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Old 24-11-2020, 22:48   #1
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Splitter for GPS antenna?

I am adding an AIS transponder on my boat and I am wondering if I could use for it the same GPS antenna that my Garmin GPS plotter is using?

Looking for splitters on the net I found a few quite expensive boxes (circa $300) but also some cheap Y cables.

Would that work?

My GPS antenna cable is fitted with a male BNC plug that goes in the female BNC socket in the Garmin plotter.

The AIS transponder also has a female socket (TNC, not BNC, but I have an adapter).

So it looks like I would need a female to dual male BNC cable. I found one on eBay. Most BNC Y cable are the reverse: male to dual female. Apparently they are meant to be used with video.

So I am puzzled. What is my best option?

For completeness, the plotter is a Garmin 276C Plus, the AIS transponder is an Emtrak B954
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Old 25-11-2020, 00:58   #2
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Re: Splitter for GPS antenna?

Connect ais to Garmin through nmea network.
The GPS data will be provided by the plotter and also the ais will send it's data to plotter. You will see ais targets on plotter
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Old 25-11-2020, 02:19   #3
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Re: Splitter for GPS antenna?

I thought that all transponders had internal GPS receivers. Are you sure that you need an external GPS for yours?

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Old 25-11-2020, 02:20   #4
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Re: Splitter for GPS antenna?

Quote:
Originally Posted by jdmuys View Post
I am adding an AIS transponder on my boat and I am wondering if I could use for it the same GPS antenna that my Garmin GPS plotter is using?

Looking for splitters on the net I found a few quite expensive boxes (circa $300) but also some cheap Y cables.

Would that work?

My GPS antenna cable is fitted with a male BNC plug that goes in the female BNC socket in the Garmin plotter.

The AIS transponder also has a female socket (TNC, not BNC, but I have an adapter).

So it looks like I would need a female to dual male BNC cable. I found one on eBay. Most BNC Y cable are the reverse: male to dual female. Apparently they are meant to be used with video.

So I am puzzled. What is my best option?

For completeness, the plotter is a Garmin 276C Plus, the AIS transponder is an Emtrak B954

By rights AIS transponders are meant to have a dedicated gps feed , NV the Emtrak 950 series has an integral GPS receiver and antenna , an external antenna is only needed of the unit is buried deep in the boat
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Old 25-11-2020, 02:21   #5
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Re: Splitter for GPS antenna?

Quote:
Originally Posted by sebba View Post
Connect ais to Garmin through nmea network.
The GPS data will be provided by the plotter and also the ais will send it's data to plotter. You will see ais targets on plotter
Class B transponders will generally not have an ability to accept external nmea based gps sentences.
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Old 25-11-2020, 08:09   #6
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Re: Splitter for GPS antenna?

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Originally Posted by goboatingnow View Post
Class B transponders will generally not have an ability to accept external nmea based gps sentences.
A prominent vendor of AIS devices notes in his FAQ section:

Quote:
Q: Can I use my existing GPS for my new Class B AIS transponder?

A: No. While this should be technically feasible, the AIS Class B specs - unlike the Class A specs - require the AIS transponder to use its own GPS system. Therefore you need to use the GPS antenna provided with the transponder. In some cases the GPS antenna is embedded in the transponder unit. These units can also use an external antenna if needed. Some transponders come with a GPS antenna. Others have a GPS antenna connector but the antenna is not included and therefore needs to be ordered separately. Be sure to check this when you order.
Source: https://www.milltechmarine.com/Class...AQs_ep_62.html

To more be more authoritative, an actual ITU recommendation should be cited. Perhaps someone can provide that.
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Old 25-11-2020, 08:25   #7
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Re: Splitter for GPS antenna?

A somewhat tricky question - here's why:


The antenna actually contains a preamp, which is powered by approximately 5 VDC on the coax. This means that if you connect two devices to one antenna in parallel, the two devices will be have their output voltages connected together - not a good thing.


So you need a splitter that will pass DC, but on only one side! These exist - I bought one on eBay a month or so ago - but they are definitely not common, and you will probably have to accept whatever connector they come with (generally SMA, but could be anything). Any splitter that works at 1.5 GHz will work, as long as the preamp in the antenna is powered up.


Here's the one I bought: https://www.ebay.com/itm/One-New-In-...AAAOSw~LtbgBva Though entering "GPS Antenna splitter" into eBay search will bring up lots of hits




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Old 25-11-2020, 08:51   #8
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Splitter for GPS antenna?

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Old 25-11-2020, 09:04   #9
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Re: Splitter for GPS antenna?

Just mount the ais antenna inside. I often mount under the deck in the electronics cabinet. Beside the ais.

New gps will shoot through Fiberglass.
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Old 25-11-2020, 09:04   #10
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Splitter for GPS antenna?

ISO 62887-1 ( and -2 for class A) use utc timecodes to synchronise the slot times , hence the spec mandates an integrated GNSS receiver to decode the time codes . It doesnít specifically mandate that the vessels position must also be derived from the interval receiver but in practice since manufacturers have to build in an integrated gnss receiver , it also provides navigation information
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Old 25-11-2020, 09:05   #11
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Re: Splitter for GPS antenna?

On a related issue:

As noted, your AIS transponder should have its own GPS. We found that our Vesper transponder internal GPS, which was down below by the nav station, lost its signal when in rough/rolly seas (one night at anchor after the wind shifted the Vesper lost gps alarm kept sounding off... rough night). We added the Vesper external gps antenna and have never had a problem since.

ALSO: if you don't have a separate antenna for your AIS receive/broadcast (no, not the GPS antenna), you will need a good splitter (unless your version of emtrak AIS has an integrated splitter).
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Old 25-11-2020, 10:36   #12
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Re: Splitter for GPS antenna?

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Originally Posted by Jim Cate View Post
I thought that all transponders had internal GPS receivers. Are you sure that you need an external GPS for yours?

Jim
not all B have internal transponder antenna- and A does not. as far as internal antenna vs external antenna, the internal is dubious accuracy thru deck- it is fine on above deck installs. the antenna is in the face plate.
The below deck concept is poorly understood/educated, but essentially for simplicity your antenna will be receiving signals that reflect down the companionway. So accuracy and reception may/will be inconsistent.
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Old 25-11-2020, 10:41   #13
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Splitter for GPS antenna?

The specification requires an internal gnss receiver it allows either internal or external gnss antenna ( class A or B ). The primary requirement is that the gnss receiver meets IEC 61108 quality standard , the iso spec does not mandate interval or external antenna specifically
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Old 25-11-2020, 11:35   #14
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Re: Splitter for GPS antenna?

Since you apparently don’t have your Nmea 2000network setup. Now would be a good time. Remember AIS has two antenna needs -GPS and VHF antenna. The VHF can be shared with a splitter but many have issues with that. I use a dedicated 3 ft steel whip type.
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Old 26-11-2020, 04:00   #15
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Re: Splitter for GPS antenna?

Thank you for all the answers.

Indeed my AIS (Em-trak B954) has a GPS receiver and an integrated antenna. Since my boat is fiberglass, I will experiment with no external antenna.

Thanks Hartley for the technical explanation of what it takes to share a GPS antenna (and for the link too).
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