A ground plane is a conductive surface, usually metal, that acts as the missing half of certain antennas. A monopole antenna, such as a whip or a roof-mounted puck, uses the ground plane beneath it as a mirror, and the two together behave like a complete dipole.

Antennas fall into two groups. A ground-plane-dependent antenna needs a metal surface of the right size beneath it to work as designed. A ground-plane-independent antenna, such as a dipole or most panel and wall-mount designs, carries everything it needs inside its own housing and can be mounted on plastic, wood or brick.

A ground plane for a ground-plane-dependent antenna should extend at least a quarter of a wavelength from the antenna in every direction at the lowest frequency in use. At 700 MHz, the lowest cellular band in UK use, a quarter wavelength is about 107 mm, so the metal surface needs to be at least about 215 mm across. A vehicle roof or a metal cabinet lid usually provides this, and the antenna datasheet states the size the maker has designed for.

A ground-plane-dependent antenna mounted without a ground plane is detuned. Its VSWR rises, its radiation pattern tilts upwards away from the horizon and signal falls, most noticeably on the low bands. Where a site has no suitable metal surface, use a ground-plane-independent antenna or fit a metal plate of the size the datasheet specifies.

Embedded antennas rely on a ground plane too, usually the copper layer of the device's circuit board. The board's size and shape therefore affect how well an embedded antenna performs, which is why embedded antenna makers specify a minimum board size and a clear area around the antenna.