Dear Boat Doctor,

I am adding an AIS transceiver to my boat. After a few close calls, I decided that not only do I want to know where other boats are, I’d like them to be able to see me as well. It looks like the AIS will work well with my chartplotter but I don’t know what I’m supposed to be doing with the antenna. I’d appreciate your guidance.

Nick Mankovich

Seattle, Washington

Dear Nick,

Automatic Identification System—used to locate and identify vessels by electronically exchanging data with each other—is great to have onboard. Each vessel transmitting an AIS signal is identified by name and its position, course and speed is displayed.  It is not a replacement for radar, which will display any boat it picks up, but a really nice supplement.

The system uses the marine VHF band to share data. In the simplest case, the AIS receiver uses its own dedicated antenna. This comes with its own complications though, as another antenna needs to be added and cable run through the boat. The higher the antenna, the better the system works, so this implies an antenna at the masthead. But you can’t really have two antennas on your masthead. Not only is there is not a lot of room for two, but the antennas will interfere with each other. If you want a dedicated antenna you’ll need to find somewhere else to put it.

An alternative is to add a VHF/AIS antenna splitter to share the antenna between the VHF radio and the AIS system. This is an active splitter, to allow each device to operate without interfering with the other.

A splitter makes the installation easier, but it’s not without compromise. The AIS and VHF can’t operate at the same time, so if you are transmitting on the VHF, the AIS can’t transmit or receive. In practice this doesn’t make much difference, since you really don’t transmit on the radio that often, especially for continuously sustained periods of time.

The other factor to consider is the insertion loss of the splitter. When you split a VHF antenna you lose a little bit of signal. This little bit can make a difference, and you can lose range on both your VHF and AIS. To counteract this, some of the AIS splitter manufacturers will add a small amplifier to overcome the loss. One of the better devices is the Vesper Marine AIS splitter (www.vespermarine.com).

To get the best performance, a proper antenna is needed. If you are using a dedicated antenna for your AIS, it is tuned to perform best at the at AIS frequency of 162MHz. If you are using a splitter, an antenna designed to operate efficiently at both the AIS and VHF frequencies will work best.