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Radio antenna theory 
In telecommunication, a ground plane is a flat or nearly flat horizontal conducting surface that serves as part of an antenna, to reflect the radio waves from the other antenna elements. The plane does not necessarily have to be connected to ground. Ground planes are particularly used with monopole antennas.
To function as a ground plane, the conducting surface must be at least a quarter of the wavelength (λ/4) of the radio waves in size. In lower frequency antennas, such as the mast radiators used for broadcast antennas, the Earth itself (or a body of water such as a salt marsh or ocean) is used as a ground plane. For higher frequency antennas, in the VHF or UHF range, the ground plane can be smaller, and metal disks, screens or wires are used as ground planes. At upper VHF and UHF frequencies, the metal skin of a car or aircraft can serve as a ground plane for whip antennas projecting from it. The ground plane doesn't have to be a continuous surface. In the ground plane antenna the "plane" consists of several wires λ/4 long radiating from the base of a quarter wave whip antenna.
The radio waves from an antenna element that reflect off a ground plane appear to come from a mirror image of the antenna located on the other side of the ground plane. In a monopole antenna, the radiation pattern of the monopole plus the virtual "image antenna" make it appear as a two element center-fed dipole antenna. So a monopole mounted over an ideal ground plane has a radiation pattern identical to a dipole antenna. The feedline from the transmitter or receiver is connected between the bottom end of the monopole element and the ground plane. The ground plane must have good conductivity; any resistance in the ground plane is in series with the antenna, and serves to dissipate power from the transmitter.
Printed circuit boards 
A ground plane on a printed circuit board (PCB) is a large area or layer of copper foil connected to the circuit's ground point, usually one terminal of the power supply. It serves as the return path for current from many different components.
A ground plane is often made as large as possible, covering most of the area of the PCB which is not occupied by circuit traces. In multilayer PCBs, it is often a separate layer covering the entire board. This serves to make circuit design easier, allowing the designer to ground any component without having to run additional traces; component wire needing grounding is routed directly through a hole in the board to the ground plane on another layer. The large area of copper also conducts the large return currents from many components without significant voltage drops, ensuring that the ground connection of all the components are at the same reference potential.
However, in digital and radio frequency PCBs, the major reason for using large ground planes is to reduce electrical noise and interference being coupled from one part of the circuit to another through the ground (ground loops), and crosstalk between adjacent circuit traces. When digital circuits switch state, large current pulses flow from the integrated circuits through the ground circuit. If the power supply and ground wires have significant resistance, the voltage drop across them may create noise voltage pulses in the ground wires, which are applied to other parts of the circuit, The large capacitance of the ground plane allows it to absorb the current pulses without much change in voltage,
In addition, a ground plane under printed circuit traces can reduce crosstalk between adjacent traces. When two traces run parallel, an electrical signal in one can be coupled into the other through electromagnetic induction by magnetic field lines from one linking the other; this is called crosstalk. When a ground plane layer is present underneath, it forms a transmission line (stripline) with the trace. The oppositely-directed return currents flow through the ground plane directly beneath the trace. This confines the electromagnetic fields to the area between the trace and the ground plane, reducing crosstalk.
Ground planes can also be placed on adjacent layers to power planes, creating a large parallel plate decoupling capacitor that prevents noise from being coupled from one circuit to another through the power supply.
Ground planes are sometimes split and then connected by a thin trace. This allows the separation of analog and digital sections of a board or the inputs and outputs of amplifiers. The thin trace has low enough impedance to keep the two sides very close to the same potential while keeping the ground currents of one side from impacting the other.
See also 
- List of electronics topics
- Power plane
- Microstrip and Stripline
- Line-of-sight propagation
- Radio electronics and Radio propagation
- Printed circuit board milling
- Antenna tuner
- Salisbury screen
- References and citations
- Groundplane antenna model FA-2 from the book PRACTICAL ANTENNA DESIGN second edition, Philippine copyright, 1990, 1994 by Elpidio C. Latorilla
- John Whitmore, sci.electronics > What is a PCB with a Ground plane?. August 11, 1992.
- Amateur Quarter Wave Ground Plane Antenna Calculator. Computer Support Group, Inc., 2006.
- What is a Ground Plane? Criterion Cellular, 2006.