AIS Reception Issue

[twistedtree]

I'm having troubles with AIS reception and would welcome any guidance on how to best diagnose the problem. All the electronics are new, so this is not something that has become broken over time. Right now, my belief is that it's antenna location/interference.

Here's some more background/detail:

- The biggest clue is that if I have the AIS on, but both VHFs turned off, the AIS appears to work well picking up plenty of targets in all directions.

- With the VHFs turned on, I lose all the targets, with only occasional detection of targets behind the boat. Note that I lose the targets even though I am not transmitting at all on the VHF. All it's doing is receiving.

- I've tried the VHFs individually, and having either one powered on causes the AIS targets to be lost.

- The antenna layout is not optimal, but I think it's rare that optimal layout can be accomplished. Realistically, it's an exercise in finding the best tradeoffs, but so for I haven't found that happy place. Here are the details on the layout.

- First, this is a power boat with an arch. In the middle of the arch is a mast about 3' tall housing the radar, and over that a Sat dome.

- There are two VHFs, each with an 8' antenna on either side of the flybridge about mid-ships. The antennas are about 10' apart, but in the same horizontal plane (which ideally they shouldn't be). Adjacent to each antenna, going up from the mount, the first two feet is the flybridge side (glass), the next 4 feet are bimini frame tubes, and the last 2 feet are in clear space. I know this is sub-optimal, and my plan is to raise the antennas using extension poles, but I don't have a good way to support the extensions, and until I do my mounts are only rated for the 8' that I have.

- The AIS antenna is a 4' Shakespeare AIS antanna mounted on the port side of the arch. It is about 8'-10' aft of the port VHF antenna, and it has a 4' extension pole. It's mounted such that the base of the antenna (not the base of the extension pole) is level with the top of the VHF antennas, so they are not in an overlapping plane.

Questions and thoughts so far.....

- With no transmissions on the VHFs, how could this be interference between the VHF and AIS antennas?

- In looking at it yesterday, I realized my bimini frame is not grounded, yet it's clearly in the line of fire of the VHFs. Could this be setting up some sort of Faraday cage that shields the AIS, even though it's all below the AIS antenna? The bimini, after all, is all forward of the AIS antenna, and the only place I pickup targets with the VHF on is aft. Have I inadvertently created an AIS cloaking device?

- Even though the VHFs are on a dedicated, and separate breaker from the AIS, could there be interference carried across the DC power lines?

- I'll have to double check, but I don't think the AIS antenna cable runs alongside either of the VHF antenna cables.

Again, I'd welcome any thoughts on how to diagnose what's going on, experiments to run, etc. So far my next step is to rig a ground wire to the bimini to see if that has any effect.

Thanks in advance. I'm an electrical engineer, but to me all this radio stuff is still witch craft.