The VHF radio horizon calculation is wrong

[Jammer]

Many sources proclaim that VHF is a line-of-sight radio technology limited in distance by the curvature of the earth. Formulas given usually state that the distance in miles is roughly 1.4 * sqrt ( antenna height in feet ). The horizon is calculated for each station and then the distances are added. This would limit maximum range between two sailboats to about 40 miles.



Real world performance of properly installed VHF radios is much better, that is, greater distances can be achieved. Further, power, sensitivity, and antenna gain affect range. There is not a signal cutoff at the limit of line of sight but rather progressive loss of signal.


I did some searching to try to better understand what happens, the theory behind it, and the actual factors that go into useful range.


I offer this article, which summarizes three models (that do not agree with each other): continuousWave: Whaler: Reference: VHF Radio Propagation Over Water


There is some discussion of the article here: Effect of Antenna Height on Signal Strength - CONTINUOUSWAVE


The key conclusions that I draw from all this are that:

• The "radio horizon" concept is oversimplified and tends to understate useful range particularly where higher antennas and higher power levels are involved
• Height affects range to a greater extent than the "radio horizon" formulas show
• Power, antenna gain, and receiver sensitivity do actually make a difference in useful range
• As is the case on land, frequencies lower in the VHF band provide greater range, due to reduced path loss. Marine VHF occupies a narrow band segment, but amateur radio operators would have access to 50 MHz/6 meters and 28 MHz/10 meters, and many boaters can and do use CB radio operating at 27 MHz. There are also Marine HF allocations at 25 MHz that effectively operate as somewhat longer-range VHF frequencies except during band openings at the peak of the sunspot cycle.

I am curious what real-world experience people have had.

[StuM]

Not "wrong", but I think everyone would agree that your alleged "proclamation" is a simplification. It's just a rule of thumb, but it's a very useful one.

There's been lots of discussion here about things like atmospheric ducting extending range and interference and antenna placement reducing it.

Sure "greater distances can be achieved." At the same time "lesser distances can be the effective range"

There are far too many variables to be able to actually predict a maximum range in a specific situation.

[deblen]

I have many years of personal experience in the install and service of standard 25watt marine vhf transceivers in the Fundy/Gulf of Maine region.
My experience has shown that,with good quality equipment & using so called 10db (ex:Shakespeare 476) at both stations & mounted at heights of 30-60ft. above water, resulted in the following:


Reliable,anytime, clear comms
approx 75 nm-in clear dry weather
approx 40 nm-in fog,rain or very humid weather.


Scratchy,fading, but useable comms
approx 90-100 nm-in clear,dry wx
approx 60 nm- in fog,rain,or very humid weather


The above are practical ranges with top eqpt.
The most important element is the antenna quality,height and coax.


My 3db Shakespeare 5215 mounted at top of 40ft mast is reliable to about 30nm in average conditions when speaking to another sailboat with same antenna. I know I xmit farther,because I have been answered from greater distances by fishermen using a big antenna. Distance of comms relies on the eqpt. at both ends.

Never found much,if any difference in the Tx/Rx performance of modern radios,as long as the radio is supplied with a good solid supply of 12V.
Note: if you are having a xmit problem,reduce your output to 1W. This requires much less current,& xmits surprisingly far. The Rx is no different on 1W.


Since my experience shows me that humid air reduces propagation over water substantially,I came to the personal conclusion that tunnelling/ducting is a major factor,and,probably some tropospheric bending.


My personal rule of thumb for propogation over averge height land is 10nm land = 1nm water.


Cheers/Len

[Brian.D]

Yeah, it is a rule of thumb. A Catalina 36 was headed to Hawaii when it lost its rudder. They called for help on VHF and the San Diego Coast Guard heard them and dispatched a helicopter. The Catalina was over 200 NM away from San Diego when the call was placed. So like a lot of 'formulas', there is always X, an unknown factor that is involved.

For the new users trying to figure out deck mount or mast mount, use the formula and maybe the X factor will be in your favor.

[chris mac]

I just replace our antenna and wire, kept the old(ish) radio
Prior, I was lucky to hear 20 miles out.
Recently I heard commercial traffic in Norfolk. That was 80 miles away!
Our antenna is roughly 64 feet up

[S/V Illusion]

A friend of mine (also a ham) has worked all 50 states on VHF with antennas typical of any boat.

Implying there is any practical relationship between height and distance is an academic over-simplification.

[Jammer]

Originally Posted by deblen

I have many years of personal experience in the install and service of standard 25watt marine vhf transceivers in the Fundy/Gulf of Maine region.


My experience has shown that,with good quality equipment & using so called 10db (ex:Shakespeare 476)

Thanks, that's insightful.


I had to look up the Shakespeare model numbers. The Shakespeare 476 is a 21 foot long antenna. It is a co-linear array of 1/2 wave dipoles, and while the manufacturer doesn't state how many, there are room for 12 given its length.



The 5215 you mention using is the base-loaded stainless steel whip.

[continuouswave]

The notion that VHF radio is limited to "line of sight" is incorrect. But this misunderstanding is often repeated among boaters.

There is a real concept of a radio horizon. There are three common horizons:
--the geometric horizon
--the optical horizon
--the radio horizon

I derive the formulas for all three in an article at

RADIO HORIZON
Classic Whaler: Boston Whaler: Reference: Radio Horizon

Both optical and radio waves depend on REFRACTION to extend their horizon. The degree of refraction and the path of refraction are also variable, influenced by weather. Everyone is (or should be) familiar with unusual optical refractions that occur due to unusual weather and air temperature boundaries. They are called mirages.

