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Old 21 February 2008, 17:01   #1
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Internal vs external aerial

Hi

I'm looking for a cheap-ish GPS unit for my boat, have found several that offer either internal or external aerials.
If i got one with an external aerial, does it have to be mounted on a horizontal plane? or can it be mounted off (about 20 degrees off level)
If i got an internal aerial, will it be affected by the wrap around plexiglass windscreen?

Which do people have, or would go for if given the choice?

Cheers

Jimmy
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Old 21 February 2008, 17:55   #2
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Hi Jimmy
I am not a GPS expert but my experience of installing various options may be of value to you.
On my last installation I mounted an external aerial on the top surface of the console (cut a 7cm hole and siliconed the "mushroom" flush with the surface!!). Above it was a full width acrylic screen with a full width S/S hand rail situated over the screen. The system worked without ever losing accurate positional information.
On our training RIB we mounted an internal aerial GPS receiver on the console top surface which also had a full width screen, again no problems with the reception.
I am of the view that with the large number of GPS transmitters normally being received by a GPS receiver at any one time, allied to the relatively slow speed of a boat, it will take quite a large degredation in reception quality before "one becomes temporarily unsure of one's position !!!"
No doubt someone will prove me wrong.
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Old 21 February 2008, 18:10   #3
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Me again,
Sorry I forgot to respond on the "horizontal" issue. Basically the nearer to the horizontal the better, however, considering the RIB will often be banked up to circa 30 degrees from the horizontal (either to port or stbd) without losing GPS positional information, it can reasonably be deduced that an aerial mounted 20-30 degrees from the horizontal is likely to continue to operate effectively.
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Old 21 February 2008, 18:30   #4
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I guess if you consider the reliability of in-car GPS units which are basically sitting inside a shielded metal box with somewhat obscured vision of the sky then a screen on a RIB shouldn't cause much concern!

I have only ever had a problem with a handheld unit when it's been placed close to a large bulkhead which mush have obscured quite a lot of the sats.

Only comment I would make is the position of you in relation to the antenna. If you block a large portion of the sky from the antenna (ie by standing directly in line of antenna to sats) then it may struggle but otherwise I don't think there should be a problem. The sats are moving a lot as well so it should just be a momentary blip. Inbuilt antenna are also a lot easier to fit :-)
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Old 22 February 2008, 08:10   #5
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AFAIK You get more signal gain from the external aerial.
Also if you were fitting one below decks on a yacht. Then you would need the external option.
Plus you have to run a cable.
More important is the sensitivity of the actual GPS receiver.

I would personally pick the most sensitive receiver with the internal aerial option( why not both I ask my self!!!).
If that was the my only criteria.

As for the inclination I dont think it makes that much difference.( within in reason)

Dont forget the cost of charts in your decision.
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Old 22 February 2008, 08:59   #6
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Quote:
Originally Posted by Rooster21 View Post

I would personally pick the most sensitive receiver with the internal aerial option( why not both I ask my self!!!).
If that was the my only criteria.
So without risking a deluge or opinions for or against a specific brand, can you reccommend a couple that would fulfill your criteria above?

Jimmy
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Old 22 February 2008, 11:26   #7
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If your looking at the cheap end then I would probably say Lowrance with chart included. They seem value for money.
Not used a Standard Horizon. Worth a look.
Garmin I like and have used most of, plus the charts are everywhere.
But can work out expensive if you buy them.

I have a Lowrance at present. Its was cheap with a UK chart and it talks to my Suz. (allegidly)
But I do prefer the garmin Bluecharts in comparison to the Nauitpath.

All the lower ranges seem to have very similar receivers. So not much to choose from there.

To some up If I didnt want NMEA 2000 I would buy Garmin.
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Old 22 February 2008, 14:11   #8
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I don't think you can go wrong with either Garmin or Lowrance and I've heard good reports about both internal aerial units. I went external simply cos the inyternal units were not available at the time.

Another consideration you may want to think about is the type of charts you want. I preferred the Navionics to the Garmin bluechart or lowrance's own and that swayed my decision (Navionics are closer in look and feel to standard admiralty chart that some of the others).

If you have multiple plotters, the garmin bluechart license allows you to install it on 2 units (it's locked to the plotter) whereas the Navionics soln is locked to the card itself so it can be moved around. Not sure if that is a consideration but worth mentioning.

When I got mine you got a better screen with the Lowrance for approx same amount of cash.

Hopefully that hasn't muddied the water even more and good luck.

Edit - Meant to say I've seen Garmin and Lowrance units working inside in display cabinets (not in sumulation) so the receivers must be pretty good!
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