T&T: AIS

[Milt Baker]

Dave,

You're moving in the right direction!  I still believe the problem is in your antenna and/or antenna cable and/or cable end termination.  By 
jumpering and using your 9db gain antenna and getting the signals, I think you've answered your own question.  Certainly in a less expensive  
AIS receiver (which seems to be what you have), receiver sensitivity will be an issue.  Pair limited receiver sensitivity and a wimpy 3db gain  antenna, and you get . . . well, you know what.

Since I now have both a Nobeltec AIS receiver (built by someone in Europe and marketed there under a different name) and a Furuno AIS 
transceiver, I can compare what the two of them receive.  The Furuno, a far more expensive unit with considerably better receiver sensitivity, 
gets more AIS signals and from farther out with a 3db gain antenna than the Nobeltec does with its 6 or 9db gain antenna (I forget which).  
Receiver sensitivity is the difference--at least that's my take. 

The problem Luis Capriles helped me solve in Venezuela last year was that I was getting a few AIS signals but only those very close.  In the 
end, we found that my installer had used a cell phone antenna (go figure!) for the AIS.  We also found that my installer had put an "extra"  
PL259 connection in the antenna line, which further attenuated the incoming signal.  As soon as we installed a new Shakespeare VHF antenna 
Luis had in stock and got rid of the extra connection, things got much better--that's when I started receiving signals from ALL the ships in the 
PLC anchorage.

Bottom line: your receiver is probably up to the job but only if you're willing to give it the mate it needs: an excellent VHF antenna.

And that's my $.02 worth.

--Milt 

Dave wrote: 

After listening to several folks re my lack of receiving many AIS signals
from ships close by issue I made up a coax jumper and tied the AIS receiver
into my 9db antenna vhf antenna. Bingo ships out to 20 miles are showing up.

So I tried calling several of the ships at 3-5 miles with my handheld on one
watt with the antenna of the handheld about 11' off the water. Contact was
made with no issue at all. This to the same ships that don't appear on the
AIS with a 3db antenna at 17'.

Does anyone know what the power of the AIS transponders is? I sure would
think that the transmit signal would reach 3 miles even to a 3db antenna at
that height...especially if a handheld at 1 watt can communicate with them
without problems.

Perhaps I have a bad receiver.....but if it works for a few ships as it does
with the 3db antenna and many more with the 9db antenna I  would rather
think it is a transmit issue.