Standard Horizon GX2000E, Руководство пользователя

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Page 11 GX2000E/GX2100E
5 GETTING STARTED
5.1 ABOUT VHF RADIO
The radio frequencies used in the VHF marine band lie between 156 and 162
MHz. The marine 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 25 W radio transmission expected distances can be greater
than 25 km, for a portable 5W radio transmission the expected distance can
be greater than 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 1 m, 3 dB gain antenna represents twice as much
gain over the imaginary antenna.
Typically a 1 m 3 dB gain stainless steel whip is used on a sailboat mast. The
longer 2.5 m 6 dB fiberglass whip is primarily used on power boats that require
the additional gain.