User's Manual

LD2103 V2.8 Page 34 of 36
SRT Marine
Instruction Manual
SRT-MTB
©
Class B Marine AIS
Appendix A
Antennas and Antenna Mounting
GPS Antenna
The GPS antenna used must be of the active type (i.e. it should incorporate an LNA) and must be suitable for marine shipboard
applications (index of protection, ruggedness, means of mounting, etc.). An antenna should be selected with a gain (in dB)
depending on the length of cable between the antenna and the AIS unit; after subtraction of cable and connector losses a
minimum total gain of 25 dB should be available at the SRT-MTB unit GPS antenna connector.
The GPS antenna to be used for AIS use must be a dedicated antenna, i.e. not shared with any other GPS receiver.
Installation of the GPS antenna is critical for the performance of the built in GPS receiver which is used for timing of the
transmitted time slots and for the supply of navigational information should the main navigational GPS fail. We strongly
recommend that:
•
The GPS antenna is mounted in an elevated position and free of shadow effect from the ship’s superstructure
•
The GPS antenna has a free view through 360 degrees with a vertical angle of 5 to 90 degrees above the horizon.
•
As the received GPS signal is very sensitive to noise and interference generated by other onboard transmitters, ensure
that the GNSS antenna is placed as far away as possible from radar, Inmarsat and Iridium transmitters and ensure the
GPS antenna is free from direct view of the radar and the Inmarsat beam.
•
It is also important that the MF/HF and other VHF transmitter antennas are kept as far away as possible from the
GNSS antenna. It is good practice never to install a GNSS antenna within a radius of 5 meters from these antennas.
VHF antenna for AIS use
The VHF antenna employed for AIS use:
•
Must be a dedicated antenna, i.e. not shared with any other VHF transmitter/receiver.
•
Must be suitable for marine shipboard applications (index of protection, ruggedness, means of mounting, etc.)
•
Should be omni-directional and vertically polarised with unity gain (0 dB) with a bandwidth sufficient to maintain VSWR
<1.5 over the frequency range 156 – 163 MHz. As a minimum the 3dB bandwidth must cover the two AIS channels
and the DSC Channel.
•
Should be mounted with at least a two metre vertical separation distance from any other VHF antenna used for speech
or DCS communication but see also the section “Radio Frequency Exposure Warning” below.