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.
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In the case of a mast radiator or radiating tower, the whole mast or tower is itself the transmitting antenna.
Mast or tower?
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 guys. Broadcast engineers in the UK use the same terminology. A mast is a ground-based or rooftop structure that supports antennas at a height where they can satisfactorily send or receive radio waves. Typical masts are of steel lattice or tubular steel construction. Masts themselves play no part in the transmission of mobile telecommunications. Masts (to use the civil engineering terminology) tend to be cheaper to build but require an extended area surrounding them to accommodate the guy wires. 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. 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 with guys which go to ground.
- Torre de Collserola, a guyed tower with a guyed mast on top where the tower portion is not free-standing.
Experimental radio broadcasting began in 1905, and commercial radio broke through in the 1920s.
Until August 8, 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 (1968.5 ft) or taller.
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.
Guyed masts 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.
In several cities in Russia and Ukraine several tubular guyed masts with crossbars running from the mast structure to the guys were built in the 1960s. All these masts, which are designed as 30107 KM, are exclusively used for FM and TV transmission and, except for the mast in Vinnytsia, are between 150–200-metre (490–660 ft) tall. The crossbars of these masts are equipped with a gangway that holds smaller antennas, though their main purpose is oscillation damping.
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.
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.
Fiberglass and other composite materials
Fiberglass poles are occasionally used for low-power non-directional beacons or medium-wave broadcast transmitters. Carbon fibre monopoles and towers have traditionally been too expensive but recent developments in the way the carbon fibre tow is spun have resulted in solutions that offer strengths similar or exceeding steel for a fraction of the weight which has allowed monopoles and towers to be built in locations that were too expensive or difficult to access with the heavy lifting equipment that is needed for a steel structure.
Wood has been superseded in use by metal and composites for tower construction. Many wood towers were built in the UK during World War II because of a shortage of steel. In Germany before World War II wooden towers were used at nearly all medium-wave transmission sites which have all been demolished, except for the Gliwice Radio Tower.
Ferryside television relay station is an example of a TV relay transmitter using a wooden pole.
Other types of antenna supports and structures
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.
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, the Willis Tower, 4 Times Square, and One World Trade Center. The North Tower of the original World Trade Center also had a 110-metre (360 ft) telecommunications antenna atop its roof, constructed in 1978–1979, and began transmission in 1980. When the buildings collapsed, several local TV and radio stations were knocked off the air until backup transmitters could be put into service. Such facilities also exist in Europe, particularly for portable radio services and low-power FM radio stations. In London, the BBC erected in 1936 a mast for broadcasting early television on one of the towers of a Victorian building, the Alexandra Palace. It is still in use.
Many people view bare cellphone towers as ugly and an intrusion into their neighbourhoods. Even though people increasingly depend upon cellular communications, they are opposed to the bare towers spoiling otherwise scenic views. Many companies offer to 'hide' cellphone towers in, or as, trees, church towers, flag poles, water tanks and other features. 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", or simply concealed cell sites.
The level of detail and realism achieved by disguised cellphone 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 artificial flora). Such towers can be placed unobtrusively in national parks and other such protected places, such as towers disguised as cacti in United States' Coronado National Forest.
Even when disguised, however, such towers can create controversy; a tower doubling as a flagpole attracted controversy in 2004 in relation to the US Presidential campaign of that year, and highlighted the sentiment that such disguises serve more to allow the installation of such towers in subterfuge away from public scrutiny rather than to serve towards the beautification of the landscape.
Disguised cell sites sometimes can be introduced into environments that require a low-impact visual outcome, by being made to look like trees, chimneys or other common structures.
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.
Telescopic, pump-up and tiltover towers
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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. They can save money by needing to withstand high winds only when raised, and as such are widely used in amateur radio.
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.
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.
In 2013, interest began in using unmanned aerial vehicles (drones) for telecom purposes.
Other special structures
For ELF transmitters ground dipole antennas are used. Such structures require no tall masts. They consist of two electrodes buried deep in the ground at least a few dozen kilometres apart. From the transmitter building to the electrodes, overhead feeder lines run. These lines look like power lines of the 10 kV level, and are installed on similar pylons.
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 equal height.
