VHF/AIS antenna length in World Sailing Offshore Special Regs

[merjan]

World Sailing Offshore Special Regs has a length requirement on the VHF/ AIS antennas, cfr 3.29.6b:

“…has a dedicated AIS antenna not less than 38 cm (15”) in length mounted with its base not less than 3 m (10’) above the waterline …”

Same length is required for VHF antennas. That rules out the common arrangement of stubby AIS antennas mounted on the push pit.

I can’t claim to fully understand what size antennas would be suitable for 162MHz waves but my guess would be an integer fraction of 1.85 metres. There is no 38cm / 15” AIS antenna that I could find in the market. So that number looks very arbitrary to me.

I was wondering if anyone could explain the reason behind the 38cm specification.

 

[Plum]

There is no 38cm / 15” AIS antenna that I could find in the market. So that number looks very arbitrary to me.

There are several on the market. For example Shakespeare RIB & Raceboat VHF Antenna | Force 4 Chandlery. AIS is within the marine vhf frequency range.

 

[wonkywinch]

A quarter wave whip for 160Mhz would be 46cm so maybe the rules written to ensure min antenna spec is a quarter wave instead of a rubber duck for better transmission over longer distances (that would certainly be the advantage of mounting it higher up).

 

[merjan]

Thank you for the replies! I somehow missed that product, and 15” doesn’t seem to have much to do with AIS wavelengths then!

 

[st599]

By adding a loading coil, you can shorten the physical antenna. Short antennas typically radiate more skywards, are harder to match to a transmitter and are more susceptible to the effect of nearby metallic objects