A guy-wire or guy-rope is a tensioned cable designed to add stability to structures (frequently ship masts, radio masts, wind turbines, utility poles, and tents). One end of the cable is attached to the structure, and the other is anchored to the ground at a distance from the structure's base. They are often configured radially (equally spaced about the structure) in trios, quads (pairs of pairs) or other sets. This allows the tension of each guy-wire to offset the others. For example, roof antennas are sometimes held up by three guy-wires.
On some very high structures flight safety markers on the guys themselves are necessary. Shorter, sturdier structures, such as electrical utility poles, may require only a single guy-wire to offset the pull of the electrical wires.
For chimneys and masts for VHF-/UHF-transmissions or non-transmission use, only the mechanical properties of the guys are important. This is not the case for mast antennas, for masts with aerials for VLF, LF, MF, and SW or for masts situated close to such aerials.
Template:See A guy is a term for a line (rope) attached to and intended to control the end of a spar on a sailboat. On a modern sloop-rigged sailboat with a symmetric spinnaker, the spinnaker pole is the spar most commonly controlled by one or more guys.
Guys for mast antennasEdit
Special attention must be paid to guy-wires of mast antennas. Guys of conductive materials such as metals and that are longer than one-fifth of the radiated wavelength can have a strong influence of the radiation pattern, especially when used as a mast antenna or carrying an aerial for VLF, LF, MF, and SW. Guy wires also sometimes interfere when situated close to such a site.When steel cable is used, the guys are divided by insulators into multiple sections, each smaller than one-fifth of radiated wavelength. Porcelain is often used for these insulators. The individual sections of the guys can develop large charges of static electricity, especially on very tall masts. The voltage caused by this static electricity can be several times larger than that generated by the transmitter. In order to avoid dangerous and unpredictable discharges, the insulators must be designed to withstand this high voltage, which results at tall masts in overdimensioned backstage insulators. At each backstage insulator, a lightning arresters in the form of an arc gap (and often additionally an arrester) is required for the purpose of overvoltage protection in case of lightning strikes. The insulators and arresters must be maintained carefully, because an insulator failure can result in a mast collapse.
Some newer mast antennas are fitted with insulators at the mast construction and are grounded via coils located near each ground anchor. In some cases it is possible to ground the guys directly at the anchor blocks. This is only possible if the guys do not disturb the radiation pattern of the mast antenna. Guys grounded via a coil or directly have the advantage that maintenance is easier, because all parts requiring maintenance are at the mast and at the anchor basement and that the insulators must only withstand the maximum transmission voltage.
On some mast antennas guys of non-conductive polymers are used. Although these guys alleviate some of the problems associated with mast antennas they are rarely used as they are not as durable and long lasting as metal guys.
On antennas for longwave and VLF, the guys may serve an electrical function, either for capacitive lengthing of the mast or for feeding the mast with the radiation power. In these cases, the guys are fixed without an insulator on the mast, but there is at least one insulator in the guy if necessary. If guys are used for feeding the mast with high frequency power it is often possible to use a grounded mast. The power to the guys is fed via conductor ropes running from the tuning unit to the feed point on the guys.
A list of famous guyed structures can be found on List of masts. There are also structures which consist of a free standing bottom and a guyed top.
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