Assembling your antenna system
Assembling of the tower (V)
The guy wires
Radio amateurs use guy wires to attach the tower supporting their antenna, but they usually know few things about the physical properties of these "cables" except that they prevent the tower of falling ! It is time to remind you some notions.
Guy wires attached at various heights along the structure restrain the tower against the strength of the wind, translating the lateral force applied upon the tower in a downward compression force onto the base.
Therefore their tension must respect the manufacturer's tower specifications. Loose guy wires are useless but guy wires tighted as strong as a piano string, enduring pulling forces exceeding 500 kg (1000 lbs.) will create too much lateral forces, creating an additional torque onto the base; by high winds this is on the contrary the best solution to see your bolts and nuts flying away and your tower topple ! Hence the necessity to respect the specifications and to tightly anchor the tower base.
If there are not enough guy wires the vertical spacing between them can be excessive and it might result in the tower buckling and an uprooting of the base. Idem if the guy wires are not equaly spaced around the tower. It is recommended to attach at each guy colar either 3 guy wires 120° apart (seen from above) or 4 guy wires 90° apart. They must be anchored in the ground at the same level as the tower base at a distance at least equal to 75% of the height of the tower.
While small and light masts 6m high require often to be guyed in the middle and on top to prevent too much swing, an heavy duty tower 10m high (33') bolten in its base can be secured in using only one set of guy wires attached on top. Usually for standard height of towers it is recommended to install 2 set of guy wire, one at half height, the second set on top. If the tower is very long or frail you have advantage to attach the guy wires at all thirds, the third one being attached on top.
Guy wires play also the role of resonant element if their length is within 10% of your working frequency. Like any wire antenna guy wire can "pick-up" some energy from the antenna and radiates for their own ! This has no other harm consequence than modifying the radiation pattern of your antenna and in a few occasions to generate RFI.
In practice, two insulators are used on each guy wire, each placed 30 cm or so (1') from the anchorage point. For more security, if you work with eggs strain insulators (made of porcelain) pass the guy wire across the insulator and tie it around it also, so that in case of break the guy wire will still hold, although loosier. To prevent any resonance cut the guy wire so that no section displays a length in feet that can be evenly divided by 16 or 22. Therefore according a study made by Jerry Hall, K1TD, the ideal guy wire lengths are 0-4.2m, 5.4-6m, 7.2-8.7m, 11.7-12m and 17.1-17.4m equivalent to 0-14 ft, 18-20ft, 24-29 ft, 39-40 ft and 57-58 ft. Thus, for a 10m high tower, which guy anchors are placed at a distance equivalent to 75% of the tower height, the longest guy wire is 12.5m long (41 ft) minus twice the 30cm required for the thimble and clamps, so we fall well in the 11.7-12m range. If the guy wire is a bit shorter than this value it is in resonance on 20m and 10m; if it is a bit longer it is in resonance on 15m and 10m too. At mid-height of this tower, the guy wire is 9m long minus twice 30 cm; here also we fall well in the good lenghts.
Guy wires are generaly made of twisted steel strands able to support a pulling of several tons. It can optionally be protected with a PVC jacket. The amateurs should use the standard "ham approved" wire of Extra High Strenght grade EHS 3/16" or 4.7mm thick. Except for light installations, do not use an aircraft cabling system as it sustains a strenght only 70% as strong. New material allow also to build "plastic" guy wires, in fact made of high resistant polyester protected with a polyamid jacket. These new design are however more expensive than the ordinary steel cable.
To transform a steel cable in a guy wire, buy the require metering to avoid the resonance and fold back the open ends of the cable on a distance of about 30 cm (12") around a thimble to prevent the wire of breaking due to the sharp bending. The excess of wire should be tightly attached along the guy wire with three of more mini-clamps, leaving a few dozen centimeters of wire free. This free end will be strengthened later, when all will be installed in the case you should have to adjust the cable lengths, using a serving tool or pliers and wrapping a thin metallic wire around the guy wire.
Note that if you plan to pass the guy wire in a close loop to attach it to an anchor (to a steel loop fixed on the colars attached on the tower or in the eye of a turnbuckle for example), don't forget to pass first the thimble and the guy wire in the opening of the loop before achieving the cable.
