5 GETTING STARTED
5.1 ABOUT VHF RADIO
The radio frequencies used in the VHF marine band lie between 156 and 158 MHz with some shore stations available between 161 and 163 MHz. The ma- rine VHF band provides communications over distances that are essentially “line of sight” (VHF signals do not travel well through objects such as buildings, hills or trees). Actual transmission range depends much more on antenna type, gain and height than on the power output of the transmitter. On a fixed mount 25W radio transmission expected distances can be greater than 25 km.
5.2 SELECTING AN ANTENNA
Marine antennas are made to radiate signals equally in all horizontal directions, but not straight up. The objective of a marine antenna is to enhance the signal toward the horizon. The degree to which this is accomplished is called the antenna’s gain. It is measured in decibels (dB) and is one of the major factors in choosing an antenna. In terms of effective radiated power (ERP), antennas are rated on the basis of how much gain they have over a theoretical antenna with zero gain. A 1 m, 3dB gain antenna represents twice as much gain over the imaginary antenna.
Typically a 1 m 3dB gain stainless steel whip is used on a sailboat mast. The longer 2.5 m 6dB fibreglass whip is primarily used on power boats that require the additional gain.
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