Radio masts and towers

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Masts of the Rugby VLF transmitter in England
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Masts of the Rugby VLF transmitter in England

Radio masts and towers are, typically, tall structures designed to support antennas (also known as aerials in the UK) for telecommunications and broadcasting, including television. They are among the tallest man-made structures. Similar structures include electricity pylons and towers for wind turbines.

Masts are sometimes named after the broadcasting organisations that use them, or after a nearby city or town.

The Warsaw Radio Mast (Warszawa radio maszt) was the world's tallest supported structure on land, but it collapsed in 1991, leaving the KVLY/KTHI-TV mast as the tallest.

In the case of a mast radiator or radiating tower, the whole mast or tower is itself the transmitting antenna.

Contents

[edit] Mast or tower?

The terms "mast" and "tower" are often used interchangeably. However:

  • In engineering terms, a tower is a self-supporting or cantilevered structure, while a mast is held up by stays or guys.
  • In colloquial American English, both types of structure are often called towers.
  • In colloquial British English, both types of structure are often called masts.

(This article reflects the engineers' distinction between the two.)

Masts tend to be cheaper to build, but because they require an extended area surrounding them to accommodate stay blocks, towers are more commonly used in cities where land is in short supply.

There are a few borderline designs which are partly free-standing and partly guyed. For example:

  • The Gerbrandy tower consists of a self-supporting tower with a guyed mast on top.
  • The few remaining Blaw-Knox Towers do the opposite: they have a guyed lower section surmounted by a freestanding part.
  • Zendstation Smilde a tall tower with a guyed mast on top (guys go to ground)
  • Torre de Collserola a guyed tower, with a guyed mast on top. (Tower portion is not free standing.)

[edit] Materials

[edit] Steel lattice

The steel lattice is the most widespread form of construction. It provides great strength, low wind resistance and economy in the use of materials. Such structures are usually triangular or square in cross-section.

When built as a stayed mast, usually the whole mast is parallel-sided. One exception is the Blaw-Knox type.

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.

[edit] Tubular steel

Some masts are constructed out of steel tubes. In the UK, these were the subject of collapses at the Emley Moor and Waltham TV stations in the 1960s.

[edit] 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 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. As well as accommodating technical staff, these buildings may have public areas such as observation decks or restaurants.

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.

[edit] Fibreglass

Fibreglass poles are occasionally used for low-power non-directional beacons or medium-wave broadcast transmitters.

[edit] Wood

There are fewer wooden towers now than in the past. Many were built in the UK during World War II because of a shortage of steel. In Germany before World War II in nearly all mediumwave transmission sites towers built of wood were used. Nowadays these towers are demolished, except for the transmitter Gliwice.
Brecon VHF-transmitter is a small VHF-transmitter using wood poles. Ferryside Relay is a TV relay transmitter using also a wood pole.

[edit] Other types of antenna supports and structures

[edit] Poles

Shorter masts may consist of a self-supporting or guyed wooden pole, similar to a telegraph pole. Sometimes self-supporting tubular galvanized steel poles are used: these may be termed monopoles.

[edit] Buildings

In some cases, it is possible to install transmitting antennas on the roofs of tall buildings. In North America, for instance, there are transmitting antennas on the Empire State Building and the Sears Tower. Such facilities also exist in Europe, particularly for portable radio services and low-power FM radio stations.

[edit] Disguised cell-sites

Many people view bare cell phone towers as ugly and an intrusion into their neighbourhoods. Even though people increasingly depend upon cellular communications, they hate the bare towers popping up across scenic views. People allow modern technology like roads, power lines, telephone poles, lamp posts, water towers, etc., and have become used to them. Many companies offer to 'hide' cellphone towers as trees, church towers[1], flag poles, water tanks, etc. There are many providers that offer these services as part of the normal tower installation and maintenance service. These are generally called "stealth towers" or "stealth installations".

