AIS Antenna Selection for Class A and Class B

AIS antennas look like any other marine VHF whip, and in many ways they are. The difference is that an AIS transponder is listening and transmitting on two fixed channels, 161.975 MHz and 162.025 MHz, all the time. A poor antenna or a bad installation shows up as reduced reporting range, gaps in other vessels' plots and, on Class A sets, alarms that no one can explain.

Class A and Class B in brief

Class A transponders are fitted to vessels that fall under SOLAS carriage requirements and transmit at 12.5 W. Class B units are aimed at smaller commercial and leisure craft and transmit at lower power, typically 2 W, or 5 W for SOTDMA Class B. Because Class B has less transmit power to work with, antenna efficiency and mounting height matter more, not less.

Dedicated antenna or splitter?

A dedicated AIS antenna gives the best performance and keeps AIS working if the VHF radio antenna is damaged. An antenna splitter lets AIS share the VHF radio antenna, which saves space on small vessels but adds a component that can fail. On Class A installations and on any vessel that relies on AIS for collision avoidance, a dedicated antenna is the safer choice.

Choosing gain and length

• AIS23: 3 dB, 0.92 m stainless steel. Light, compact and suited to smaller craft or crowded masts.
• AIS74: 4 dB, 1.36 m glass fibre. A balanced choice for most workboats and pilot vessels.
• AIS76: 6 dB, 2.6 m fibreglass. Maximum range on larger, more stable vessels.

Higher gain flattens the radiation pattern. That increases range on a stable vessel but can lose signal when the vessel rolls hard. For small workboats that see heavy motion, 3 to 4 dB is usually the better compromise.

Mounting height and separation

Range at VHF is largely line of sight, so height wins. Mount the AIS antenna as high as practical, clear of metal structures and away from radar scanners. Keep it separated from the VHF radio antenna, ideally by vertical spacing on the same mast, to reduce interference when either transmitter is keyed. Follow the transponder manufacturer's guidance on minimum spacing.

Remember the GNSS antenna too. Class A transponders require their own GNSS input, and that antenna needs a clear view of the sky. Our guide to marine AIS, GPS and VHF systems covers how the three fit together.

Mounts and cable

Use stainless steel rail and mast mounts rated for marine use, and keep cable runs as short as the installation allows. Every metre of coax adds loss, which reduces both transmit range and receive sensitivity.

Frequently asked questions

Can I use a standard marine VHF antenna for AIS?

Yes, provided it covers the AIS channels at 161.975 and 162.025 MHz. Many marine VHF antennas cover 156 to 162.5 MHz. A dedicated AIS antenna tuned to the band gives the best match.

How high should an AIS antenna be?

As high as practical. VHF range depends largely on antenna height, so moving the antenna up the mast often improves range more than choosing a higher-gain model.

Does a Class B transponder need its own GNSS antenna?

Most do, either built in or as an external antenna. Check the transponder documentation, and mount the GNSS antenna with a clear view of the sky.

For help selecting AIS antennas across a fleet, see the maritime range or contact the team at sales@renair.co.uk.