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.