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Notes on the use of AIS units

GPS antenna
Comar Systems stipulate that for the CSB200 transponder the GPS antenna should not be shared with any other device.  For this reason we bundle the recommended antenna with the AIS transponder.

VHF antenna for AIS
There are generally two options concerning the antenna used for an AIS transceiver:
         • a separate AIS antenna mounted on the pushpit
         • a masthead VHF antenna shared with the marine radio using a splitter

Comar Systems advise us that the standard marine VHF antenna is tuned to the mid band of the VHF range 156.00 – 162.00 and performance at AIS frequencies 161.975/162.025 tends to be reduced.  AIS antennas are tuned to 162.00.

Due to the curvature of the earth it is necessary to have VHF antennas as high as possible to obtain good range.  A VHF antenna at the masthead with a range of up to 25 miles compares with only around 6 miles for an antenna mounted on the pushpit.

Comar specify that if a separate antenna is used for the AIS transceiver there should be a vertical separation of at least 2 metres between this and the marine VHF radio antenna. This is because if two separate antennas are used, interference is likely if they are mounted side by side.  As range is very important for the marine VHF radio when it is used in emergency situations this restricts the options for an AIS antenna on a single masted yacht.

Weatherdock has developed the EasySplit OCB splitter specifically for use with AIS transceivers.  This splitter enables an AIS transceiver and plotter, marine VHF radio and fm radio to all share the same antenna. 

The EasySplit OCB splitter operates by detecting and momentarily breaking any transmission signal on the marine VHF radio to allow the AIS Class B transmission which occurs at 30 second intervals.  The data in the AIS transmission is sent digitally and the interruption to the VHF transmission is usually no more than a few milliseconds.  Echopilot, who distribute the EasySplit OCB, tell us that the break in transmission is so quick that it is not usually audible.

We have set up systems using the OCB splitter and a standard VHF antenna at the masthead and have received AIS data from vessels at distances of over 20 miles - a significant number of our customers are using this arrangement.

The Comar splitters are designed fail safe and the vhf radio will transmit when there is no power to the splitter.