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Discussion Starter · #1 ·
Guys...

I am about to buy a chart plotter.
I wondered if it were really necessary to fit an external antenna as it means another juggle with the through-deck fittings.
Would it perform OK with an internal antenna?
It would make life easier if it did.
The boat is GRP.

Thanks.

Rockter
 

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GPS performance will be better with the antenna free and clear. It will usually work from inside the cabin, better under the dodger, but not as well as with a separate antenna.

You can always start with the internal antenna and add an external one later.
 

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My understanding is that the main difference is this : an external antenna will allow much more frequent position updates, eg 10 hz vs. 1hz for the internal antenna. I'm told this can be handy, giving you, for example, better speed data, but I've never missed it. (helm mounted Lowrance HDS 5m with internal antenna. Uncannily accurate, so far)

BTW, some GPS antennas can be mounted on the underside of the deck, and will still perform better in that location than an internal one.

Here are the installation instructions for the Garmin 19X.

http://static.garmincdn.com/pumac/GPS19x_HVS_INST_ML.pdf

Note reference to under-deck mounting.
 

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Discussion Starter · #5 ·
Guys :

Thanks.
I like the idea of the internal mount, but the deck is teak with a balsa sandwich, and it might be a little thick.
I can give it a try internally to see how things go.
Rockter.
 

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My understanding is that the main difference is this : an external antenna will allow much more frequent position updates, eg 10 hz vs. 1hz for the internal antenna....
Only if the GPS chip is designed to put out 10 Hz. I am not aware of any marine (or even automotive) GPS that puts out more than 1 Hz. Totally unnecessary, IMO. Maybe helpful in a jet plane, but certainly not on a sailboat.
 

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Guys :

Thanks.
I like the idea of the internal mount, but the deck is teak with a balsa sandwich, and it might be a little thick.
I can give it a try internally to see how things go.
Rockter.
I doubt you'll have any problems. I took my VHF into the bowels of a Hurley 22, stuck it in every corner I could get it into, while shaded under trees on a trailer to test the GPS on the radio and it locked in no later than 45 seconds or so at the longest (was the initial power up). I would think that the plotter would do so just fine as well. But, worst case, can always go with the external later. My bet is that it works just fine though and saves ya a few bucks :)
 

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Only if the GPS chip is designed to put out 10 Hz. I am not aware of any marine (or even automotive) GPS that puts out more than 1 Hz. Totally unnecessary, IMO. Maybe helpful in a jet plane, but certainly not on a sailboat.
The NMEA 2000 version of the Garmin 19x does 10hz, and the Maratron GPS 200 does 5hz. For the HDS gen 2 series Lowrance quotes 5hz with internal antenna and 10 hz with external. Seems internal antennas have improved since I bought my plotter.

Personally, if I ever upgraded I'd go with something like the Lowrance Point 1 that has a GPS antenna and heading sensor/compass in the same unit. Both 10hz update.
 

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Depends. My new Lowrance HDS has an internal antenna. They claim the external antenna has a faster update rate (which for us slow boats is a plus). If your going to eventualy have radar... and you want to have the "overlay" feature, you need a heading reference. In my case, the Lowrance has to use the external GPS to allow this.

But I concure with most everyone... unless you have a real need for it... it's just an extra.
 

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My Garmin GPSmap 478 works the same (accuracy & acquisition time) at the pedestal mount, or below deck at the nav station.
 

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As everyone else has said, it depends. Most likely you'll be okay. But that's definitely not a guarantee. Do you have a handheld GPS that you could try? Or even an automotive GPS? If it gets a good signal then that's a very strong indication that your marine GPS will also get a good signal in the same location.

Best, of course, would be to buy a GPS that has an option for an external antenna. Then you can buy it and install it, and if it has problems you can always add the antenna later.

Good luck.
 

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Depends. My new Lowrance HDS has an internal antenna. They claim the external antenna has a faster update rate (which for us slow boats is a plus)...
Why is it a plus for slow boats? The slower the boat, the less distance you travel in a given time, therefore the less you need faster update rates. Faster updates are helpful for cars and jet planes. Sailboats, no so much.

...If your going to eventualy have radar... and you want to have the "overlay" feature, you need a heading reference. In my case, the Lowrance has to use the external GPS to allow this...
A compass gives you heading, a GPS gives you COG. The two might be the same under certain circumstances, but often not if you have side current or slippage with a side wind. More importantly, if you're not moving your GPS will not give you any directional information. I'd suggest using your autopilot's compass for heading, instead of the GPS. Do you have that option?
 

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A compass gives you heading, a GPS gives you COG. The two might be the same under certain circumstances, but often not if you have side current or slippage with a side wind. More importantly, if you're not moving your GPS will not give you any directional information. I'd suggest using your autopilot's compass for heading, instead of the GPS. Do you have that option?
Yes I agree, for radar overlay you need a heading sensor. If you've already got a Lowrance HDS plotter, something like the Lowrance Point 1 is a no-brainer, combining as it does a heading sensor, GPS antenna, and NMEA 2000 connectivity.
 

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Guys :

Thanks.
I like the idea of the internal mount, but the deck is teak with a balsa sandwich, and it might be a little thick.
I can give it a try internally to see how things go.
Rockter.
I think the answer also depends on the number of bikinis on boaard. For instance, if your nav station is to starboard the weak GPS signal may be blocked like it was for this poor guy.


And this guy doesn't have a problem now, but will in the very near future. He will definitely have problems maintaining an undistracted course, right when he needs a reliable autopilot!


And this guy tried 15 times to find a place where the GPS would work below deck. He finally threw the unit out, figuring it was broken. He wasn't really planning to go anywhere anyway.


Regards,
Brad

P.S. Dont even get me started on partial shading of solar panels. This guy gets some unusually small amounts of charging even on sunny days. His Victron battery monitor must need calibration. Either that or because his site says that's Miss December and everyone knows those panel don't work well under December conditions.
 
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I have the Lowrance HDS7, it shoots right through my Morgan 33 Out Island balsa cored cabin roof as if it were not even there. As for the update speed, it really doesn't make a bit of difference when you are only traveling along at 5 MPH - does it?

Gary :cool:
 

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A faster refresh rate will always be more acurate, slow boat or not. Don't want to squable about that. The Lowrance system has to have it for radar overlay.
 

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I have two Garmin chartplotters side by side in my cockpit. I have two large solar panels mounted overhead. One has an external antennae and the other one doesn't and I have never been able to tell any difference between them as far as reception or performance.

I always thought the antennae was probably on really needed if the chartplotter was somewhere like a metal cabin.

(But, I kind of feel like nobody is even going to notice my post just a few inches below Bene505's).

:)
 
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