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3.3 Installation of VHF / GPS Antennas
Interference to the Ship's VHF Radiotelephone
The AIS ship borne equipment, like any other ship borne transceiver operating in the VHF
maritime band, may cause interference to a ship's VHF radiotelephone. Because AIS is a
digital system, this interference may occur as a periodic (e.g. every 20 seconds) soft clicking
sound on the ship's radiotelephone. This affect may become more noticeable if the VHF
radiotelephone antenna is located close to the AIS VHF antenna, and when the
radiotelephone is operating on channels near the AIS operating channels (e.g. channels 27,
28 and 86).
Attention should be paid to the location and installation of the various antennas, in order to
support the antenna characteristics in the best possible way.
VHF Antenna Installation
Antenna Location
Location of the mandatory AIS VHF-antenna should be carefully considered. Digital
communication is more sensitive than analogue/voice communication to interference created
by reflections caused by obstructions such as masts and booms. It may be necessary to
relocate the VHF radiotelephone antenna to minimize interference effects.
To minimise interference effects, the following guidelines apply:
•
The AIS VHF antenna should have omni directional vertical polarisation providing 3 to 5
dB gain.
•
The AIS VHF antenna should be placed in an elevated position, as free standing as
possible, with a minimum of 2 metres in horizontal direction from constructions made of
conductive materials. The antenna should not be installed close to any large vertical
obstruction. The AIS VHF antenna should have a visible sky of 360°.
•
The AIS VHF antenna should be installed at least 3 meters away from interfering high-
power energy sources such as radar and other transmitting radio antennas, and out of the
way of the transmitting beam.
•
There should not be more than one antenna on each level. The AIS VHF antenna should
be mounted directly above or below the ship's primary VHF radiotelephone antenna, with
no horizontal separation and a minimum of 2 metres vertical separation. If it is located on
the same level as other antennas, the distance apart should measure at least 10 metres.
See also sample for antenna layout in the Appendix.
Cabling
The cable should be kept as short as possible to minimise attenuation of the signal. Double
shielded coaxial cables equal to or better than RG214 are recommended.
RG214 at VHF attenuation per meter of app. 0,07 dB/m (45m = 3,15db)
VHF AIS frequency app. 162MHz
All outdoor connectors on the coaxial cables should be fitted with preventive isolation, such
as shrink-stocking with silicone to protect the antenna cable against water penetration.
TM
GlobalWatch
AIS Installation Manual
2
6
Version 1.0
Y1-03-0183-1 Rev. A
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