Ground Plane - Radio Antenna Theory

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.