Getting Started; About Vhf Radio; Selecting An Antenna - Standard Horizon Eclipse GX1300 Owner's Manual

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
marine VHF band provides communications over distances that are essen-
tially "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 25 W radio transmission expected distances can be greater
than 15.5 miles (25 km), for a portable 5 W radio transmission the expected
distance can be greater than 5 miles (8 km) in "line of sight".

5.2 SELECTING AN ANTENNA

Marine antennas are made to radiate signals equally in all horizontal direc-
tions, 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 3.28 feet (1 m), 3 dB gain antenna
represents twice as much gain over the imaginary antenna.
Typically a 3.28 feet (1 m) 3 dB gain stainless steel whip is used on a
sailboat mast. The longer 8.2 feet (2.5 m) 6 dB fiberglass whip is primarily
used on power boats that require the additional gain.
3dB
6dB
9dB
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