Standard Horizon MATRIX GX1280S, Руководство пользователя

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GX1280S Page 54
8.9.4 Safety Call
Used to transmit boating safety information to other vessels. This message
usually contains information about an overdue boat, a derelict afloat, loss of
a navigation aid or an important meteorological message.
8.10 ADDITIONAL DIGITAL SELECTIVE CALLING INFORMATION
For additional information the USCG has an excellent site that should be
visited at www.navcen.uscg.mil/marcoms/gmdss/dsc.html.
8.11 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
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 15 miles, for a portable 5W radio transmission the ex-
pected distance can be greater than 5 miles in “line of sight”.
8.12 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 foot, 3dB gain antenna represents
twice as much gain over the imaginary antenna. The length of the antenna
you choose, however, must also be related to the size of your boat.
Typically a 3 foot 3dB gain stainless steel whip is used on a sailboat mast.
The longer 8 foot 6dB fiberglass whip is primarily used on power boats that
require the additional gain.