Antennas too close together?

[lustyd]

Why not if they are on two separate antenna? If using a splitter yes the receive will be disabled for the AIS. If, as the OP says, he has two separate antennas then they need greater separation than he has at present. That was his question. the reason for the separation is to prevent damage to the AIS system, antenna included, when transmitting on VHF.

If they are close then the AIS won’t receive if VHF transmits because of the relative power. The weak AIS from other vessels and relatively weak signal from an AIS transmit will be drowned out with 25W of VHF. It’s like having a quiet conversation with someone a few metres away while standing next to a nightclub sound system.
While the signals can cope with some interference, they don’t cope well with massively more powerful signals right next door. Even properly spaced this is likely to affect it so a splitter has almost no real world downside.

 

[Alex_Blackwood]

If they are close then the AIS won’t receive if VHF transmits because of the relative power. The weak AIS from other vessels and relatively weak signal from an AIS transmit will be drowned out with 25W of VHF. It’s like having a quiet conversation with someone a few metres away while standing next to a nightclub sound system.
While the signals can cope with some interference, they don’t cope well with massively more powerful signals right next door. Even properly spaced this is likely to affect it so a splitter has almost no real world downside.

But the AIS is still working. My point is that is possible the AIS receiver, or transceiver, ( Not sure if OP said which it is) could be damaged by a strong VHF signal from an antenna in close proximity. I have experience of it happening. Albeit on much more powerful equipment but the separation was also much greater and professionally installed. Had been like it for years until one transmitter was used on full power and took out the Base tuning unit of the other antenna. Don't say it can't happen, unlikely, maybe. It's a bit late when you find out it can!

 

[lustyd]

But the AIS is still working.

Not while the VHF transmission happens, so from that perspective it's identical to a splitter. The VHF antenna will also create a blind spot in both directions for the AIS, so in that respect would be much worse than a splitter.
Yes, damage might be caused, but that's a secondary issue in my opinion since having two antennas close to one another is slightly worse than a splitter. In an ideal world we'd all have two masts at different heights, but since most of us don't, a splitter is the more sensible option for most.