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Old 15-10-2010, 17:36   #1
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Question GPS Antenna Location

Anyone with thoughts on best location for a GPS antenna? I have a Hunter 31. Do have a radar mast that I could attach it to, or is it better lower down where more stable? Does it really matter what height it is, so long as it is relatively unobstructed?
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Old 15-10-2010, 17:57   #2
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Height does not matter. It wants to see the sky clearly. Where would you put your head to see all the stars? That's a good spot. Mine in on the cabin top where I suppose the boom and mast shade a few satellites some of the time.

Is really near the radar a good idea? Is there a warning in any manual? That's a bunch of power to select a signal thru...
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Old 15-10-2010, 18:00   #3
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I use the top rail of the push pit.
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Old 15-10-2010, 18:11   #4
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The satellites are 26,000 miles or so abovr you so the height on your boat is not going to be a factor. If it's on the spreaders, you can loose the signal in heavy seas due to the antenna swinging about. Generally you get a signal from 5 plus satellites and you're always in the process of dropping a signal to one and picking up another. I haven't heard of any warnings about radars, but several years ago the USCG put out a warning that certain TV antennas, while in operation, would interfer with the GPS signals.
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Old 15-10-2010, 18:40   #5
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I have a GPS at my nav station where I am right now. The antenna is fitted flush through the coach roof above. I have a second one at the pedestal guard but that one has a built in antenna. As has been said, all that matters is the GPS have a clear view of the sky. I have not noticed any difference with the biminni up or down.
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Old 15-10-2010, 18:58   #6
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I mounted my GPS on a swinging arm just inside the comp.way entrance, It makes no diff if I close the hatchets and sit inside or if I swing it outside with that clear view everyoneīs talking about, I still get a good signal from the satīs.
I did get an external antenna when I bought the thingy but never bothered to mount it since I havenīt encountered any problems.

Just fired the thing up and I got a fix on 12 satīs out of the 16 total, sitting inside the boat. 3 dgr below freeze, itīs safe to say all the hatchets are closed
Swinged it out and still the same 12 fixes with the same strength.

Iīd say put the antenna wherever it fits best
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Old 15-10-2010, 19:48   #7
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- with clear view of the sky,
- away from low flying objects,
- away from radiating antennas,
- not high.

b.
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Old 15-10-2010, 20:34   #8
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Quote:
Originally Posted by John A View Post
The satellites are 26,000 miles or so abovr you so the height on your boat is not going to be a factor. If it's on the spreaders, you can loose the signal in heavy seas due to the antenna swinging about. Generally you get a signal from 5 plus satellites and you're always in the process of dropping a signal to one and picking up another. I haven't heard of any warnings about radars, but several years ago the USCG put out a warning that certain TV antennas, while in operation, would interfer with the GPS signals.
FYI, GPS satellites are not in geostationary orbit. They are at medium earth orbit - 12,552 miles. Geostationary satellites orbit at 22,236 miles. Iridium birds are at LEO - 485 miles, Globalstar's are at about 870 miles.

And clarifying the CG warnings - they were about pre-amplifiers in certain antennas, not the antennas themselves.

As far as radar, you don't want to mount a GPS antenna at the same height as a radar antenna. Even though they're different frequencies, having 2kW peak pulsed power blasting into your GPS antenna is not good for the RF front-end.
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Old 15-10-2010, 21:04   #9
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On both my boats I use the GPS' internal antenna, even below deck. Works quite well. Never had an issue with losing contact or errors.

as far as I can see, fiberglass is close to transparent to the GPS signal. Our deck is plywood, glass and teak (3 layers) and the gps doesn't seem to care. If it can get through that, it should be able to get through straight glass/cored deck ,etc. I get 12 strong signals almost instantly after turning it on.

The real reason is I can't get myself to run an antenna and drill a hole in the deck for an external antenna I would have if the GPS didn't work well.. but to my relief, it works flawlessly
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Old 15-10-2010, 21:30   #10
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We have our gps antenna mounted on the stern pulpit, top rail. Never had any problems.
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Old 15-10-2010, 22:32   #11
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We have ours inside the transom (Transom slopes down from the cockpit to a swim platform). As Phantomracer has discovered, GPS works fine through fibreglass. It means we have one less thru deck cable and one less thing to knock off the pushpit.
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Old 15-10-2010, 23:51   #12
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Quote:
Originally Posted by Beausoleil View Post
FYI, GPS satellites are not in geostationary orbit. They are at medium earth orbit - 12,552 miles. Geostationary satellites orbit at 22,236 miles. Iridium birds are at LEO - 485 miles, Globalstar's are at about 870 miles.
Sorry for the error. The number 26,000 had stuck in my head from talking to the young techs contracted by NASA to montior satelites. It appears that that's the maximum distence in kilometersa.
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Old 16-10-2010, 06:16   #13
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Middle rail of the pushpit.

Deck mounted is too easy to kick and top rail is too easy to hit with so many things, such as yourself when you stumble. Beside the radome is even better - if you have one.
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Old 16-10-2010, 06:30   #14
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Originally Posted by barnakiel View Post
- with clear view of the sky,
- away from low flying objects,
- away from radiating antennas,
- not high.
b.
Those are the answers - Depending upon the size of the boat the higher the antenna the more lateral swinging the antenna experiences as the boat rolls. Lateral swinging will induce false positions and affect the true directional movement of the boat. Albeit the effect is small it can be noticeable when zooming way in.
- - The problem is getting a reasonably clear view of the sky without thick hull/metal/etc. interfering with signal reception. With modern GPS's with the ability to track 12 or 20 satellites this problem has been greatly reduced. The older GPS's could not receive signals through the canvas of a bimini. Now you can get usable signals inside the cabin of a FRG boat. My Garmin 76 series units get lots of satellites even inside an aluminum pilot house, but it does have rather large windows.
- - The new auxillary external Garmin antennas are very low profile - less than 1/2 inch high and curved. I have one mounted on the toe rail on the transom to feed a GPS in the aft cabin for anchor watching. The giant "domes" are really passe. Like modern audio equipment for your home, when you open the unit there is more empty space inside than components.
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Old 16-10-2010, 09:00   #15
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It doesnt even need a clear view of the sky!!

I finally found mine a few months ago... after a 2 year search! I is in the aft lazarette so far aft UNDER the stern swim platform!

Obviously put there so charterers couldn't rip it off in their drunken frenzies.

So, low to the water (I think its under the waterline) and under a few layers of FRP.

It works fine. Well, I ain't been on no reef yet.
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