Standard Horizon Quantum GX5000S, Руководство пользователя

Страница: 8 / 100

GX5000S Page 8
7 GETTING STARTED
7.1 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 15 miles, for
a portable 5W radio transmission the expected distance can be greater than 5
miles in “line of sight”.
7.2 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 foot, 3dB gain antenna represents twice as much
gain over the imaginary antenna.
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.