- A guyed mast needs additional land to accommodate the guys, and is thus best suited to rural locations where land is relatively cheap. An unguyed tower will fit into a much smaller plot.
- A steel lattice tower is cheaper to build than a concrete tower of equal 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.
- Concrete towers can be built with aesthetic design, especially in Continental Europe. They are sometimes built in prominent places and include observation decks or restaurants.
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.
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, also called a service elevator.
Aircraft warning features
Tall structures in excess of certain legislated heights are often equipped with aircraft warning lamps, usually red, to warn pilots of the structure's existence. In the past, ruggedized and under-run filament lamps were used to maximize the bulb life. Alternatively, neon lamps were used. Nowadays such lamps tend to use LED arrays.
Height requirements vary across states and countries, and may include additional rules such as requiring a white flashing strobe in the daytime and pulsating red fixtures at night. Structures over a certain height may also be required to be painted with contrasting color schemes such as white and orange or white and red to make them more visible against the sky.
Light pollution and nuisance lighting
In some countries where light pollution is a concern, tower heights may be restricted so as to reduce or eliminate the need for aircraft warning lights. For example, in the United States the 1996 Telecommunications Act allows local jurisdictions to set maximum heights for towers, such as limiting tower height to below 200 feet (61 m) and therefore not requiring aircraft illumination under US Federal Communications Commission (FCC) rules.
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. One example is the Gerbrandy Tower in Lopik, Netherlands. Further towers of this building method can be found near Smilde, Netherlands and the Fernsehturm in Waldenburg, Germany.
Hazard to birds
Radio, television and cell towers have been documented to pose a hazard to birds. Reports have been issued documenting known bird fatalities and calling for research to find ways to minimize the hazard that communications towers can pose to birds. There have also been instances of rare birds nesting in cell towers and thereby preventing repair work due to legislation intended to protect them.
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- "Some New York City TV and Radio Stations Off The Air After World Trade Center Collapse". Archived from the original on 2006-12-31.
- CARC - UNC-Charlotte - Class 1000 Clean Room Facilities Archived 2006-09-12 at the Wayback Machine
- Cell Phone Trees
- "Stealth towers".
- "Telecom-equipped drones could revolutionize wireless market". azcentral.com.
- Shire, Gavin G.; Karen Brown; Gerald Winegrad (June 2000). "Communication Towers: A Deadly Hazard to Birds" (PDF). American Bird Conservancy. Earthjustice. Retrieved 2010-09-29.
- "Avian Collisions at Communication Towers - Sources of Information". U.S. Fish and Wildlife Service. June 1, 2009. Retrieved 2010-08-13.
- "Nesting falcon hits Vodafone customers in Southampton". BBC News. 15 April 2013. Retrieved 20 May 2013.
- Ray, Bill (17 April 2013). "Angry Birds fire back: Vulture cousins menace UK city's mobiles". The Register. Retrieved 20 May 2013.
- Sreevidya, S., and Subramanian, N., Aesthetic Appraisal to Antenna Towers, Journal of Architectural Engineering, American Society of Civil Engineers, Vol. 9, No. 3, September 2003, pp. 102–108
|Wikimedia Commons has media related to Communications towers.|
- All US Towers over 200 feet, transmitting on certain frequencies, or have certain transmitters over a certain power, must be registered in the US. This is the online directory.
- The Transmission Gallery: Broadcast Transmission Sites in the UK
- The Transmission Gallery: Constructing Stayed Masts
- Scott Fybush, international tower photographer who has documented thousands of towers in his travels
- Tom Bosscher of Western Michigan's website on towers of Michigan
- Mike Fitzpatrick's NECRAT.US tower based website contains tower pictures from New England, New York, and beyond.
- Turkish towers (in Turkish)
- Amateur Radio Tower construction project
- Displays over 12,000 80+ meter tall towers used in wind resource assessment
- French towers (in French)
- The Legal Landscape When a Tower Collapses
- Richard Moore's Anorak Zone Photo Gallery of UK TV and Radio transmission sites
- Mobile Phone Masts & Radio Base Station Planning UK
- Various Communication Masts &Towers in Atlanta, Georgia, US
- UK masts and towers at thebigtower.com
- Searchable map of all registered transmitters in Australia