More VHF and AIS range problems come from the antenna, its cable, and its connectors than from the radio itself. A brand new set fitted to a tired coax run with a corroded PL259 will transmit and receive noticeably worse than a modest radio on a properly made installation, and the fault often does not show up as a total failure, it shows up as reduced range that gets blamed on the wrong thing.
This guide covers antenna siting, coax choice, connector work, VSWR checking, and the decision between an AIS splitter and a second dedicated antenna, the areas that decide whether a VHF or AIS installation performs to its potential.
VHF range is overwhelmingly a function of antenna height above the water on both ends of the conversation, not transmitter power. Mounting height at the masthead on a sailing boat gives noticeably better range than a rail-mounted antenna on the same boat, which is why masthead VHF antennas are the default on cruising yachts.
On a motor boat without a mast, the highest practical point clear of other antennas, radar, and structure that would sit in the pattern is the right choice. Keep separation from radar scanners in particular, and avoid mounting directly beside another vertical antenna where the two can detune each other.
Cable loss matters more than most owners expect, especially on longer runs to a masthead antenna. A low-loss coax such as RG213 or an equivalent marine-grade cable is worth the extra cost and slightly larger diameter on any run over about ten metres, thinner cable like RG58 loses a meaningful proportion of signal over a long masthead run and should really only be used for short connections.
Route coax away from other cabling where practical, avoid tight bends that exceed the cable's minimum bend radius, and support it properly through the mast and below decks so its own weight and boat motion do not eventually work a connector loose or crack the cable jacket at a fixed point.
PL259 connectors are the standard for VHF but they are not weatherproof on their own and they are not difficult to get wrong, a poorly soldered or crimped centre pin, or braid strands bridging to the centre conductor, causes intermittent faults that are maddening to trace later. Self-amalgamating tape over the joint, applied properly and reapplied when it perishes, is what actually keeps water out over years of use, not the connector itself.
Where the antenna cable uses an N-type connector, common on some AIS and higher frequency equipment, the same principles apply, correct strip lengths, no stray strands, and a proper weatherproof seal at any exposed joint. Never leave a coax joint unsealed and assume it will be fine because it is under a deck fitting, capillary action will still pull water in along the braid.
Voltage standing wave ratio tells you how well the antenna, cable, and radio are matched, and it is the single most useful test after any antenna installation or suspected fault. A VSWR meter or a radio with a built-in VSWR function, checked with the antenna in its normal working position, should read low, typically under about 1.5:1 on a healthy VHF installation, rising VSWR points to a damaged antenna, water ingress in a connector, or a cable fault.
It is worth checking VSWR as part of any commissioning, not just when there is already a suspected problem, a baseline reading taken when everything is known to be good makes it far easier to recognise deterioration later.
AIS can share the VHF antenna through a splitter, which switches priority to VHF transmission when the radio keys up and otherwise lets AIS receive and transmit through the same antenna. This works well on most leisure boats and avoids a second masthead cable run.
A dedicated AIS antenna, lower down and separate from the VHF, is worth considering where VHF traffic is heavy enough that splitter switching could plausibly clash with an AIS transmission, or where running a second cable is straightforward anyway during a refit. For most cruising boats a good quality splitter on a well-installed masthead antenna is a perfectly sound choice.
Reduced range after a period of normal use usually points to water ingress in a connector, a coax joint that has corroded, or antenna damage, rather than a fault in the radio itself. A VSWR check will usually confirm whether the antenna and cable are the problem.
Yes, with a splitter designed for the purpose. It prioritises VHF transmission and shares the antenna the rest of the time, and is a well proven approach on leisure boats.
As a rough guide, under about 1.5:1 indicates a healthy match on a typical VHF installation. Readings climbing towards 3:1 or higher point to a fault worth investigating before it gets worse.
Yes, height above water is the dominant factor in VHF range because the signal is largely line of sight. A masthead position on a sailing boat typically gives significantly better range than a lower mounted antenna, all else being equal.
A visual check of exposed connections and their weatherproofing each season is sensible, along with a VSWR check if you notice any change in range or if the boat has had rig work that involved disturbing the cable.
We check VSWR, inspect coax and connectors, and fit or repair VHF and AIS antenna systems properly, not just swap the visible parts. Get a free assessment.