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Clock Radios, Talking Clock Radios, Portable Radio, Large Button Clock... independentliving.com | Hospital Radio Ipswich :: Radio Shows :: Radio Show Detail hospitalradioipswich.org.... |
 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 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.
[edit] 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. Masts tend to be cheaper to build but require an extended area surrounding them to accommodate the 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:
[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 steelSome 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. At several cities in Russia and Ukraine, guyed masts were built between 1960 and 1965 with crossbars running from the mast structure to the guys. All these masts are tubular structures, used exclusively for FM/TV transmission. Except for the mast in Vinnytsia, these masts have heights between 150 and 200 metres.  [edit] Reinforced concreteReinforced 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. Many are still in use today. 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] FibreglassFibreglass poles are occasionally used for low-power non-directional beacons or medium-wave broadcast transmitters. [edit] WoodThere 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 wooden towers were used at nearly all medium-wave transmission sites, but all of these towers have since been demolished, except for the Gliwice Radio Tower. Ferryside Relay is an example of a TV relay transmitter using a wooden pole. [edit] Other types of antenna supports and structures[edit] PolesShorter 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] BuildingsIn 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 Sears Tower, and formerly on the World Trade Center towers. When the buildings collapsed, several local TV and radio stations were knocked off the air until backup transmitters could be put into service.[1] 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 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 as trees, church towers, flag poles, water tanks and other features.[2] 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 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[3]). Such towers can be placed unobtrusively in national parks and other such protected places, such as towers disguised as cacti in Coronado National Forest.[4] Even when disguised, however, such towers can create controversy; a tower doubling as a flagpole attracted controversy in 2004 in relation to the U.S. 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.[original research?][5] [edit] Mast radiatorsMain article: Mast radiator 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. [edit] Telescopic, pump-up and tiltover towersMain article: Cell on wheels 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:
[edit] Balloons and kitesA 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] Other special structuresFor two VLF transmitters wire antennas spun across deep valleys are used. The wires are supported by small masts or towers or rock anchors. See List of spans: Antenna spans across valleys. The same technique was also used for the Criggion VLF transmitter. 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. [edit] Design features[edit] Economic and aesthetic considerations
[edit] Masts for HF/shortwave antennasFor 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 riggersBecause 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 featuresTall 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. 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. [edit] Light pollution and nusiance lightingIn 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 and therefore not requiring aircraft illumination under FCC rules. The limit is more commonly set to 190 or 180 feet to allow for masts extending above the tower. [edit] Wind-induced oscillationsOne 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 best 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-Württemberg, Germany). [edit] Catastrophic collapsesMain article: List of catastrophic collapses of radio masts and towers [edit] See also
[edit] References[edit] Additional reading
[edit] External links
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