R.A. Miller Industries, Inc.

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Tips for Ground Planes for Aircraft Radio Antennas
Most radio antennas require a ground plane.  The AV-534 is a good example of an antenna that performs best with a ground plane.  A ground plane is a metal surface that is connected to the antennas ground (common).  There is no need for a ground plane when mounting an antenna on a metal aircraft, but it can be tough to accommodate in a fiberglass aircraft such as a sailplane. 

Note: Radio antennas can be mounted inside a fiberglass aircraft, but they would not work well if mounted inside a carbon fiber aircraft. 

Below is a note from RAMI about ground planes for aircraft radio antennas.

A proper ground plane is one that extends 1/4 wavelength (at the operating frequency) outward in all directions from the feed point or base of the antenna. 1/4 wavelength at aircraft com frequencies is approximately 24 inches. So the ground plane would be approximately 4 ft. X 4 ft. (or 4 ft. diameter) conductive surface with the antenna mounted in the center. You can also accomplish this with (2) four foot ( 1.22 m) long conductive wires running perpendicular to each other and electrically connected to the shield of the transmission line at the base of the antenna at their mid points. Similar to a base station antenna seen at an airport terminal building. The wires can be run inside the fuselage forward and aft and down the inside wall. 

Sincerely,

Ben Ennenga
Engineering Manager / GAA Product Line Manager
RAMI

Tips for Ground Planes for Transponder Antennas
I talked to Ben Ennenga at RAMI about transponder antenna ground planes.  Some interesting notes from that conversation are below.  Thank you Ben for the friendly and helpful support!

• A ground plane is required (of course) with both rod style and blade style transponder antennas.
• The smallest recommended ground plane would be 12 inches in diameter (6 inches radius). 
• A ground plane works about equally well regardless of whether it is made from a solid plate (such as aluminum) or instead made using 2 wires in a "+" arrangement. In either case, the radius should be at least 6 inches.
• When working with carbon fiber glider fuselages, the antenna must be mounted on the outside of the fuselage, but it is OK to mount the ground plane on the inside.
• The mounting screws for the RAMI AV-74 antenna are connected to the antenna ground.  Therefore, you can connect the ground plane to either or both of the mounting screws.
• It is OK for the ground plane wires ("+" configuration) to curve and follow the inside of the glider fuselage.