Radio masts and towers
Radio masts and towers are typically tall structures designed to support antennas for telecommunications and broadcasting, including television. There are two main types: guyed and self-supporting structures. They are among the tallest human-made structures. Masts are often named after the broadcasting organizations that originally built them or currently use them.
A mast radiator or radiating tower is one in which the metal mast or tower itself is energized and functions as the transmitting antenna.
Terminology
The terms "mast" and "tower" are often used interchangeably. However, in structural engineering terms, a tower is a self-supporting or cantilevered structure, while a mast is held up by stays or guy-wires.; A mast: is a guyed mast, a thin structure without the sheer strength to stand unsupported, that uses attached guy lines for stability. They may be mounted on the ground or on top of buildings. Typical masts are of steel lattice or tubular steel construction. Masts tend to be cheaper to build but require an extended area surrounding them to accommodate the guy wires.
; A tower: is a self-supporting structure, possibly also placed on a rooftop, that accomplishes the same purpose of raising actual radiating antennas to a functional height. Since it does not require land area from which to anchor guy lines, towers are more commonly used in cities where land is in short supply.
There are a few borderline designs that are partly free-standing and partly guyed, called additionally guyed towers. Examples:
; Gerbrandy tower: consists of a self-supporting tower with a guyed mast on top.
; Blaw-Knox towers: Those few of the towers still standing do the opposite: They have a guyed lower section surmounted by a freestanding part.
; Zendstation Smilde: is a tall tower with a guyed mast on top with guys which go to ground.
; Torre de Collserola: is a guyed tower with a guyed mast on top where the tower portion is not free-standing.
History
The first experiments in radio communication were conducted by Guglielmo Marconi beginning in 1894. In 1895–1896 he invented the vertical monopole or Marconi antenna, which was initially a wire suspended from a tall wooden pole. He found that the higher the antenna was suspended, the further he could transmit, the first recognition of the need for height in antennas. Radio began to be used commercially for radiotelegraphic communication around 1900.The first 20 years of commercial radio were dominated by radiotelegraph stations, transmitting over long distances by using very long wavelengths in the very low frequency band – such long waves that they are nearly unused at present. Because the extreme wavelengths were one to several kilometers long, even the tallest feasible antennas by comparison were still too short, electrically, and consequently had inherently very low radiation resistance. In any antenna, low radiation resistance leads to excessive power losses in its surrounding ground system, since the low-resistance antenna cannot effectively compete for power with the high-resistance earth. To partially compensate, radiotelegraph stations used huge capacitively top-loaded flattop antennas consisting of horizontal wires strung between multiple steel towers to increase efficiency.
Image:Wire T antenna station WBZ 1925.jpg|thumb|Multiwire broadcast T-antenna of early AM station WBZ, Springfield, Massachusetts, 1925.
AM radio broadcasting began around 1920. The allocation of the medium wave frequencies for broadcasting raised the possibility of using single vertical masts without top loading. The antenna used for broadcasting through the 1920s was the T-antenna, which consisted of two masts with loading wires on top, strung between them, requiring twice the construction costs and land area of a single mast. In 1924 Stuart Ballantine published two historic papers which led to the development of the single mast antenna. In the first he derived the radiation resistance of a vertical conductor over a ground plane.
He found that the radiation resistance increased to a maximum at a length of so a mast around that length had an input resistance that was much higher than the ground resistance, reducing the fraction of transmitter power that was lost in the ground system without assistance from a capacitive top-load. In a second paper the same year he showed that the amount of power radiated horizontally in ground waves reached a maximum at a mast height of
File:Hillmorton radio masts.jpg|thumb|left|Masts of the Rugby VLF transmitter near Rugby, England
By 1930 the expense of the T-antenna led broadcasters to adopt the mast radiator antenna, in which the metal structure of the mast itself functions as the antenna.
One of the first types used was the diamond cantilever or Blaw-Knox tower. This had a diamond shape which made it rigid, so only one set of guy lines was needed, at its wide waist. The pointed lower end of the antenna ended in a large ceramic insulator in the form of a ball-and-socket joint on a concrete base, relieving bending moments on the structure. The first, a half-wave mast was installed at radio station WABC's 50 kW transmitter at Wayne, New Jersey in 1931.
