Thread: AIS Questions
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Old 04-12-2021, 03:23 PM
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Originally Posted by cmb1998 View Post
I'm in the process of a complete electronics replacement on my 30ft fishing boat (old setup was mid 90's vintage). The boat has a fly bridge and then a main helm in the cabin. I'm planning to put a VHF at each station for redundancy. My plan was to use a dual VHF / AIS antenna for each radio, and wire them with main helm connected to the NEMA network and then the other would be wired separately and not on the network (but same radio so could switch out in a pinch if needed). My question - if I go this route, when you use the main helm VHF will it stop transmitting AIS? I've got two antennas on the radar arch (both are getting replaced), would I be better served by making one AIS only and the other VHF only? Thank you!
If you want to use a single antenna for VHF and an AIS transponder, you need an active antenna splitter or an AIS such as the Digital Yacht AIT5000 that has a built-in splitter. Then what happens is that the AIS will briefly stop transmitting while you have the microphone keyed to transmit on the VHF. It works seamlessly and you can't really tell that there is a pause in the AIS transmission.

Normally you would not have two AIS transponders on board because it would create lots of confusion to have two units sending out the data for a single MMSI. Two VHFs and two VHF antennas is a very important form of redundancy, however.

Maybe it will help you to describe how my boat is set up, which also has two helms. I have a VHF mounted at the main helm that has a wired remote mike at the tower helm. I have second VHF mounted down below that has a wired remote mike at the main helm. There are two 17 foot VHF antennas. One is connected only to the VHF at the main helm. The other is connected to the built-in splitter on my AIS transponder and shared between the AIS and the VHF mounted down below in the salon.

I don't think it is a good idea to try and set things up so you can "mix and match" the VHF radios and the antennas. You introduce a lot more connectors into the antenna wiring as well as needing a switch. All of that can lead to impairing the signal quality and increase the risk of failure. Modern VHF radios are extremely reliable if you get a quality unit such as Icom or Standard Horizon. They just don't fail very often.