Yes, the AV-75 external FLARM antenna needs a groundplane to operate correctly. In most aircraft, the skin serves as the groundplane. Bonding the antenna to the conductive surface is essential. If the skin is not metal or carbon fiber, you will need a separate groundplane.
When required, a 25 cm (minimum) diameter symmetric, conductive disc mounted on the inside with the antenna mounted on the outside should be sufficient. The antenna connector and mounting hardware should run through the center of the conductive disc and make a good electrical connection to it.
The antenna installation must be done according to AC 43.13-2B Chapter 3.
Note: If the skin of the aircraft is carbon fiber, this should be sufficient as groundplane, but we suggest consulting a specialist before installation.