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Old 29-05-2014, 14:59   #1
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One GPS Antenna, multiple receivers

I just bought an em-trak class B AIS transponder. It comes with it's own mushroom GPS antenna. I already have a mushroom GPS antenna installed for my chartplotter system. Do I need to install this second antenna for the AIS or is there some way I can use the antenna already installed for both by using a splitter or some other device?
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Old 29-05-2014, 15:43   #2
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Re: One GPS Antenna, multiple receivers

The AIS must have its own antenna/receiver by regulation. You may be able to feed the GPS data from the AIS to your chart plotter and remove the old mushroom. For now I would suggest you install the new mushroom and get everything working together. Then investigate the options for removing the old one.
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Old 29-05-2014, 16:18   #3
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Re: One GPS Antenna, multiple receivers

I have the same AIS unit, installed about a month ago. Works like a charm.

You do need a dedicated GPS antenna, though. It may be a bit of a hassle to install, but my suggestion is that you bite the bullet and just install it.

BTW, you CAN use the AIS antenna on the EM-Trak unit for other things. Mine feeds the chart plotter software (Rose Point Coastal Explorer) on my Win7 laptop via a USB connection.

Previously, I was using another GPS unit (Furuno GP-31) to feed both my VHF/DSC transceiver and the laptop. Don't need it for the laptop any more.

Bill
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Old 30-05-2014, 07:42   #4
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Re: One GPS Antenna, multiple receivers

Bill, what do you use the AIS antenna for in CE? Do you mean the AIS data?

Mark
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Old 30-05-2014, 08:28   #5
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Re: One GPS Antenna, multiple receivers

Most GPS antennas are active. They contain active components that are powered from the receiver. The lead from your GPS receiver to your antenna thus also carries some DC current.
You can imagine this could lead to trouble if you connect two receivers to the same antenna.
There exist splitters however that will decouple the DC feed on one of the outputs. You'll need something like that.
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Old 30-05-2014, 10:17   #6
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Re: One GPS Antenna, multiple receivers

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Originally Posted by colemj View Post
Bill, what do you use the AIS antenna for in CE? Do you mean the AIS data?

Mark
Mark,

Not just AIS data. The Em-Trak AIS unit has two antennas: a GPS antenna and a VHF antenna for reception/transmission of AIS data.

GPS location data derived by using the Em-Trak GPS satellite antenna, is passed to the laptop thru the same USB port as the AIS data. It is used to display my vessel's position in Coastal Explorer.

Here's a current screen shot: my boat is shown in red, and the green boats are emitting AIS signals.

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Old 30-05-2014, 11:12   #7
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Re: One GPS Antenna, multiple receivers

What KVB said, plus... I have done this in land-based facilities, but you need to make sure the feed voltage is the same. It's typically +5 vdc, but I have heard of some that require +12vdc. We use GPS as a timing reference and I had come across a situation where a technician had combined the two platforms requiring GPS timing to a single antenna when one had failed. He put a simple splitter between the two receivers. The receivers both worked fine, but we would get alarms because the circuits monitored current draw and since two receivers were outputting the same voltage, and both were expecting the full current draw, a low current alarm would come in because the current was being split between the two receivers. To fix this, I installed a DC blocker inline with one of the receivers. It's possible, but more likely that AIS receivers and chartplotters don't monitor for low current on the antenna anyway.
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Old 30-05-2014, 15:44   #8
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Re: One GPS Antenna, multiple receivers

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Originally Posted by btrayfors View Post
Mark,

Not just AIS data. The Em-Trak AIS unit has two antennas: a GPS antenna and a VHF antenna for reception/transmission of AIS data.

GPS location data derived by using the Em-Trak GPS satellite antenna, is passed to the laptop thru the same USB port as the AIS data. It is used to display my vessel's position in Coastal Explorer.

Here's a current screen shot: my boat is shown in red, and the green boats are emitting AIS signals.

Bill
Ah, that makes sense. You wrote AIS antenna but meant to write GPS antenna, so I thought you may have been talking about AIS data when you were really talking about GPS data.

All clear here now!

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Old 31-05-2014, 13:42   #9
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Re: One GPS Antenna, multiple receivers

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Originally Posted by El Rubio View Post
What KVB said, plus... I have done this in land-based facilities, but you need to make sure the feed voltage is the same. It's typically +5 vdc, but I have heard of some that require +12vdc. We use GPS as a timing reference and I had come across a situation where a technician had combined the two platforms requiring GPS timing to a single antenna when one had failed. He put a simple splitter between the two receivers. The receivers both worked fine, but we would get alarms because the circuits monitored current draw and since two receivers were outputting the same voltage, and both were expecting the full current draw, a low current alarm would come in because the current was being split between the two receivers. To fix this, I installed a DC blocker inline with one of the receivers. It's possible, but more likely that AIS receivers and chartplotters don't monitor for low current on the antenna anyway.
Instock make a splitter that does this built in.
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