Guy-wire

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A guy-wire or guy rope is a metal wire used to aid stability in tall structures (frequently radio masts). It is attached to the structure on one end and the ground on the other. They are almost universally used in counter-balancing sets or pairs, with one wire on one side of the structure, and one on the other side.

A guy-wire is often placed at the end of a string of power poles to counterbalance the pull of the wires.

Roof antennas are usually held up by three guy-wires.

On some high structures flight safety markers in the guys are necessary. 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.

[edit] Guys for mast antennas

Special attention must be paid for guys 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.

On most mast antennas steel guys are used. The guys are divided by insulators into multiple sections, each smaller than one-fifth of radiated wavelength. Steatite 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 caused by the transmitter. In order to avoid discharges the insulators have to 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, which are situated next to the anchor blocks at which the guys are fixed on insulators. 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 stand the maximum voltage, which occurs at transmission operation.

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.