T&T: AIS

[Peter Bennett]

Sunday, January 7, 2007, 2:15:39 PM, Dave wrote:

DC> <Milt (and others) wrote: I think you have something wrong.>

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

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

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

According to info I've found, and AIS transmitter should be 12.5 watts
- half the power of a voice transmitter.

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

No - it is a receive issue.  The ships transmit regularly, every few
seconds, whether they receive anything from you or anyone else.

>From other messages in this thread, I suspect that the cable from the
3 dB antenna to the AIS receiver is faulty - disconnect it from both
the receiver and the antenna, and measure the resistance between
center conductor and shield at either end - the resistance should be
infinite. Also, if possible, measure the resistance of the center
conductor between the ends of the cable - that should be almost zero.
If you can't get the ends close enough to do this measurement, have
someone short center to shield at one end while you measure center to
shield at the other - you should read almost zero resistance.



--
Peter Bennett, VE7CEI    Vancouver, B.C., Canada
Lien Hwa 28 (AKA Polaris 30) "Sea Spray"
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca