The ground plane need not be a continuous surface. In microstrip antennas and printed monopole antennas an area of copper foil on the opposite side of a printed circuit board serves as a ground plane. At upper VHF and UHF, the metal skin of a car or aircraft can serve as a ground plane for whip antennas projecting from it. For higher frequency antennas, in the VHF or UHF range, the ground plane can be smaller, and metal disks, screens and wires are used as ground planes. 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. 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 radius. For a monopole antenna (a), the Earth acts as a ground plane to reflect radio waves directed downwards, making them seem to come from an image antenna (b).
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