For VHF communication the most significant factor in establishing range is your antenna’s height above sea level. This assumes, of course, your system consists of a good antenna with the right size cabling and a properly functioning radio or AIS unit.
Think of radio range as your radio horizon plus the radio horizon of the station you are communicating with. Radio horizon is an invisible circle around your boat, the perimeter of which is the distance to which the radio signal from your antenna will reach. The station with which you are communicating will have its own circle, the extent of which will depend on its antenna’s height above sea level. When the two circles meet you can communicate. This is your range. So, your communicating range varies, depending upon the radio horizon of the station with which you are communicating.
Your radio horizon in nautical miles is 1.4 x root of antenna height in feet above SL. For example, if your antenna is at the masthead, 49 feet above sea level, your radio horizon will be 1.4 x 7 = 9.8 nm. If you are communicating with an identical boat, your combined range will be twice this figure, about 20 miles. However, if you are communicating with the QE2 where her antenna is nearly 200 feet above sea level, her radio horizon will be about 20 miles so you’ll be able to communicate at closer to 30 miles.
Coastguard stations have powerful transmitters located high up on headlands and have large radio horizons. An antenna at 900 feet above sea level would have a range of 1.4 x 30 = 42 miles, so you could communicate at about 50 miles.
Clearly, the ideal location for your VHF antenna is as high as you can get it and, on a sailing boat, that means the masthead. Nowadays you’ll probably be using AIS and radio so the masthead location can create a problem – how to locate two antennas far enough apart so they don’t interfere with each other. If you can get the antennas over 0.75 meters apart, they should function fine. Some experts say they need to be more than 1m apart but anecdotal evidence seems to suggest the 0.75m figure is good enough. If you can’t get this degree of separation your choices come down to using an active splitter or mounting one of the antennas (AIS) at the next highest location – mizzen mast, radar-arch/pole or the pushpit.
Having a second antenna is a wise choice because it means you aren’t interrupting AIS data when using the radio and the second antenna provides redundancy. Arrange the cabling so that either antenna can be connected to either radio or AIS. Carry an SO239/BNC adapter if the input to your AIS is BNC. Your radio is always PL259.
If you have a combined radio/AIS it will usually be equipped with a splitter – be sure to use a full range VHF antenna, 156 to 163 MHz, so as not to compromise reception.