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

 

[William_H]

The clue is in the words "not less than" As said 46 cms would be a good 1/4 wavelength wire. Much shorter can be tuned to 160mhz so 38cms chosen as a limit on shortening antennae for the purpose of the regulations. ol'will

 

[Martin_J]

The cynical side of me, going back to when AIS transmission was first mandated within the offshore special regulations, was just thinking that this antenna length and height above the waterline were to prevent an all out lightweight boat/crew from simply installing an AIS transmitter with a short stubby antenna lying somewhere under the chart table..

Yes, they'd be transmitting alright, but wouldn't be very visible to either the race organisers or [offshore] to other competitors.

How good an antenna and feed cable you now choose to install depends on how visible you want to be and how far away you want to receive distant vessel transmissions from.