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Re: GPS Antennas



> Date:          Sun, 25 Feb 2001 19:02:55 +0000
> From:          Roy Welch <rdwelch@swbell.net>
> Subject:       [amsat-bb] GPS Antennas
> To:            amsat-bb@AMSAT.Org

> Does anyone here know what kind of GPS antenna is used on vehicles with
> built in Navigation systems?  By that I mean, is it an integral part of the
> cell phone antenna, etc. or is it a seperate antenna altogether.  I have
> seen some cell phone antennas with a small bulge at the base off to the
> back side.  I wondered if this was the GPS antenna.  I would like to mount
> an external GPS antenna but since it is on the family auto, I want it to be
> as unobtrusive as possible with no external coax hanging on the car.
> -- 
> 73, Roy
> 
Dear Roy,

For my mobile GPS antenna, I use an external antenna made by Motorola.
I bought it at a hamfest, I don't recall the price.  Its a gray plastic device 
with sloping sides about 1.5" square if that.  It adheres to the car with
a powerful built in magnet.  I added some double sided tape under the 
magnet to protect the car's finish.  Its been on the car for three years now
and fits so tight that I have named it the "barnicle."

My car is a hatch back, so I placed the antenna on the roof right above the
trunk opening seam.  The small coax it uses fits right through with no problems.
The antenna is so small that you don't really notice it unless you look for it.

The antenna itself contains a small patch antenna that is about a inch
square.  Its made from a special high dialectric ceramic material.  There 
is also a  pre-amp that is powered through the coax.  It works very well.

I have also experimented with a larger marine GPS antenna.  This one is
a plastic dome about 6" in diameter.  I had it sitting inside the car under
the rear hatch back near the window.  This arangement did not work as well
as the external antenna has.  

 In effect, the antenna is sitting in a metal box that is open on
the top in this arrangement.  Seeing as the box is higher on one side then
the other, the antenna cannot see the entire sky, that is, it is directional.
You turn a corner, and loose some of the sats.  Not good.

QST published an artical on a homebrew GPS patch antenna a few years
ago.  I have not tried it.  I suppose you could use it with a MMIC like a ERA-1
or one of the better HP gasfet MMIC's and have the equivelent of the 
commercial patch antenna.  It would make a very flat package that you could
lay on the rear window shelf or attach to the car's roof externally.

--73-- David WA0AUQ

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