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Old 06-04-2013, 05:04   #1
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so, I bought a Garmin GPSMap 420s off eBay...

I also got a passive antenna for it. The gps has a transom mounted transducer and, of course, no instructions. I plan to go to garmin's website and find a manual.

My real questions are about routing the transducer wiring and placement for the antenna. I was thinking of putting the antenna on top of the mast by the vhf antenna. I am rewiring the mast this spring anyway....

My boat is a bristol 24.

Any advice?

Thanks,

Dan
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Old 06-04-2013, 05:17   #2
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Not sire height is going to give u an advantage w the antenna. You could mount it on the bimini or a stern rail.
Transducer could be scuffed up and glied / mounted inside the hull just not right on the keel
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Old 06-04-2013, 05:36   #3
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Re: so, I bought a Garmin GPSMap 420s off eBay...

I believe that unit has an internal antenna, and unless you are mounting it below you are very unlikely to need the external antenna.

My 540s lives below decks right now on the Pearson 424 I am aboard, and on a swing out mount on my Ariel. Never, ever looses a signal.... Same unit as mine with a smaller screen.

If you use the external. Antenna there is really no advantage to mounting it on the mast.
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Old 06-04-2013, 06:54   #4
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Re: so, I bought a Garmin GPSMap 420s off eBay...

Since the GPS receiver antenna is NOT line of sight, there is nothing to be gained by mounting it on top of the mast, as there is with a line of sight VHF transceiver antenna. There is a remote possibility that your GPS signal may be degraded by a top of the mast mount due to the motion of the mast top. Out of my depth here, help me out, electronics types!
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Old 06-04-2013, 07:03   #5
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I was thinking of mounting it on the mast to keep it out of the way more than for better reception. The B24 has a beam of 8' & is only 24'-9" over all: Not a ton of room. The remote antenna is small though - so not a real issue.

In fact, I am thinking of hanging a solar panel off the stern rail. I could mount the antenna near that. Routing the wire would be simple, then, too.

Hmnnn....
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Old 06-04-2013, 07:10   #6
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Re: so, I bought a Garmin GPSMap 420s off eBay...

Quote:
Originally Posted by gts1544 View Post
Since the GPS receiver antenna is NOT line of sight, there is nothing to be gained by mounting it on top of the mast, as there is with a line of sight VHF transceiver antenna. There is a remote possibility that your GPS signal may be degraded by a top of the mast mount due to the motion of the mast top. Out of my depth here, help me out, electronics types!
Actually, GPS is line of sight. Any radio signal higher in frequency than MF behaves like light. The difference is that the transmitter is aboard a satellite soaring overhead so the line of sight is upwards.
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Old 06-04-2013, 08:02   #7
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Re: so, I bought a Garmin GPSMap 420s off eBay...

Donning my nerd hat, lower frequency radio signal behaves like light, too. It's just that longer wavelengths have higher dispersion angles. Which is why a ray of white light passing trough a prism turns into a rainbow, and why mile-long radio waves go around the earth radius, while much shorter waves don't.

Agree that raising a GPS antenna shouldn't make much of a difference, because you should almost always have a good line of sight to four GPS satellites from the sea level. Possible exceptions: passing between two cliffs, or if a World War Three has just started.
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Old 06-04-2013, 08:16   #8
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Re: so, I bought a Garmin GPSMap 420s off eBay...

As Jedi says it is line of sight upwards.

Mounted at mast height will do more than degrade the signal, it will make it unusable unless its filtered over several seconds.

The GPS antenna needs to be as low as possible and can be inside the boat where it can still receive through fiberglass. Mine is inside the boat, amidships, on top of a cupboard near the chain plates and its fine.

On top of a 60 foot high mast the tip would swing 30 feet left and 30 feet right every 12 seconds in a long swell while your boat is only doing 600 feet per minute.
Work that out.... 6 knots left to right as you do 6 knots forward.....


Mount low


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Old 16-04-2013, 03:43   #9
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Re: so, I bought a Garmin GPSMap 420s off eBay...

Dan, before you fit it to the boat I would update the opperating software on it. I have just done our Garmin 555s which was a bif of drama but there has been some important upgrades to the O/S. Use a car battery to power it and a 1 or 2 gb SD card to download the update. Swtich the GPS on and only then insert the SD card to load the update.

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