Selection of the VHF antenna is critical to achieving the optimum receive and transmit range. We would recommend choosing an antenna that covers 156-162 MHz (Marine VHF & AIS) tuned to a Centre frequency of 159 MHz.
VHF radios operate at the other end of the marine band (156MHz), so for AIS as well via a splitter you will need to check that it performs across both frequency ranges.
Siting of an antenna is crucial as even a well suited antenna may not radiate a good signal if installed in a poor location. Put simply, keep the antenna upright, clear of anything that could touch it (rigging/other antennas etc), keep a reasonable distance from any other devices/antenna arrays like Radar/HF/VHF radio that may be operating at high power and also keep clear of any other metallic obstructions.