The boat’s VHF or AIS no longer works (or has poor range)

The VHF is the safety equipment you use least… until the day you need it. The AIS works silently: you often discover it had stopped transmitting when another boat says it cannot see you. In both cases, the fault is rarely in the set itself: it is most often in the power supply, the antenna or the cable that runs up the mast. This guide helps you look in that order, safely. The manufacturer’s manual for your radio and your AIS always has the last word.

1. What you notice

• The VHF does not switch on, or switches off when you press the transmit key.
• People hear you badly or not at all, or you only hear stations very close by.
• The radio shows an antenna alarm or gets hot when transmitting.
• Other boats cannot see you on AIS, or you no longer see anyone.
• The digital calling key (DSC) does not work or asks for an MMSI number.

2. Safety first

Never test the distress key. A distress call, even one sent by mistake, mobilises the rescue services: if one goes out, cancel it immediately by voice on channel 16, following the procedure in your manual. Do not transmit without an antenna connected, as this can damage the transmitter, and do not touch the antenna while transmitting. Before working on the wiring, switch off the power at the panel and beware of short circuits on the battery side. Finally, antenna connectors are often at the masthead: never go up alone, never at sea, always with a harness and a competent crew member on the winch, in calm weather — or take advantage of the mast being unstepped at the yard.

3. Likely causes, from the most common to the rarest

• A power supply that is too weak — when transmitting, a VHF draws much more current than when listening: a low battery, a corroded lug or a cable that is too thin is enough for it to switch off or have poor range as soon as you press the key (see batteries that run down).
• A fuse or breaker — on the panel or on the set’s power lead, sometimes hidden behind the console.
• The antenna cable — the coaxial cable running up the mast lets water in, gets crushed at the deck entry or at the mast foot, or is cut when the mast is unstepped: reception and transmission drop sharply.
• Corroded connectors — at the masthead, at the mast foot and behind the radio: a green or damp connector loses a large part of the signal.
• The antenna itself — whip bent, broken or torn off by a halyard, cracked base; a lightning strike can also destroy it.
• The VHF/AIS splitter — when one antenna serves both, a unit shares it between them; if it is badly powered or faulty, one or the other (or both) performs poorly.
• An MMSI number that is missing or wrongly entered — without it, digital calling (DSC) does not work and an AIS transponder does not transmit. A wrongly entered MMSI is often hard to correct yourself: many sets only accept the entry once.
• No GPS position for the AIS — an AIS transponder needs its own position: with the GPS antenna disconnected, damaged or badly placed, it stops transmitting or transmits without a position.
• A fault in the set — rarer: frozen screen, transmitter out of action after water ingress or a voltage surge.

4. What you can check yourself, safely

• The voltage — with a multimeter, at the battery terminals, then, if it can be reached without dismantling anything, where the radio’s power supply arrives. A voltage that collapses at the moment of transmitting points to the power supply.
• Fuses and breaker — power off: by eye, a blackened fuse or an open breaker.
• Accessible connectors — power off: behind the radio and at the mast foot, look for corrosion, damp or a loose connector. Note what you see.
• The antenna seen from the deck — with binoculars from the pontoon: antenna straight, present, base intact. An emergency antenna, mounted on the pushpit, lets you compare in a few minutes.
• The MMSI number — in the radio’s and the AIS’s menu: it must be present and match exactly the one on your station licence.
• The AIS GPS position — in the set’s menu: position displayed, number of satellites, status of the GPS antenna.
• A proper radio check — call the harbour office or a friendly boat on a working channel agreed in advance, not on channel 16, and ask how you are received; try again a few miles away. Some radios offer a digital test call to another MMSI number: follow the procedure in your manual. Never the distress key.
• AIS visibility — ask a neighbouring boat whether it can see you on its screen, or check an online AIS tracking site: some signals only appear there after a delay, and only in areas covered by shore stations.

5. When to stop and call a professional

Call in a marine electronics technician if the radio switches off when transmitting despite a good voltage, if the antenna alarm persists, if the coaxial cable or the mast connectors need replacing, if the MMSI needs correcting or clearing, if the VHF/AIS splitter is at fault, or after a lightning strike. A professional measures the standing wave ratio (what comes back from the antenna instead of going out), checks the cable along its whole length and sets up the AIS correctly. As for regulations, carrying a VHF depends on the navigation area and the type of boat, and using a fixed VHF or an AIS generally requires a station licence and an operator’s certificate: in France, check with the Agence nationale des fréquences (the national frequency agency), and see mandatory safety equipment.

6. Stopping it from happening again

Do a radio check at the start of the season and before every long passage. Protect the mast-foot connectors from water, and have those at the masthead checked whenever the mast is unstepped. Keep a charged handheld VHF on board as a backup. Check your MMSI after any work on the electronics, and keep the station licence on board. Before a long voyage, the guide preparing a transatlantic crossing goes through all the communication equipment.

Record in the Hullteam log every check, every replacement of an antenna, cable or connector and every AIS setting, with the date and the invoice: it is this history that will make the boat’s value at resale — see which proofs count.