The AIS system works on two channels in the VHF frequency range and in order to receive or transmit AIS information it is necessary for the AIS unit to be connected to a VHF antenna. One option is to fit a VHF splitter such as our SPL2000, which allows both the VHF radio and the AIS to use the boat’s main VHF antenna. The other option is to fit a second VHF antenna which is then just connected to the AIS.
Any VHF antenna can be used, although some antennas are “AIS tuned” to get maximum gain on the two AIS channels and also have a BNC type connector fitted (common on AIS units), rather than the traditional PL259 VHF connector.
If you decide to fit an additional VHF antenna for the AIS, you want to mount it as high as possible but you should be aware that it cannot be mounted right next to the existing VHF antenna and a certain amount of separation must be maintained to avoid the 25 watts of transmit power from your VHF radio going straight in to the AIS receiver circuitry, potentially damaging it. The diagram below shows the recommended minimum separation of the antennas.