What does the ground plane have to do with the antenna?

In the case of a 1/4-wave antenna, the ground plane acts as the counterpoise to form, in essence, a centered 1/2-wave dipole. Since this plane is the other half of the antenna, its size and proximity are essential. Often an antenna can appear smaller than its specified wavelength. This is due to internal mechanical tricks such as helical windings that can dramatically reduce the antenna's physical size. This does not mean that the same size is appropriate for the ground plane. A compromised ground plane can affect antenna stability and operational frequency. Review AN-00500: Antennas: Design, Application, and Performance and AN-00501: Understanding Antenna Specifications and Operation for a greater understanding of antenna issues.