Radio masts and towers
Radio masts and towers are, typically, tall structures designed to support antennas for telecommunications and broadcasting.
In the case of a mast radiator, the whole mast or tower is itself the transmitting antenna.
| Contents |
Mast or tower?
The terms "mast" and "tower" are often used interchangeably. But in engineering jargon, a tower is a self-supporting or cantilevered structure, while a mast is held up by stays or guys.
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 hybrid 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.
Materials
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.
Tubular steel
Some masts are constructed out of steel tubes. In the UK, these were the subject of spectacular collapses at the Emley Moor and Waltham-on-the-Wolds TV stations in the 1960s.
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. They can form prestigious landmarks, such as the CN Tower in Toronto.
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.
Fibreglass
Fibreglass poles are occasionally used for low-power non-directional beacons or medium-wave broadcast transmitters.
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.
Other types of antenna supports and structures
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.
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, flag poles, water tanks, etc. There are many providers that offer these services as part of the normal tower installation and maintenance service.
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.
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.
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 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.
Design features
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 structuress, which tend to require more frequent maintenance, may have stairs and sometimes a lift.
Aircraft warning lamps
Taller structures are often equipped with 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.
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).
See also
- List of towers
- List of masts
- Mast radiator
- Radio masts and towers - catastrophic collapses
- Lattice tower (here also list of radio towers built of wood)
External link
The Transmission Gallery: Broadcast Transmission Sites in the UK