AIS (Automatic Identification System) is a technology that has revolutionised collision prevention at sea.  

More recently, the advent of satellite AIS receivers permits tracking vessels on ocean passages. This allows Rescue Coordination Centres to not only see a vessel in distress, but also those in the vicinity who may be able to assist.

At Pacific Maritime Radio, we also have global satellite AIS coverage, and we use it extensively to monitor recreational vessels who have purchased our Passage Guardian service.

The most common configuration is for the VHF radio and AIS transponder to share a single antenna fitted to the mast head.  A splitter is used to connect the two devices to the antenna.

There are a number of problems with this configuration.  

  • The antenna deployed is usually optimised for the VHF radio but not AIS.  This means there is a mismatch for the AIS transponder, and not all of the energy is transmitted or received.
  • The coaxial cable is a long run, and is often of an inferior type, or is aged and therefore is not transferring all of the energy.
  • A fault developing in the cable or the mast head antenna is almost impossible to correct at sea.
  • Not all splitters are the same, and some introduce losses on both transmit and receive.
  • If you lose your rig in heavy weather, you have no VHF radio or AIS.

So, what to do?  Here's what I suggest.  Do them all, otherwise you may not observe a significant improvement.

  • Replace the mast head antenna with an antenna designed for both VHF radio and AIS.
  • Replace the coaxial cable with Ancor RG-213 marine grade coaxial cable.
  • Replace your splitter with a Vesper SP-160 splitter.  Trust me on this.
  • Add a second antenna of the same type to your radar arch or mounted on a pole aft and use a coaxial switch to select either the mast head or radar arch antenna.  Use the same type of coaxial cable as you used for the mast head.

If a fault develops in the masthead antenna system, or the rig is damaged / lost, you can quickly switch to the antenna on the radar arch.  Both VHF radio and AIS remain operational.

You also have the option of running the VHF radio on the mast head and the AIS transponder on a dedicated AIS antenna on the radar arch if you discover that range is acceptable.

If a fault develops with the radar arch mounted antenna, it is within relative easy reach, and so more likely to be able to be repaired at sea (on a calm day!)

When Vesper Marine's new Cortex Class B SOTDMA smartAIS transponder with integrated splitter becomes available, it has output power of 5 watts instead of 2 watts.

The combination of all of these technologies will optimise your vessel AIS for both collision avoidance and satellite visibility.

Disclaimer:  Pacific Maritime Radio does not receive any financial reward for endorsing products from Vesper Marine.  We use their products in our maritime coast station and experience tells us they deliver excellent results.