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What is the possible range of a VHF/AIS antenna? It is often stated to be about 10 to 25 miles.
What affects the range? Heights of the antennas (both transmit and receive) and the terrain between the two.
VHF radio's signal travels in a straight line and because of the curvature of the earth, the height of the antenna is the single most important factor to consider.

There is a straightforward calculation that can be used to identify the potential range in nautical miles - that is, how far your radio can 'see' to the horizon is equal to 1.23 times the square root of the height of your antenna in feet. (note that there are a variety of multipliers used, depending whether your measurement is in miles/feet or km/ metres or statute miles/feet, but in practice, these do not vary much and so the impact on the range is very small.
As an example - antenna height is 60 feet, the distance it can transmit to the horizon is ~ 9.5 nautical miles. (1.23 x √60 = 9.5)
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In addition to this, the vessel to which you are transmitting also has an antenna above the water. For example, this antenna is 18 feet above the water, which, using the same formula, can potentially transmit about 5.2 miles, adding the two ranges together gives a total of 14.7 nautical miles as a maximum potential vhf/ AIS range between the two vessels.

A land based station is usually much higher. An antenna on a hill on a tower might be very high, for Example it's 400 feet. So theoretically, the transmit range to the horizon is 24.6 miles. Add that to your own vessel range to horizon 9.5 miles, this will create a maximum potential range between the two antennae of 34.1 nautical miles.
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This latter case clearly exceeds the roughly 25 mile maximum range limit described above. There are many other factors which can affect range and performance. The first of which is the rapidly diminishing strength of the transmitted signal with distance. The power density of an electromagnetic wave is proportional to the inverse of the square of the distance from the source. So, for every 4 miles your signal travels, its strength is reduced by a factor of 16.

Several other factors including the type of antenna affect range. (The sensitivity of the receiving radio, background electromagnetic noise, land masses, weather, are a few others.)

One thing to note is the loss of signal through the coaxial cable (coax) which leads from the radio to the antenna. A powerboat's cable is often only a few feet, so the loss is negligible. However, on a sailboat, the antenna cable is much longer. To make up for this, a heavier cable, with less loss is required.

The cable for a power boat may be only 20 feet, and RG-58U cable could be used with only a -1.2dB loss. A sailboat using that same cable but running it 60 feet would experience a -3.6dB loss. Using a heaver cable, such as RG-213, a sailboat running the coax that same 60 feet would see only -1.6dB loss. (A -3dB loss halves the signal. )
There are many other things which can adversely affect the transmission range: weather, land masses such as islands or peninsulas, over which you have little or no control.

Sometimes you'll hear a radio transmission quite clearly which is many, many miles beyond normal transmission capabilities of your normal VHF radio. This can be caused by skip or ‘ducting effect’, or in the case of Coast Guard, possibly repeater stations. Don't bother trying to talk back. Such transmissions are anomalous and usually only in one direction
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Another area of potentially high loss is in the connections. A sailboat needs to have a junction at the base of the mast so that the mast may be removed when necessary. The connectors at this and all junctions or terminals should be gold plated and installed carefully using the correct tools.

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