During the 1930s it was found that the diamond shape of the Blaw-Knox tower had an unfavorable current distribution which increased the power emitted at high angles, causing multipath fading in the listening area. By the 1940s the AM broadcast industry had abandoned the Blaw-Knox design for the narrow, uniform cross section lattice mast used today, which had a better radiation pattern.
The rise of FM radio and television broadcasting in the 1940s–1950s created a need for even taller masts. The earlier AM broadcasting used LF and MF bands, where radio waves propagate as ground waves which follow the contour of the Earth. The ground-hugging waves allowed the signals to travel beyond the horizon, out to hundreds of kilometers. However the newer FM and TV transmitters used the VHF band, in which radio waves travel by line-of-sight, so they are limited by the visual horizon. The only way to cover larger areas is to raise the antenna high enough so it has a line-of-sight path to them.
Until 8 August 1991, the Warsaw radio mast was the world's tallest supported structure on land; its collapse left the KVLY / KTHI-TV mast as the tallest. There are over 50 radio structures in the United States that are 600 m or taller.
Materials
Steel lattice
The steel lattice is the most widespread form of construction. It provides great strength, low weight and wind resistance, and economy in the use of materials. Lattices of triangular cross-section are most common, and square lattices are also widely used. Guyed masts are often used; the supporting guy lines carry lateral forces such as wind loads, allowing the mast to be very narrow and simply constructed.When built as a tower, the structure may be parallel-sided or taper over part or all of its height. When constructed of several sections which taper exponentially with height, in the manner of the Eiffel Tower, the tower is said to be an Eiffelized one. The Crystal Palace tower in London is an example.
Tubular steel
s are sometimes also constructed out of steel tubes. This construction type has the advantage that cables and other components can be protected from weather inside the tube and consequently the structure may look cleaner.These masts are mainly used for FM-/TV-broadcasting, but sometimes also as mast radiator. The big mast of Mühlacker transmitting station is a good example of this.
A disadvantage of this mast type is that it is much more affected by winds than masts with open bodies. Several tubular guyed masts have collapsed. In the UK, the Emley Moor and Waltham TV stations masts collapsed in the 1960s. In Germany the Bielstein transmitter collapsed in 1985.
Tubular masts were not built in all countries. In Germany, France, UK, Czech Republic, Slovakia, Japan and the Soviet Union, many tubular guyed masts were built, while there are nearly none in Poland or North America.
Several tubular guyed masts were built in cities in Russia and Ukraine. These masts featured horizontal crossbars running from the central mast structure to the guys and were built in the 1960s. The crossbars of these masts are equipped with a gangway that holds smaller antennas, though their main purpose is oscillation damping. The design designation of these masts is 30107 KM and they are exclusively used for FM and TV and are between tall with one exception. The exception being the mast in Vinnytsia which has height of 354 m and is currently the tallest guyed tubular mast in the world after the Belmont transmitting station was reduced in height in 2010.
Reinforced concrete
Reinforced concrete towers are relatively expensive to build but provide a high degree of mechanical rigidity in strong winds. This can be important when antennas with narrow beamwidths are used, such as those used for microwave point-to-point links, and when the structure is to be occupied by people.In the 1950s, AT&T built numerous concrete towers, more resembling silos than towers, for its first transcontinental microwave route.
In Germany and the Netherlands most towers constructed for point-to-point microwave links are built of reinforced concrete, while in the UK most are lattice towers.
Concrete towers can form prestigious landmarks, such as the CN Tower in Toronto, Canada. In addition to accommodating technical staff, these buildings may have public areas such as observation decks or restaurants.
The Katanga TV tower near Jabalpur, Madhya Pradesh, in central India hosts a high-power transmitter for the public broadcasters Doordarshan and Prasar Bharati.
The Stuttgart TV tower was the first tower in the world to be built in reinforced concrete. It was designed in 1956 by the local civil engineer Fritz Leonhardt.