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Clay Johnson

Garmin GDL 82 antenna location

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I’m looking for advice from those who have installed the GDL 82.  Did you install the GPS antenna on the exterior or inside the cabin, e.g. on the glare shield?

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I got an MRA to install the GDL 82 and the GDL 39 on a tray I built that sits just aft of, and attaches to, the mount of the 696. Works fine. The ADS-B flight check had no problems. No holes in the roof and nothing visible on the glare shield except the residue of the goo left over from the adhesive left over from removing the XM antenna. If I were to do it over again,IMG_1210.thumb.JPG.fbe5cdfd65f8ef9a0d7228af32b7cb8e.JPG I would make the tray shorter and sit slightly lower (longer tab that attaches to the 696). The tray comes close to the firewall and doesn't need to be quite that big.  The tray acts as the ground plane for the antenna and the box for the GDL 82 is mounted beneath.  FD-USA put the caveat in the MRA that if it failed the ADS-B flight test I would have to do something different.

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The antenna won't work well unless it has a certain number of square inches of metal in contact with it. Foil tape under the mushroom worked just fine in a friend's CT. I have seen a Citabria that used the foil tape as well. The trick would be to ensure electrical contact of the tape with the antenna.  Maybe put an aluminum doubler under the mushroom which the antenna mounting screws attach. I believe the GA-35 antenna specs out a 7" diameter disc at a minimum. One would have to do the math to get equivalent or better square inches of foil tape.

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I know a ground plane is specified, but I'm not sure if it's actually required for it to work.  I have a GPS antenna on my glareshield now that came with my 496 and it works fine.  I'd have to check to know for sure, but I don't think it has a ground plane under it.

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According to the GDL-82 install manual, the ground plane is required. "The GPS/SBAS antenna requires a minimum ground plane radius of 7.5 inches around the perimeter of the antenna." From Page 6-8 of the GDL-82 installation manual.

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Yep, there it is. They seem to be talking about a top of the airplane install, but it should be the same requirement. I wonder if my XM/GPS on the glare shield actually has one?

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