Thanks, here is an expensive splitter:

digitalyacht.net/2014/03/27/isaf-offshore-special-regs-now-mandate-an-ais-splitter/
digitalyacht.co.uk/product/spl2000/

It is necessary to think carefully about how the splitter is being used. I would be concerned that using a passive splitter designed to split a very weak signal received from an aerial into 2 or more very weak signals that are received by 2 receivers that is then applied to share an aerial between 2 transceivers, or 1 transceiver and a receiver, may result in overloading the receiver or the second transceiver input when the first transceiver transmits. As the transmitter power is very much greater than the very weak signal from the aerial.

wongaga said..
Passive splitters will also introduce some signal loss. This isn't a problem if you're sailing around Sydney Harbor or Port Phillip, but if you're way out there on the blue, it could make the difference between hearing or being heard when you most need it.

point taken

tomooh said..
when I got a ais transciever I decided it was easier and better to just get a complete new aerial than to try to share the existing one at the masthead. Offshore you need redundancy and you don't want to be climbing the mast to try and get a broken connection repaired when with 2 aerials you could just swap over to the ais aerial or use the handheld and keep the ais running.

And carry an emergency antenna to stick on the pulpit or somewhere after the mast has been cut or carried away following a rig failure or 360.