When you are installing your VHF, arguably the most important consideration is where to place the antenna. You know it should be as high as possible, but mounting it high introduces difficulties in the installation itself because it is harder to get to.
Inevitably, this leads to you questioning whether it really is that important to mount the antenna up high.
The height of a VHF radio antenna is important because VHF is considered a “line-of-sight” communication technology. Essentially, you can only communicate between two antennas that are within sight of one another. The higher each antenna is, the further you can communicate.
It is easiest to imagine by considering how far you can see. If you are standing on the ground, you can only see a short distance. From the top of a building, or a hill, you can see significantly further. This is your line of sight.
VHF radios work in the same way. Their signal is transmitted out of their antenna and travels in a straight line. If there is another antenna within sight, it will receive the signal.
An antenna mounted on the ground has a shorter line of sight than an antenna high up, mounted on the top of a building or at the top of a mast on a boat.
Not only is the height of your own antenna important, but the height of the receiving antenna also plays its part.
In the above image, the transmitting VHF antenna is blue. Its signals propagate in a straight line away from the antenna, limited by the curvature of the earth.
The green antenna is tall enough to receive the signal. The top of the green antenna can “see” the top of the blue antenna.
Conversely, the shorter red antenna cannot “see” the blue antenna so they cannot communicate with each other. Even though it is closer to the blue transmitter than the green antenna is, its lower height means it has less range.
Calculating VHF range from antenna height?
We have seen that the height of the antenna has a direct impact on the possible range of communication with VHF radio because it uses line-of-sight communication.
This means that it is possible to directly calculate the expected range of your VHF based upon the height of the antenna and the curvature of the earth.
A common formula that you may have seen before is
where “d” is the expected range in statute miles, and “h” is the antenna height in feet.
At sea, we use nautical miles instead. As 1 nautical mile is approximately 1.15 statute miles, we can simplify the equation to be
where “d” is the expected range in nautical miles, and “h” is the antenna height in feet.
Of course, this formula only gives you the range of a single antenna. For example, if you assume a handheld radio being held at sea level, you will probably have a height of 3 ft.
Our formula only gives you a range of approximately 1.7M.
To get an accurate range, however, we need to account for the horizon of the receiving antenna as well. You simply add the two ranges together. Therefore, you can expect two handheld VHF radios held approximately 3ft above the water will be able to communicate over 3.4M.
For a handheld radio communicating with a large ship’s antenna, we get
assuming the handheld radio is 3ft (1m) above the water and the ship’s antenna is 100ft (30m) above the water.
What is the ideal height of a VHF antenna?
In an ideal situation, a VHF antenna will be mounted as high as possible. Typical locations include the top of a mast, or on the roof of a cabin.
Using the equations we have already discussed, we can see that the height of your VHF antenna has a direct impact on the range of transmission.
On a typical 20ft boat, with an 18ft mast, you can expect to add around 5 miles to your transmission by mounting the antenna at the top of the mast compared to mounting it on the cabin top.
Not only do you benefit from the increased range, but the added distance between the antenna and other electronic sources on board will reduce the chance of interference with your VHF transmission.
I have written about ways to minimise interference with VHF communications in this article (How To Reduce Interference On A Marine VHF Radio) if you would like to find out more about sources of interference from within your own boat.
Is antenna height more important than power when determining VHF range?
It might seem a little counter-intuitive that antenna height is the most important consideration for determining the range of your VHF communication.
Logically, you would think that the power of your transmission is more important. After all, there is a button on your radio that switches it between “high power” and “low power” mode.
As it happens, however, antenna height is far more significant than the power of your transmission when determining range.
The reason for this is that even a low powered VHF transmission has the potential to carry a long distance.
For example, the VHF transmitter carried in the suit of the Apollo astronauts was around 0.5W and could be detected by antennas on earth when the astronaut was on the moon.
Reference: Apollo Operations Handbook
The low power mode of a marine VHF is around 1W, so will have no trouble transmitting across the typical range of 20 nautical miles or so that you would expect between two yachts.
The line of sight between the two antennas is the most important consideration when determining the range of a VHF transmission.
Why can VHF only communicate through line-of-sight?
VHF radio communication takes place around the 150 MHz frequency within the electromagnetic spectrum.
Due to its frequency, VHF transmissions tend to travel in straight lines, without refraction of the signal occurring.
As the frequency reduces, the radio waves gain the ability to refract more. As you get down below 3 MHz, they can refract much better.
This is why HF communications can take place over a much longer range than VHF.
The best know HF frequency, 2182 kHz, falls below the critical 3 MHz that we identified as having the ability to refract.
This means that it propagates as a ground wave instead of simply using line-of-sight. Basically, HF transmissions can bounce off different layers within the atmosphere, giving them the ability to travel around the earth.
VHF signals, with a higher frequency than HF signals, can only travel in straight lines.
The only way to transmit a VHF signal further is to increase the height of your own antenna (the transmitter) and the other vessel’s antenna (the receiver).
You can always get away with mounting your VHF antenna low down, but you will sacrifice range by doing so.