The VHF radio horizon calculation is wrong

[continuouswave]

Originally Posted by kas_1611

VHF Radio signals, like all electromagnetic radiation, decays with the cube root of distance....

You may be the only person alive who believes that. There is no decay in the radiation. See my comments above re spreading loss.

[Brian.D]

That "shield" in your microwave is called a Faraday Cage or Faraday Shield and is "tuned" (those tiny holes) to prevent RF leakage. Common walls do not have such a "shield". Now, SCIFs do have Faraday Cages to prevent "listening in" using RF. So, yes, RF can penetrate normal walls that are not made totally of metal. Your cell phone is living proof.

[Sailing Ohm]

Sounds like someone needs to take an amateur radio operator course... everything will be explained

[Lodesman]

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.

.

Don't know whose sailboats you're using, but two boats with 65' masts would have the limit a little over 20 NM (by your formula). The radio waves don't stop at 20 miles, they're just going over your head. VHF is line-of-sight, with a little bending. Small hills don't usually block transmissions, but in a mountainous fjord you can be cut-off. VHF ranges can be greatly extended by surface ducting - this is prevalent in areas that see frequent inversions. Someone mentioned San Diego - and I also had a crystal clear VHF conversation with a USN warship in the opareas off San Diego, while we were heading south off Mexico, over 300 NM from them. It happens.

[Cadence]

Originally Posted by Sailing Ohm

Sounds like someone needs to take an amateur radio operator course... everything will be explained

Ref 14, you and I agree.

[Jammer]

Originally Posted by Lodesman

Don't know whose sailboats you're using, but two boats with 65' masts would have the limit a little over 20 NM (by your formula).

Right, my mistake. I saw it earlier and had missed the edit window, and the discussion seemed to have moved on so I didn't try to correct it until now.

The essential point remains that I am observing, and more sophisticated models are predicting, coverage far beyond the "radio horizon" given by the formula. At first I thought this was just because of terrain effects -- ridge diffraction and so on -- but there are many sources that show this happening over the ocean also.

Anyway, this matters for a couple of reasons:


1. The standard advice is that antenna gain and feedline quality matter little on a boat because range is limited by the radio horizon. It appears to me that this isn't true. A careful installation that picks up a few dB from a better antenna and a dB or two less feedline loss, will have better range


2. There are situations where being able to get reliable 40 mile VHF coverage would eliminate the need for other, more costly communications systems (HF or satellite)

[Simi 60]

Originally Posted by 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.

We were at Cape Gloucester recently and hearing the Marine Recue put out reasonably clear morning weather reports from 1770.
Thats about 350nm away

Usually I am happy if we get 40nm

[Jammer]

Originally Posted by Sailing Ohm

Sounds like someone needs to take an amateur radio operator course... everything will be explained

There is much that I have learned and continue to learn from other amateur radio operators. But not everything they do applies to the marine environment. Propagation is different over water than over land. Marine radio, like commercial LMR, uses techniques that provide dependable communications rather than techniques that provide opportunities for exceptional communications.

[Jim Cate]

Another long range VHF anecdote:

A few years back for two nights running we heard the evening "hand-over" broadcast from the VMR at Tin Can Bay, Queensland. We were anchored in New Caledonia with a range of over 800 miles. We did not hear any other Aussie stations at the time, nor on any other days. Very episodic!

Jim

[StuM]

Originally Posted by Jammer

2. There are situations where being able to get reliable 40 mile VHF coverage would eliminate the need for other, more costly communications systems (HF or satellite)

Ay, there's the rub

[David B]

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.

It's a useful 'rule of thumb'. Not wrong, when taken in that context.
In normal conditions, after line of sight, there is bending and scatter, then in abnormal conditions, I have on a number of occasions had perfect coms at marine VHF frequencies over hundreds, and in some cases, thousands of miles.

[Brian.D]

You want 'line of sight' communications? Talk to the moon on VHF. 270,000 miles, 540,000 miles round trip.

[StuM]

Originally Posted by Brian.D

You want 'line of sight' communications? Talk to the moon on VHF. 270,000 miles, 540,000 miles round trip.

Or talk to Voyager on 2.1 GHz


We're still picking up their 23W transmissions at 14 billion miles!

[GordMay]

Originally Posted by StuM

Or talk to Voyager on 2.1 GHz
We're still picking up their 23W transmissions at 14 billion miles!

But, not with a 3 Ft whip antenna.

[GordMay]

The Deep Space Network (DSN) has three permanent stations around the globe: in Goldstone, California; Madrid, Spain; and Canberra, Australia.
Each location has one 70-meter parabolic dish standing 20 stories high and weighing a staggering 6 million pounds. These dishes are the DSN’s workhorses, built between 1966 and 1974. The rest of the network is made up of dishes with 34-meter antennas that can either talk to spacecraft on their own, or can “array together” to form a more powerful signal. In total, there are 13 dishes positioned 120 degrees apart around the world talking to 35 different spacecraft.
“DSN NOW” shows the downlinking and uplinking of data in real time.
➥ https://eyes.nasa.gov/dsn/dsn.html