While VHF communication range is dependent on many variables, we will just discuss the three most common and what many consider to be the most important factors.
All marine VHF’s available to the average boater offer transmitter power of between 1 watt and 25 watts. All else being equal, the more power you send to the antenna the further your signal will reach.
More important than all other factors is the height of your antenna. Simply put, the higher your antenna the further your signal will reach.
While under many circumstances some bending of VHF radio waves occur, however slight, generally speaking VHF is considered to be line of sight radio. What this means is that if your antenna cannot see the antenna of the vessel you want to communicate with, chances are your signal will not reach them. It also means that if there are any intervening obstructions, your signal will also fail to reach its destination.
For those that want some idea of what communications range they can expect from their radio installation, there are numerous formulas available to compute this for you. I will offer up only 2 that should provide you with a reasonable guide of what theoretical range you can expect from your particular installation.
For the conservatives among us this first formula does not take into account any bending of the radio waves:
d = 1.17 * √h
Where d = range in nautical miles and h = the height of your antenna in feet.
Another formula that does take into consideration the slight bending of the radio wave is:
d = 1.42 * √h
As an example: An antenna mounted at 64’ above sea level (typical on many sailboats):
d = 1.42 * √64
d = 1.42 * 8.0
d = 11.36 NM
Keep in mind when communicating with another station; you must factor in their antenna height as well. Add these together and you will have the distance that you can expect to be able to initiate communications. So using the example above if both vessels had 64’ antenna heights, they would likely be able to establish communications at almost 23 nautical miles.
So, what does all of this mean? All other things being equal, the higher you can place your antenna will result in increased range of your radio.