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A dipole antenna is a straight electrical conductor measuring 1/2 wavelength from end to end and connected at the center to a radio-frequency (RF) feed line. This antenna, also called a doublet, is one of the simplest types of antenna, and constitutes the main RF radiating and receiving element in various sophisticated types of antennas. The dipole is inherently a balanced antenna, because it is bilaterally symmetrical.
Ideally, a dipole antenna is fed with a balanced, parallel-wire RF transmission line. However, this type of line is not common. An unbalanced feed line, such as coaxial cable, can be used, but to ensure optimum RF current distribution on the antenna element and in the feed line, an RF transformer called a balun (contraction of the words "balanced" and "unbalanced") should be inserted in the system at the point where the feed line joins the antenna. For best performance, a dipole antenna should be more than 1/2 wavelength above the ground, the surface of a body of water, or other horizontal, conducting medium such as sheet metal roofing. The element should also be at least several wavelengths away from electrically conducting obstructions such as supporting towers, utility wires, guy wires,and other antennas.
Dipole antennas can be oriented horizontally, vertically, or at a slant. Thepolarization of the electromagnetic field (EM) radiated by a dipole transmitting antenna corresponds to the orientation of theelement. When the antenna is used to receive RF signals, it is most sensitive to EM fields whose polarization is parallel to the orientation of the element. The RF current in a dipole is maximum at the center (the point where the feed line joins the element), and is minimum at the ends of the element. The RF voltage is maximum at the ends and is minimum at the center.
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