Should I use a splitter or a dedicated VHF antenna?

There are trade-off's between having a dedicated AIS antenna and sharing one with your existing VHF system through a splitter. A dedicated AIS-tuned antenna is an excellent choice and will typically out-perform a standard VHF antenna due to being tuned specifically for the high end of the VHF band where AIS is located. It can also serve as a backup antenna for your VHF. However, a dedicated antenna may be harder to install since you need to mount it and run the cable, etc. You should place it as high as reasonably possible but as far away from masts and other antennas, particularly other VHF antennas.

A splitter allows you to use your current VHF antenna instead of fitting another one which will generally lend itself to an easier installation. You'll probably get the best performance out of your AIS system this way too because your existing antenna is likely to be mounted up high. If you decide to get a splitter, you must use one designed for AIS. If you have a transponder you must use a splitter designed specifically for use with AIS transponders.

However, your VHF antenna may not be well tuned at AIS frequencies which will lead to poor transmit performance. You can check your antenna performance at AIS frequencies by looking at the VSWR curves provided by the antenna manufacturer (they can sometimes be found on the manufacturers website). Look for a VSWR of 2:1 or less at 162MHz. Most VHF antennas are well tuned for 156MHz but will start to degrade at the extremes of the VHF band.

Using a splitter also means you can't transmit on your VHF and receive AIS updates at the same time. In practice this usually isn't a big deal since on many boats transmitting on the VHF radio is limited. The WatchMate 850 or WatchMate Vision coupled with the Vesper Marine Antenna Splitter (SP160) gives you an indication on the AISWatchMate screen when the VHF radio is being used. This will also indicate if your MIC or VHF radio is stuck.

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