Furuno FM-4000 - Manual (Page 8)

FM-4000 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
25 W radio transmission expected distances can be greater than 15 miles, for
a portable 5 W 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, 3 dB gain antenna represents twice as much
gain over the imaginary antenna.
Typically a 3-foot 3 dB gain stainless steel whip is used on a sailboat mast. The
longer 8-foot 6 dB fiberglass whip is primarily used on powerboats that require
the additional gain.
7.3 COAXIAL CABLE
VHF antennas are connected to the transceiver by means of a coaxial cable –
a shielded transmission line. Coaxial cables are specified by their diameters
and construction.
For runs less than 20 feet, RG-58/U, about 1/4-inch in diameter, is a good
choice. For runs over 20 feet but less than 50 feet, the larger diameter RG-8X
or RG-213/U should be used. Cable runs over 50 feet should use RG-8X. For
installation of the connector onto the coaxial cable see the figure below.
To get your coaxial cable through a
fitting and into your boat’s interior, you
may have to cut off the end plug and
reattach it later. You can do this if you
follow the directions that come with the
connector. Be sure to make good sol-
dered connections.
1/16''
3/4''
1 1/8''
3/4''
Adapt er
1/8''
5/8'' 3/8''