I ran a few experiments today, and here are the raw results:
- Connected the AIS system to the ICOM antenna, and left both VHFs disconnected from any antenna and powered off.
-> Results, LOTS of targets. I'd say more and further away than ever before. I picked up several boats 12+nm away, and even picked up the stationary target on Deer Island at the entrance to Boston outer harbor. The targets were in a wide variety of directions.
- Reattached the VHF antennas. Ray on it's antenna, and ICOM on the AIS antenna. Ray powered on, ICOM powered off. Ray on 13 to hear the bridge chatter.
-> Results, after a few minutes all the targets were still active. The MDF12 puts an X through the target when it doesn't report in as expected, and eventually drops it from the display, but I have to wait a few minutes before they start going MIA. CH 13 chatter is clear.
- Turned on the ICOM, set Ray to 16 and ICOM to 13.
-> Results, still no loss of targets. But in some cases I was only hearing the bridge operator and not the boats he was talking to. But I wasn't paying a lot of attention to the chatter and can't really say if it was an isolated case.
At this point I was running out of time so from here on I was a bit rushed, but here's what happened.
- I turned on the Autopilot
breaker. This breaker powered the AP brain (Simrad AC42) and the hydraulic pump. The control panels
(2 x AP24) are connected to and powered by the N2K bus, but both were turned off.
=>Results, after a minute or so targets started dropping.
- Shut off the AP breaker.
=> Results, Targets all came back
- Turned AP breaker back on
=> Results, targets start dropping again.
So, what to make of all this? Here's my interpretation so far:
1) I think I have a problem with the AIS antenna or cable. I conclude this because after placing the AIS on the ICOM antenna I picked up targets much further away than ever before, with or without VHFs on.
2) I'm not sure what to make of the AP behavior. It might be that the AP is the real source of interference
, but I think it's more likely that it's complicit along with the VHFs and antennas in some way. When I first discovered the VHF impact on target reception
, I can't say for sure if the AP was on or off, but I know turnign the VHFs on and off impacted reception
of targets, so I think there is some combination if things at play.
The good news is that now I know how to pickup a handful of targets which of course if critical to narrowing down the minimum set of circumstances to reproduce the problem.
My next step will be to try various combinations of AP and VHFs to figure out the minimum number of things that need to be powered on to reproduce the problem.
This remains a real puzzler, but having a reproducible set of circumstances makes me confident I'll sort it out soon.