A Ground Plane is a flat surface of ferrous metal (galvanized, steel, etc) that a Magnet Mount or Fixed Mount antenna uses as an integrated part of it (e.g., forms a reflector or director for an antenna).

For a fixed mount antenna, other metals may be used as it does not have a magnet that needs to stick to it.  A Ground Plane does not need to be electrically grounded. It simply needs to be a plane of metal.

Some antennas have "internal" ground planes and do not need to be mounted on metal surfaces.  Others use "spikes" or wires eminating from the base of the antenna to create a ground plane.  Check with your antenna manufacturer for the recommended minimum size of the ground plane.  For most cellular antennas, a 6 inch diameter plate is the minimum.

Common objects can serve as antenna ground planes:

• A metal car or truck body
  
• The metal top of a vending machine
  
• A filing cabinet
  
• A ground plane can even be a cookie sheet!

A ground plane is NOT a metal roof gutter, a window frame, a downspout or any other piece of metal that is long and narrow. 

The antenna should be mounted in the CENTER of the ground plane.

Antenna gains are calculated with an infinite ground plane. Best antenna performance is achieved with large ground plane surfaces.