GPS aerial Q's

[Mickldo]

I just bought a Garmin GPSMAP 60CSx today and want to get an external aerial for it to mount to the top of my snorkel. I was looking on ebay for aerials and found two different types. There was a 28dB and a 55dB model.

What are differences between the different dB ratings? Pro's/ con's?

Would one be better for tighter terrain than the other? ie hills, tree canopies, built up areas, etc.

I will be running this GPS with my Toshibe Satellite laptop and Ozi-explorer software in my 80.

I am also thinking of getting some RAM mounts to mount all this stuff as well as my mp3 player. Who has some good photos or links for their set ups?

[e9999]

do you really need an external antenna?

The 60CSX is supposed to have a terrific internal one that can give you readings inside a house...

maybe try first without, if you haven't already?

[AJP]

I bought a Garmin external antenna (and the lighter plug cable) for a GPS76C .. it makes a difference for me and picks up more satellites than the unit by itself... GA 27C was the code and has a magnet or suction cup mount fwiw
the 60CSx is supposed to have a more sensitive chip though

[Bob_Garrett]

The internal antenna is very good, but when you're using it inside the vehicle the roof can block the view of some satellites. Combine that with other obstructions (trees, terrain, buildings, etc) and it can be difficult to acquire and maintain a signal lock.

The gain of the external antenna only needs to be high enough to overcome the loss in the cable between the antenna and the gps. For a vehicle application 28 dB should be plenty. I would avoid the 55dB gain model as you could overload the front end of the receiver. The important numbers for an external antenna are:

1) noise figure - the lower the number, the better. Should be below 2dB. Most current models are around 1 dB

2) antenna gain and pattern - antenna should have very good pattern near the horizon. Being able to pick up satellites near the horizon gives you better positional accuracy, but are harder to receive due to greater distance and terrain/obstacle attenuation.

[Mickldo]

Originally Posted by Bob_Garrett

The internal antenna is very good, but when you're using it inside the vehicle the roof can block the view of some satellites. Combine that with other obstructions (trees, terrain, buildings, etc) and it can be difficult to acquire and maintain a signal lock.

The gain of the external antenna only needs to be high enough to overcome the loss in the cable between the antenna and the gps. For a vehicle application 28 dB should be plenty. I would avoid the 55dB gain model as you could overload the front end of the receiver. The important numbers for an external antenna are:

1) noise figure - the lower the number, the better. Should be below 2dB. Most current models are around 1 dB

2) antenna gain and pattern - antenna should have very good pattern near the horizon. Being able to pick up satellites near the horizon gives you better positional accuracy, but are harder to receive due to greater distance and terrain/obstacle attenuation.

Thanks Bob

That explains the questions I need the answers for.

I do a lot of 4wding in rainforests and heavily timbered areas. These are usually very mountainous as well so I'll probably need all the help I can get.

Also the way I figured it by using the external aerial I can mount the GPS in a more convenient location rather than the top of the dash.

[Bob_Garrett]

One thing that I forgot to mention was if you are mounting the antenna to the top of the snorkel, and assuming that it's non-metallic, you will need to make a ground plane for the antenna. If you have a Safari snorkel, you can make a ground plane from a piece of thin steel, cut to the size of the of the top of the snorkel head.

[Doug_S]

I have a GPSMAP 60CSx and love it. I find that it does not need an external antenna since the one built into it is fantastic. It acquires several satallites within a minute and a half generally.

[e9999]

even my little Venture Cx does fine sitting in the center console (not even on the dash). At least it did for all the wheeling I used it for so far, admittedly not thick forests or deep canyons.

[Bob_Garrett]

The antennas used by Garmin on their units are very good. Combine that with improved receive sensitivities in the newest chips and the improvement in coverage is significant in difficult areas. Also, a new generation of gps satellites with improved antennas currently being built and launched, which should improve coverage as well.

[rusty_tlc]

An option to a hard wired antenna is a re-broadcast antenna. The unit has two antennas, one outside the rig and another inside the rig. In between is an amp the bufferes the signal from the satalites and rebroad cast it to the interior of the rig.


My Garmin Legand worked well in the 40 (fiberglass roof) but not so well in the GMC or Car. It also had a hard time under heavy foliage or in canyons. My newer Magellian Platinium performs much better. I have heard thet the Garmin mobil units work a lot better than thier hand held ones.

[Bob_Garrett]

Originally Posted by rusty_tlc

An option to a hard wired antenna is a re-broadcast antenna. The unit has two antennas, one outside the rig and another inside the rig. In between is an amp the bufferes the signal from the satalites and rebroad cast it to the interior of the rig.

These devices are ILLEGAL for sale or use in the US. The FCC has already hit sellers with some VERY large fines. As far as I know, no users of these devices have been cited, but that will likely change the 1st time someone interferes with commercial navigation.

[rusty_tlc]

Originally Posted by Bob_Garrett

These devices are ILLEGAL for sale or use in the US. The FCC has already hit sellers with some VERY large fines. As far as I know, no users of these devices have been cited, but that will likely change the 1st time someone interferes with commercial navigation.

I was unaware of that. I was looking at them a couple of years ago and I found no indication that they were illegal. I just checked some of my bookmarks and sure enough they have all been pulled off the market.
Sorry about that.

[CDN_Cruiser]

I use an external antenna with my 76CS and quite like it. As Bob mentioned, the one thing to be concerned with a high dB antenna is if it's highly directional

[Mickldo]

Thanks for the replies guys.

The GPS turned up in the post yesterday. After I turned it on for the first time to initiallise it, it managed to locate the satellites even though I was inside the house.

I haven't used it in the cruiser yet but have a trip planned this weekend.

I wasn't aware I needed a ground plane for it. Yes, my snorkel is a Safari item. Maybe a little square of Stainless Steel sheet will be enough.