The level of detail and realism achieved by disguised cell phone towers is remarkably high; for example, such towers disguised as trees are nearly indistinguishable from the real thing, even for local wildlife (who additionally benefit from the faux flora[2]). Such towers also have the added advantage of being able to be placed in national parks and other such protected places without being obtrusive, such as towers disguised as cacti in Coronado National Forest[3]. Even when disguised, however, such towers can create controversy; such a tower doubling as a flagpole attracted controversy in 2004 in relation to the U.S. Presidential campaign of that year, and also highlighted the sentiment that such disguises serve more to allow the installation of such towers in subterfuge away from public scruitiny rather than to serve towards the beautification of the landscape.[4]

[edit] Mast radiators

A mast radiator is a radio tower or mast in which the whole structure works as an antenna. It is used frequently as a transmitting antenna for long or medium wave broadcasting.

Structurally, the only difference is that a mast radiator may be supported on an insulator at its base. In the case of a tower, there will be one insulator supporting each leg.

Technical details are outside the scope of this article and can be found on the mast radiator page.

[edit] Telescopic, pump-up and tiltover towers

A special form of the radio tower is the telescopic mast. These can be erected very quickly. Telescopic masts are used predominantly in setting up temporary radio links for reporting on major news events, and for temporary communications in emergencies. They are also used in tactical military networks.

Telescopic masts consist of two or more concentric sections and come in two principal types:

  • Pump-up masts are often used on vehicles and are raised to their full height pneumatically or hydraulically. They are usually only strong enough to support fairly small antennas.
  • Telescopic lattice masts are raised by means of a winch, which may be powered by hand or an electric motor. These tend to cater for greater heights and loads than the pump-up type. When retracted, the whole assembly can sometimes be lowered to a horizontal position by means of a second tiltover winch. This enables antennas to be fitted and adjusted at ground level before winching the mast up.

[edit] Balloons and kites

A tethered balloon or a kite can serve as a temporary support. It can carry an antenna or a wire (for VLF, LW or MW) up to an appropriate height. Such an arrangement is used occasionally by military agencies or radio amateurs. The American broadcasters TV Martí broadcast a television program to Cuba by means of such a balloon. A captive balloon was also used for the British GQV experimental transmitter in 2003.

[edit] Design features

[edit] Economic and aesthetic considerations

  • The cost of a mast or tower is roughly proportional to the square of its height.
  • A guyed mast is cheaper to build than a self-supporting tower of equivalent height.
  • A guyed mast needs additional land to accommodate the guys, and is thus best suited to rural locations where land is relatively cheap. A tower will fit into a much smaller plot.
  • A steel lattice tower is cheaper to build than a concrete tower of equivalent height.
  • Two small towers may be less intrusive, visually, than one big one; especially if they look identical.
  • Towers look less ugly if they and the antennas mounted on them appear symmetrical.

[edit] Masts for HF/shortwave antennas

For transmissions in the shortwave range, there is little to be gained by raising the antenna more than a few wavelengths above ground level. Shortwave transmitters rarely use masts taller than about 100 metres.

[edit] Access for riggers

Because masts, towers and the antennas mounted on them require maintenance, access to the whole of the structure is necessary. Small structures are typically accessed with a ladder. Larger structures, which tend to require more frequent maintenance, may have stairs and sometimes a lift.

[edit] Aircraft warning lamps

Taller structures are often equipped with aircraft warning lamps, usually red in colour, to warn pilots of the structure's existence. In the past, ruggedized and under-run filament lamps were used to maximise the bulb life. Nowadays such lamps tend to use LED arrays.

[edit] Wind-induced oscillations

One problem with radio masts is the danger of wind-induced oscillations. This is particularly a concern with steel tube construction. One can reduce this by building cylindrical shock-mounts into the construction. One finds such shock-mounts, which look like cylinders thicker than the mast, for example at the radio masts of DHO38 in Saterland. There are also constructions, which consist of a free standing tower (usually from reinforced concrete), onto which a guyed radio mast is installed. The most well-known such construction is the Gerbrandy Tower in Lopik (the Netherlands). Further towers of this building method can be found near Smilde (the Netherlands) and Waldenburg (Baden-Wuerttemberg, Germany).

[edit] See also

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[edit] External links

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