OK, I'm back with more info, but unfortunately I left my notes on the boat, but I'll do my best to summarize.
I started again with the AIS and M504 antennas swapped, i.e. the specialized AIS antenna attached to the M504 and the 5225 attached to the AIS. Everything else was off at the breaker panel.
Results: Lots of targets all the way to Boston harbor nearly 25nm away.
Turned on the two VHFs and there was no loss of targets
On my handheld VHF I shut off the squelch and listened to 87 and 88, then turned on the AP. There was no change in the background noise on 87 and 88, and no loss of targets.
I then played with the SWR meter that I bought, but it indicated infinite SWR. I attribute it to operating well above it's 30Mhz limit, and disregarded the results.
Next I went to swap back the antennas, and this time I still had the AIS powered up, so I took another look at the LEDs. There were no errors indicated (and never have been), and the RX light was flashing every second or so which seems about right for the dozen or so targets I was picking up. I also observed a TX flash as my unit broadcast.
When I swapped the antennas putting the AIS antenna back on the AIS box, the RX flashes dropped way off. I still got a few, but I'd say it was 80% less than with the other antenna. When I got back to the plotter, targets were dropping, and the only one I ended up with was the closest one about a mile or 2 away.
I then measured the SWR on the 5225 antenna and got infinite SWR which is when I concluded the meter was operating outside it's range.
I then switched the antennas back (AIS antenna on M504, 5225 antenna on AIS) and all my targets came right back.
So today the only variable that impacted targets was which antenna I used on the AIS.
My thinking now is that my AIS antenna, cable, or connector is faulty and that I'm operating on a half-baked antenna that is so weak that the heal of the boat impacts it. Also, when I look back at all the things I did installing this system, there was plenty of room for error rigging
the antenna cable. I installed the PL-259 plug
, and also had to splice some extra length of RG8X to reach the AIS box. I could have screwed up either of those. I'm guessing that with the AIS antenna, it's just dumb luck when I pick up any targets, and dumb luck when I lose them. First it's only targets behind the boat, then it's the VHFs that kill targets, then it's the AP - I don't buy it anymore. I'll bet I could use the head
and drop targets (no pun intended).
The behavior reminds me of a digital circuit when there is a floating input and the circuit's behavior goes psycho. You chase signal integrity, power supply noise, timing, and all sorts of esoteric theories only to find someone forgot to tie off an unused input. Once fixed, sanity returns and the laws of physics once more apply.
A few other things:
I'm not pursuing any data communications
theories for now. Nothing is reporting any errors, and I can access the AIS via the plotter over ethernet without any issues. If those packets get through without issue, I have to believe that the target reports are too.
I have a detailed schematic of the whole system, both communications
and power. The engineer
in me couldn't resist. The system is actually quite simple. All the furuno
stuff is on Ethernet, and everything else is on N2K. The N2K attaches to one of the plotters, and it repeats all the data across the ethernet. The only place 0183 exists (good riddance) is for the VHFs since they don't support N2K, and that's handled by an N2K to 0183 converter. The N2K bus appears quite happy with zero transmission
or receive errors.
I re-ran the transceiver tests and GPS tests on the AIS, and all pass with flying colors. Today I was running only on the AIS GPS, and it pegged my location and the other boat locations without problem I could see a couple of tanker/freighters running the shipping
lanes in/out of Boston and they were right where they belonged. I also believe the error LED lights
or blinks if the AIS loses GPS lock.