posted 03-07-2015 04:11 PM ET (US)        

To conservatively estimate a range of communication, you combine your radio horizon distance with the radio horizon distance of the other station.

The best way to improve your radio range of communication is to improve your radio horizon by increasing the height of your antenna. Note that it is not necessary to increase the length of your antenna to increase its height. You can just move the mounting base higher and not increase the length of the antenna.

In general, as antenna length increases there is an expectation that the gain of the antenna will increase. There is no real overall gain in an antenna. There is only an alteration of the pattern of radiation so that more of the signal is concentrated in one direction at the expense of reducing the signal in another direction. Antennas with gain will have their radiation concentrated into narrower beams than antennas with less gain, and in that narrower beam the signal is stronger. Due to the motion of the antenna on a small boat, it is not especially useful to have an antenna with much gain; the movement of the antenna causes the signal to be directed away from the horizon, and this will actually reduce the effective radiated power to distant stations, not improve it, because the signal directed at the horizon may become weaker as the main beam of the antenna moves.

If you plan to have a radio antenna that will be in a stowed position while the boat is underway, you must consider the possibility of breakage due to someone stepping on the antenna or grabbing it as a hand hold when it is stowed. Antennas encased in fiberglass tubes are more likely to be damaged than antennas using external radiators made from steel.

Anecdotal reports of tremendous improvements of radio communication following installation of a particular antenna are most likely the result of comparing an antenna that is actually working (the new antenna) to an antenna that was not working at all or perhaps was not even actually connected to the transmitter (the old antenna), and I recommend ignoring most of these as they are not particularly reliable.

Regarding reports of radio communication at ranges that exceed the combined radio horizon distance of the two stations, this is certainly possible. Communication does just not suddenly stop at the radio horizon distance, but beyond this distance there will be much greater path loss. The radio signal can scatter and travel beyond the radio horizon, but its intensity or strength is reduced. In this fringe area communication is still possible, but signal levels will be much lower and there may be rapid fading of the signal.

Regarding communication to a rescue agency like the Coast Guard, in the USA the Coast Guard has a coastal radio system which provides excellent radio coverage to a distance of at least 20-miles offshore in about 95-percent of coastal areas. Their RESCUE 21 system is able to detect a 1-Watt transmitter making a 1-second transmission with an antenna at a height of 6-feet to a range of 20-miles to seaward. If your boat has a 25-Watt transmitter, an antenna at least 6-feet above the sea, and you make a transmission longer than 1-second, there is an extremely high probability the Coast Guard will hear you and be able to communicate with you.