Mast radiator

A mast radiator (or radiating tower) is a radio mast or tower in which the entire structure functions as an antenna. This design, first used in radiotelegraphy stations in the early 1900s, is commonly used for transmitting antennas operating at low frequencies, in the VLF, LF and MF ranges, in particular those used for AM broadcasting. The metal 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 insulated from the ground at the base. 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. 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 RF voltage applied by the transmitter. The RF power to drive the antenna is supplied by an antenna tuner unit, usually housed in a small building called a helix building next to the mast, and the cable supplying the current is simply bolted to the tower. The actual transmitter is usually located in a separate building, which supplies RF power to the helix building via a transmission line. Free-standing towers are also used as radiating structures. These towers can have a triangular or a square cross section, with each leg supported on an insulator. One of the best-known radiating towers is the Blosenbergturm in Beromünster, Switzerland. Fiberglass masts are sometimes used for small constructions. The ideal height of a mast radiator depends on transmission frequency, demographics for the location, and terrain. For radio systems in the longwave and mediumwave range, the value of the height should be in the range between one sixth and five eighths of the wavelength, with preferred values at the quarter or the half of the radiated wavelength. When this is not possible, masts with a loading coil, 'capacity hat' or similar loading arrangement are used. The height of the mast determines the radiation properties. For high power transmitters in the MW range, masts with heights around half of the radiated wavelength are preferred because they focus the radiated power better to the ground than structures with heights of quarter wavelengths, which are preferred for economical reasons for low power medium wave transmitters. A focus of radiated power towards the ground is much desired on frequencies below 3 megahertz, because groundwave propagation is very stable. Masts longer than five eighths of the wavelength are normally not used, because they show bad vertical radiation patterns, so masts for mediumwave transmitters do not normally exceed 300 metres. For longwave transmitters, however, the construction of masts with heights of half-wave wavelength is generally not economically viable and in most cases impossible. The only longwave radio mast with a height of the half length of the radiated wavelength was the Warszawa Radio Mast at Konstantynów, Poland. At the time of its collapse in 1991 it was the tallest manmade structure in the world, at 646.38 metres (2,120.67 ft) tall, for a wavelength of 1292.76 metres (frequency 232 kHz). For frequencies below longwave, masts are electrically enlarged by loading coils or capacity hats on the top, because masts of even quarter wavelength would be too high to be practical.