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A transmission tower is a structure that is usually erected for the purpose of supporting and conveying electric power transmission lines. The term can also refer to the towers used in the transmission of electromagnetic signals for telecommunications, such as cell phone towers, and those towers used for the transmission of television and radio signals. In some areas of the world, towers used for electric power transmission may be called by other names such as pylons and hydro towers.
An electric transmission tower can be found almost anywhere in the world. In most industrialized countries, they are common sights and can be very large. The most common construction method for a transmission tower today is called lattice construction, in which a tower made up of angled struts and trusses, usually of solid steel or steel tubing. Some transmission towers, however, may be constructed from very large hollow steel poles, from wood, or from concrete. Most of these types of towers are self supporting, but some designs may be stabilized by guy wires.
Generally, these large structures are used for carrying only the largest of power lines, most often those that carry over 100,000 volts. Designs vary widely from place to place, and a typical transmission tower may support as few as one or two high voltage lines, or dozens. While most are under 300 feet (100 meters) in height, some very large examples can much taller.
The term transmission tower can also refer to those towers built and used for telecommunications and broadcasting purposes. Cell phone towers, for example, relay, or transmit cell phone signals. These towers, much like electric towers, can be lattice or single pole construction. Cell phone transmission towers often have supporting guy wires.
Some extremely tall transmission towers are used for radio and television signal transmission. These towers can be over 0.6 miles (1 km) high. They are almost always constructed from metal trusses, assembled in large sections on the ground before final construction. A transmission tower of this type is always stabilized with several strong guy wires to prevent swaying in strong winds. Warning lights are installed to warn aircraft of its presence. Occasionally, radar installations use such towers as well.
In Great Britain and parts of Europe, a transmission tower is often called a pylon, a term which is avoided in the United States, as the term pylon has other meanings there. In Canada and Australia, such a structure may be called a hydro tower, denoting the hydroelectric source of the power it carries. Towers are sometimes sub-classified according to the method by which wires are supported. Strain towers are designed to reduce sag in the power lines, for example, and angle towers are used when the power lines change direction.