Here are a couple of articles you may find informative when you’re designing or troubleshooting your boat’s vhf radio/AIS antenna systems:
VHF antennas and their installation
Troubleshooting your vhf antenna system
They both highlight the importance of using good quality cable of the correct size, making good connections and using a top class antenna to extract the full potential from your radio.
You might be able to receive vhf signals on a bent coat hanger but you certainly can’t transmit effectively on anything except a proper antenna. As you transmit, the antenna coil heats up and unless it’s of substantial gauge with soldered joints and supported properly it will distort, degrading performance.
A top quality antenna will milk the full potential from whatever radio you have – hence the expression “a penny in the antenna is worth a pound in the radio”.
You can get a Metz antenna and a full range of accessories at the Salty John shop: www.saltyjohn.co.uk