It's been my experience that the ground plane doesn't care if it's inside or out side of the airplane. The important issue is to make sure the ground plane is sufficient size for the antenna and that the base of the antenna bonds to the ground plane. Most of the antennas I have worked with have a bonding seat where the screw holds the antenna to the airframe is part of the base and that in turn would bond to a ground plane placed inside the airplane through the washers and nuts.
Jon Hadlich
Panel Installer

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