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Using a VHF Antenna Splitter or a Dedicated VHF Antenna on a Garmin VHF or AIS
Using a splitter
There are trade-offs between having a dedicated AIS antenna and sharing one with the existing VHF system through a splitter. A dedicated AIS-tuned antenna is an excellent choice. It will typically outperform a standard VHF antenna because it is tuned specifically for the high end of the VHF band where AIS is located. It can also serve as a backup antenna for the VHF. However, a dedicated antenna may be harder to install due to mounting and cabling runs. The GPS antenna location should be as high as reasonably possible but as far away from masts and other antennas, particularly other VHF antennas.
A splitter utilizes the current VHF antenna instead of fitting another one, which generally lends itself to a more straightforward installation. A splitter must be designed for AIS; likewise, if using a transponder, the splitter used is explicitly designed for use with AIS transponders.
However, the VHF antenna may not be well-tuned at AIS frequencies, leading to poor transmit performance. Check the antenna performance at AIS frequencies by looking at the VSWR curves provided by the antenna manufacturer, often found on the manufacturer's 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.
Limitations
Using a splitter also means it can't transmit on the VHF and receive AIS updates simultaneously. Transmission on the VHF radio on many boats is limited. The WatchMate® 850 or WatchMate® Vision coupled with the Vesper® Marine Antenna Splitter (SP160) indicates the AISWatchMate® screen when the VHF radio is used and if the VHF radio is used MIC or VHF radio is stuck.