range is only 20 miles with VHF it's because they have problems with their antenna and feedline, something you mention later in your post.

When someone writes that "many sources proclaim that VHF is a line-of-sight radio technology limited in distance by the curvature of the earth.", I actually find that misinformative, somewhat misleading, certainly misunderstanding, etc....I mean, I mean any legit source / authority of radiowave propagation has known/understood otherwise for about 100 years now, so...

Again, for clarity, the problem that I have is that lay summaries for those not involved in the art of radio are oversimplified.

you make the assumption that everyone thinks this is the limit of VHF comms range

My "everyone" is not RF engineers. My everyone is sailors.

- some that say that there are too many variables to ever predict a maximum range, when in reality it is actually an easy thing to calculate (calculate a 100% reliable maximum range), as this is what is done all the time in most tropo-scatter comms (whether VHF links to offshore oil platforms, or VHF / UHF Radio and TV broadcasting, etc., etc. etc...), this actually done all the time....

I agree except that I'm going to call you out on the 100%, because you can engineer a link for 99.99%, and sometimes you can engineer a link for 99.999%, and with diversity you can maybe get a little more. Real world links are not 100%

and then we have some recounting their own experiences (which is fine, but unfortunately drifts the discussion further afield, and doesn't provide any understanding of the how's and why's)....

What it does is provide data points that validate the theory, in practice, on sailboats.

"10dbi" antennas at 20m high, are not just not typical, but unlikely on our boats

What is atypical but commercially available and very practical is low-loss feedline and a 3 dBd omni. These are available from many sources, both marine and non-marine, and have a beam width sufficient for good performance at reasonable angles of heel (~20-25 degrees).

....and, while I freely admit that I dislike longly-rice modeling, it's not too far off actual measured field-strengths, but if you've got a VHF marine radio system on your mid-size cruising boat with a 100% reliable range of 55nm (with no atmospheric enhancements), please post the specifics, 'cuz I just don't see it possible with off-the-shelf radio/antenna?

I did some simulations on Radio Mobile Online using publicly known information about Rescue 21 sites and a 3 dBd omni 60 feet up with RG-213, 25 watts, and published sensitivities for Icom and Standard Horizon radios now in production. I used the default path reliability of 70%.


I also did sailboat-to-sailboat simulations and was getting around 45nm.


I could re-run them with a higher path reliability. It would be interesting to see how the results change. Since the practical alternatives to VHF are themselves not 100% reliable it is not reasonable to insist upon 100% reliability when evaluating VHF coverage.

So, what am I to do?
I want to be of assistance to my fellow sailors, and point all of you to some factual information, without being an ass....and, without hurting anyone's feelings, etc...

I'm glad you're here and that you're engaged in the conversation. I am not a snowflake.

I typically have reliable (100% always there) VHF marine comms to other pleasure boats / sailboats of about 35nm to 45nm, or so (depending on the other boat's system), that's 24/7/365, rain or shine....

I think that is really valuable information and I'm glad you posted it.

BTW, New-ish, and/or clean, and properly installed coaxial cable connectors (and decent, but not "exotic" / low-loss, coaxial cable), and a new-ish antenna at the masthead, are all important things here, in order to optimize your VHF marine radio system....I recommend replacing the antenna every 5 - 10 years, whether you think it needs it or not, and if not also replacing the coax and connectors at that time, at least inspect / clean the coaxial connectors at that time, and if there is much noticeable corrosion on them, replace the coax and connectors as well....doing these things will usually have much more positive effect than whatever minor changes (if any) due to using a higher-gain antenna or lower-loss coax....

I think that's good advice and I think that sailors who aren't routinely getting the sort of 35-45 nm range you mention should take it fully on board.



Which is really my point. Sailboats that aren't getting that sort of range should figure out why. Because we have other posts on CF from people who think that PL-259s (the ones you're supposed to solder, not the crimp ones) work just fine without solder because they still get 20 miles. They believe that's all they can achieve, so why try harder?