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VHF antennas are a crucial part of your VHF installation, and understanding how they work and how to select the correct antenna can greatly improve your ability to communicate with other boaters and shore stations. The main purpose of a VHF antenna is to provide a radiator for the power that the transmitter in the radio produces, and to radiate this energy in the correct direction. Antennas also have to be enclosed in a weather-protected enclosure (or be made of a corrosion resistant metal and plastic) and must be able to withstand the substantial forces generated by a boat pounding in heavy seas.

Selection criteria:

dB Rating: An antenna’s dB rating (or antenna gain) indicates the apparent increase in transmitting power due to its ability to focus energy. Antennas with a high dB rating concentrate energy perpendicular to the antenna shaft in a field that is shaped like a disk. This makes your radio signal appear stronger to receiving stations around you.
It also reduces the amount of energy transmitted above or below the antenna, which can be a problem if your boat is pitching or rolling in seas. A concentrated signal can actually fade in and fade out as the boat rolls and pitches. As the dB rating of the antenna increases, so does the height of the antenna, but the horizontal angle diminishes. The broad radiation pattern from a low dB antenna allows a sailboat to heel over and still send signals toward the horizon. Sailboats should use a 3dB antenna mounted at the top of the mast whenever possible.

Gains and losses: A gain increase of 3dB means a doubling of signal strength: 6dB is a fourfold increase, and 9dB is an eightfold increase. The actual watts transmitted doesn’t increase; it’s just that the power is concentrated, much like a reflector on a flashlight, in a more concentrated beam.  Therefore, a radio with a 6dB antenna will sound like it has a larger transmitter than the same radio with a 3dB antenna.

Elements inside the antenna: There are three main styles of electrical elements: those using cut lengths of coax cable, those using a simple brass radiator, and those using a more complicated copper and brass radiator. While all provide acceptable performance, better antennas use brass or copper inside the fiberglass for maximum strength and durability.

So what kind of range can I expect?

Marine VHF radios for recreational boats are limited to 25 watts of output so, unlike Single Sideband or Ham radios, you cannot boost your range with a more powerful transmitter.
VHF radios operate on a line of sight principle between stations, meaning that the signals do not bend around obstructions or over the horizon.
Antenna height, more than any other factor, determines how far you can transmit. An antenna mounted up high can “see” farther over the horizon.
Antenna gain is important. If several otherwise equal radios try to contact a single receiving radio, the higher gain antenna is the one the receiving radio will hear.