2.2 VHF Antenna Installation
Clasification Society 2024 - Version 9.40
Statutory Documents - IMO Publications and Documents - Circulars - Safety of Navigation - Circular SN/Circ.227- Guidelines for the Installation of a Shipborne Automatic Identification System (AIS) – (6 January 2003)Amended by SN/Circ.245 - Annex - Guidelines for the Installation of a Shipborne Automatic Identification System (AIS) - 2 AIS Installation - 2.2 VHF Antenna Installation

2.2 VHF Antenna Installation

Parent topic: 2 AIS Installation

2.2.1 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 in obstructions like 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 omnidirectional vertical polarisation.
  • The AIS VHF antenna should be placed in an elevated position that is as free 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 objective for the AIS VHF antenna is to see the horizon freely through 360°.
  • The AIS VHF antenna should be installed safely away from interfering high-power energy sources like radar and other transmitting radio antennas, preferably at least 3 m away from and out of the transmitting beam.
  • Ideally there should not be more than one antenna on the same level. The AIS VHF antenna should be mounted directly above or below the ship's primary VHF radiotelephone antenna, with no horizontal separation and with a minimum of 2 m vertical separation. If it is located on the same level as other antennas, the distance apart should be at least 10 m.

2.2.2 Cabling

 The cable should be kept as short as possible to minimise attenuation of the signal. Double screened coaxial cables equal or better than RG214 are recommended.

All outdoor installed connectors on the coaxial cables should be waterproof by design to protect against water penetration into the antenna cable.

Coaxial cables should be installed in separate signal cable channels/tubes and at least 10 cm away from power supply cables. Crossing of cables should be done at right angles (90°). Coaxial cables should not be exposed to sharp bends, which may lead to change the characteristic impedance of the cable. The minimum bend radius should be 5 times the cable's outside diameter.

2.2.3 Grounding

 Coaxial down-leads should be used for all antennas, and the coaxial screen should be connected to ground at one end.

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