What is a Ground Plane?
A Ground Plane is a flat surface of ferrous metal (galvanized, steel, etc) that a Magnet Mount or Body Mount antenna uses as an integrated part of it. For a Body mount antenna other metals may be used as it does not have a magnet that needs to stick to it.
A Metal Car Body is a ground plane.
The metal top of a refrigerator is a ground plane.
A filing cabinet is a good ground plane.
A tin roof is a Ground Plane.
A ground plane can 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 thin.
A ground plane MUST satisfy BOTH length and width dimensions outlined below. Antenna gains are calculated with an infinite ground plane. Performance can be achieved with the minimums outlined below.
Just as long as it is big enough:
for Dual Band/800 MHz/Nextel = 14" x 14" Minimum (28" x 28" recommended)
for 1900 MHz only = 6" x 6" minimum (12" x 12" recommended)
A Ground Plane does not need to be electrically grounded. It simply needs to be a plane of metal.
Your local hardware store sells galvanized roof flashing for cheap that can be used as a ground plane.
Please note, the antenna should be mounted in the CENTER of the ground plane for a circular pattern.