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Concept
Mast radiator
Summary
A mast radiator (or radiating tower) is a radio mast or tower in which the metal structure itself is energized and functions as an antenna. This design, first used widely in the 1930s, is commonly used for transmitting antennas operating at low frequencies, in the LF and MF bands, in particular those used for AM radio broadcasting stations. The conductive steel mast is electrically connected to the transmitter. Its base is usually mounted on a nonconductive support to insulate it from the ground. A mast radiator is a form of monopole antenna.
Most mast radiators are built as guyed masts. Steel lattice masts of triangular cross-section are the most common type. Square lattice masts and tubular masts are also sometimes used. To ensure that the tower is a continuous conductor, the tower's structural sections are electrically bonded at the joints by short copper jumpers which are soldered to each side or "fusion" (arc) welds across the mating flanges.
Base-fed masts, the most common type, must be insulated from the ground. At its base, the mast is usually mounted on a thick ceramic insulator, which has the compressive strength to support the tower's weight and the dielectric strength to withstand the high voltage applied by the transmitter. The RF power to drive the antenna is supplied by a impedance matching network, usually housed in an antenna tuning hut near the base of the mast, and the cable supplying the current is simply bolted or brazed to the tower. The actual transmitter is usually located in a separate building, which supplies RF power to the tuning hut via a transmission line.
To keep it upright the mast has tensioned guy wires attached, usually in sets of 3 at 120° angles, which are anchored to the ground usually with concrete anchors. Multiple sets of guys (from 2 to 5) at different levels are used to make the tower rigid against buckling. The guy lines have strain insulators inserted, usually at the top near the attachment point to the mast, to insulate the conductive cable from the mast, preventing the high voltage on the tower from reaching the ground.
Official source
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