Shakespeare

Choosing antenna length and gain

A longer, higher-gain antenna concentrates more signal into a narrower vertical beam, which suits a stable powerboat running mostly upright, while a sailboat that heels under sail does better with a shorter, lower-gain antenna whose wider beam keeps working as the mast leans over. Mounting height affects VHF range more than antenna length does, since range is largely limited by the radio horizon between the two stations.

Cable run matters too: signal loss increases with cable length and rises sharply if a low-quality coax is used over a long run from a masthead antenna down to the radio, so a low-loss cable is worth the extra cost on longer runs. AIS can usually share a VHF antenna through a splitter rather than needing a second antenna, provided the splitter is rated for both the VHF and AIS frequencies in use. Get in touch if you are unsure which length suits your boat.