If electro-magnetic waves at light frequencies can be bend, then certainly radio waves can also be bent.

[GordMay]

Originally Posted by continuouswave

... Both optical and radio waves depend on REFRACTION to extend their horizon...
... If electro-magnetic waves at light frequencies can be bend, then certainly radio waves can also be bent.

While it’s true that both light & radio can be subject to refraction (bending), it’s in no way “certain”, that because one can, so, the other must too.
For instance, radio waves pass through typical walls, but light cannot.

[trifan]

Rule of thumb: VHF distance = line of sight + 30% (plus propagation effects such as atmospheric ducting). Ducting is a real helper; I know hams who have used strong frontal boundaries to work other hams several hundred miles away on VHF. Radio wave propagation is a science unto itself.

[kas_1611]

VHF Radio signals, like all electromagnetic radiation, decays with the cube root of distance. But with a sensitive enough receiver we can pick up radio signals from Jupiter and we are still receiving radio signals from the Voyager probes who are currently between 11.5 and 14 BILLION miles from Earth.

And the ISS has a VHF radio on board so as long as it is above your horizon they can pick up a Distress Call on Ch 16 from 180 miles away.

[clarkj]

There is a great program called Splat! that you can use to calculate propagation for VHF/UHF/Microwave systems. I wrote an article on how to use this program for AIS Class B 2W transmitters. You can enter all the various parameters including power, antenna gain etc. I discuss optical and radio horizons.
Best Regards



https://jeremyclark.ca/wp/telecom/rf...lations_splat/

[continuouswave]

Originally Posted by GordMay

While it’s true that both light [and] radio can be subject to refraction (bending), it’s in no way “certain”, that because one can, so, the other must too.

You are going to have to explain what supposition further. Radio waves and lightwaves are forms of electromagnetic energy. Any variation in their properties is due to the frequency. Radio waves--in the context of this forum and the in the context of this discussion which EXPLICITLY mentions VHF--are in the frequency range of 150 to 160-MHz, and they are certainly subject to refraction.

As the frequency of radio waves increases, the waves become less likely to be refracted. The VHF range is 30 to 300-MHz. Above 300-MHz is the UHF band, where the waves tend to be less affected, a tendency that continues as frequency increases.

Originally Posted by GordMay

For instance, radio waves pass through typical walls, but light cannot.

Sorry, but that is not nearly as constant a phenomenon as refraction. I just warmed up my coffee in a microwave oven. I could easily see through the glass door of the oven and watch the cup turning around, but the microwave radiation was blocked by the aperture grid in the glass.

At some frequencies, even clear glass will reflect radio waves but allow light waves to pass.

Again, the behavior depends of the frequency of the electromagnetic radiation.

[Cadence]

Originally Posted by Jammer

Many sources proclaim that VHF is a line-of-sight radio technology limited in distance by the curvature of the earth. Formulas given usually state that the distance in miles is roughly 1.4 * sqrt ( antenna height in feet ). The horizon is calculated for each station and then the distances are added. This would limit maximum range between two sailboats to about 40 miles.



Real world performance of properly installed VHF radios is much better, that is, greater distances can be achieved. Further, power, sensitivity, and antenna gain affect range. There is not a signal cutoff at the limit of line of sight but rather progressive loss of signal.


I did some searching to try to better understand what happens, the theory behind it, and the actual factors that go into useful range.


I offer this article, which summarizes three models (that do not agree with each other): continuousWave: Whaler: Reference: VHF Radio Propagation Over Water


There is some discussion of the article here: Effect of Antenna Height on Signal Strength - CONTINUOUSWAVE


The key conclusions that I draw from all this are that:

• The "radio horizon" concept is oversimplified and tends to understate useful range particularly where higher antennas and higher power levels are involved
• Height affects range to a greater extent than the "radio horizon" formulas show
• Power, antenna gain, and receiver sensitivity do actually make a difference in useful range
• As is the case on land, frequencies lower in the VHF band provide greater range, due to reduced path loss. Marine VHF occupies a narrow band segment, but amateur radio operators would have access to 50 MHz/6 meters and 28 MHz/10 meters, and many boaters can and do use CB radio operating at 27 MHz. There are also Marine HF allocations at 25 MHz that effectively operate as somewhat longer-range VHF frequencies except during band openings at the peak of the sunspot cycle.

I am curious what real-world experience people have had.

You are trying to compare marine FM to Am or Am SSB. It's apples and oranges. Get an old HAM to explain it. And I mean old and devoted.

[continuouswave]

There are additional phenomenon that change the path of VHF radio waves. Again, weather is a factor. Temperature inversions in the troposphere tend to bend VHF radio waves back towards earth. If the refraction matches the earth curvature, long distance paths can occur. This method is known as Tropo-scatter or Tropo-ducting. Oher phenomenon occur, such as greatly enhanced ionization in the ionosphere.

It is also important to understand the phenomenon of path loss. There are many formulas for calculating a parameter often referred to as "path loss." However, when radio waves propagate in free space there is NO path loss. The usual calculations for loss of received signal compared to transmitted signal are simply accounting for BEAM SPREADING and are more properly called spreading losses.

When radio waves propagate through a medium other than the vacuum of space, there is loss. The loss varies with the medium and with the frequency. Propagation through standard temperature air with little moisture does not introduce much loss.

For a better understanding, see this ITU publication available for download at no cost:

Recommendation ITU-R P.341-7 (08/2019)
The concept of transmission loss for radio links
P Series--Radiowave propagation
https://www.itu.int/dms_pubrec/itu-r...8-I!!PDF-E.pdf