NorthStar Navigation GM1708 961XD - Manual (Page 26)

SECTION TWO - Installation
Page 16 961 INSTALLATION MANUAL Revision A
antenna is desired. The AN205-P doesn’t require any ground and is porta-
ble, an advantage when it must be moved from one vessel to another. The
“combo” antenna should be located where it has a clear view of the hori-
zon, but is not the highest point on the vessel. Keep the combo antenna
at least six feet away from objects that can “shade” GPS or differential sig-
nals.
If poor GPS Signal-to-Noise Ratio (SNR) readings are obtained after the
unit has been running for several minutes, check that you have the
proper length of cable, and verify the quality of the antenna location and
the quality and proper termination of the connectors. SNR should be as
high as possible. Values of 15 and higher are preferable; anything below
10 could indicate poor reception.
About the
antenna cabling
Supplied with your 961 system is a 50 foot-length of RG-59 coaxial cable
to use with either the AN150 or the AN205-P. The length of coax cable to
the AN150 antenna must be no less than 50 feet and no more than 100 feet;
to the AN205-P, no less than 20 feet and no more than 50 feet. When
installing antenna cable, don’t bend it tightly in any places, and fasten the
cable along its length to avoid chafing or whipping of any kind. Coil up
any unused length of cable. Secure the cable well (the center conductor is
solid wire), and securely fasten all cable connectors.
Installing the
AN150 antenna
Whereas a loran or differential antenna should be mounted high on the
vessel for best performance, the GPS antenna should be mounted as low
as possible and out in the open to avoid “shading” (placement of the
antenna where it is partially obscured by another object from the signals
it must receive). If mounting on top of a tower or mast, understand that
the unit will be affected by the pitch and roll of the vessel. Often the bow
or stern will provide a location where shading is minimized, while serving
to keep the antenna low. Be sure that any directional L-band transmitting
antennas (such as radar or satellite communication antennas) can never
point at the GPS antenna--its internal preamplifier is quickly destroyed by
such radiation.