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A monopole antenna is a radio antenna that is one half of a dipole antenna combined with a right-angle ground plane of considerable length in place of its other half. A dipole antenna has two halves, while a monopole antenna replaces one of the halves with an electrically conductive surface known as a ground plane, which behaves like the other half of a dipole antenna. With a large enough ground plane, the monopole antenna can be as strong as a dipole antenna. Automobile antennas are a common example of a monopole antenna, with the metal of the automobile itself forming the ground plane.
There are two basic types of monopole antennas: solid and planar. The solid-type, although more expensive to manufacture, is known for having good bandwidth and being completely omni-directional. The more commonly used and less expensive planar-type also has good bandwidth, but experiences distortion at higher frequencies, thereby making it non omni-directional. A cross-shaped, planar-type monopole antenna has been designed and implemented in recent years that overcomes this frequency distortion while continuing to retain good bandwidth. A dual-frequency cross-shaped monopole has also been fabricated and tested in an attempt to reduce frequency distortion even further.
Monopole antennas can be used in a range of frequencies from several hundred kHz to a few GHz. In telecommunications terminology, a monopole usually refers to an antenna that stands alone and is self-supported or guy-wire supported, meaning it's supported by the tension of attached cables which are anchored to the ground. A group of monopole antennas used to manage the direction of long and medium wavelength radio frequencies is called a directional antenna array.
Antennas are used to transmit as well as receive electromagnetic waves, which are converted either from or to an electrical current. The same antenna can be used to either transmit or receive because the electromagnetic characteristics of all antennas are identical. Since antennas are typically used to transmit and receive radio waves, they are considered a key component of radio equipment.
Antennas are not only employed in television and radio broadcasting, but also in point-to-point communication as well. Cell phones, wireless Local Area Networks (LAN), spacecraft communication, and radar all require antennas in order to operate. The low cost and quick installation of monopole antennas has made them the first choice for mobile and Internet networks throughout the world, especially in developing countries.
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