When the end of the guy wire is "clampsed" and well fixed in its thimble, it can be attached to the anchorage system. Use for example the classic turnbuckle, this sort of double-eye-screw allowing to adjust the tension on the guy wire without twisting the cable. Select a model which "eyes" are compatible with the size of the thimble or smaller than the size of the bolts if yoy attach them to the loop of an anchor. In case of problem you can also use snap-links. At last, to prevent the turnbuckle from working loose, it is recommended to pass a safety cable in a figure-height fashion in the eyes of the turnbuckle.
The guy anchors and colars
The guy wires must be tightly attached, one side at several places on the tower, the other side to guy anchors buried in the ground. The guy anchors should be made of stainless steel bars, 1m long (40") for 3-4 cm wide (1-2"). You have the choice of the model : plane, in form of L-bracket, like a stick with a loop at one end or even using tubing. There must have one or more large holes on top to attach a heavy bolt or any other fixing means. If you need to attach more than one guy wire to the anchor, buy an single equalizer plate accepting 3 or more guy wires and attach this plate to the anchorage system.
First drive a hole 1m depth (40") and 50 cm wide (20") in the ground, if possible tilted at 45° in the direction opposite to the tower. Fill it with 30 cm (1 ft) of concrete and sunk the anchors on 1/4 to 1/3d of their length. The emerging part of the anchor must not exceed 10 to 30% of the anchor length. As displayed in the above graphs, you can either tilt the anchor in the direction perpendicular to the pulling force, thus the feets of the anchor to the tower (left drawing), or place it in the same direction as the guy wires (right drawing). When the concrete is hard, fill the hole with soil and mark it well as an obstacle. In the dark you could forget it and fall head on on it !
Very few anchor shafts are galvanized. When driven in the ground and partly sunk in the concrete, they can corrrode. Indeed, the shaft itself acts as both anode and cathode and its high conductivity makes it a perfect electrical path between the two parts. The concrete and soil act also as two different electrolytes. We are thus in presence of chemical reactions able weaken the metal resistance.
In addition, the soil has less oxygen just above the concrete anchorage and consequently, less resistivity. This phenomenon causes corrosion that is most strongly in effect in the shaft underground area located between the ground level and the concrete. Experience has shown that this is the area most likely to deteriorate and cause the tower to fail.
The anchor can also experiment corrosion due to "stray current" if for example your tower is located close to electrified railways or if there are welding or plating works. These operations put also direct current into the ground.
There are product to protect your anchors against these effects. They consist in coating the shaft before driving it into the ground. Anchor Guard for example provide such products against corrosion and very instructive pictures showing damage that might happen. They are serious... Products exist also to protect steel against rust using cold galvanizing to name Brite products.
If the depth is limited to a few dozen cm use half-lenght anchors. Fill the middle of the hole with concrete so that 1/3 of the guy anchor is sunk in the concrete. Note that free-standing towers do not require guy wires as the wind acts in compression and uplift upon the structure.
Do not attach immediately the guy wires. Attach first the guy colars on the tower as suggested by the manufacturer. For large towers they come often like rotating steel rings to bolt to the tower tubular structure. Without these accessories and more instructions, I suggest you a method similar to the drawings displayed below.
Place around the tower, at each third of the heigth or at the middle and on top, a large colar. You can use either rotating guy colars or steel strips 5 mm thick (2"), 10 cm wide (4") and 20 cm (8") longer than the length of the tower sides (so 10 cm on each side).
Now, you have several possibilities as displayed above :
- Solution A, drill two holes at the end of the colar, the innermost attaching the colar to the tower tubular structure. Attach a torque bracket around the axis the second bolt, the guy wire being attached at the other end of the torque bracket. This solution supports the best the strains;
- Solution B, use a steel strip with a notch. Drill a hole on both sides of the notch, the first to bolt the colar on the tower, the second to receive a locker. Place a turnbuckle attached to the guy wire in the noch and secure it in closing down the locker;
- Solution C, drill two holes at the end of the colar and use a snap-link to attach the guy wire;
- Solution D, drill a hole on both sides of the notch, the first to bolt the colar on the tower, the second to receive a locker. Insert a close steel loop in the notch an secure it. Attach then a guy wire to the loop.
As usual, the simpler solution using the less accessories is always the best.
Of course in all cases you need to install the thimble in the loop before achieving the guy wire. When all corners of the tower are bolten and the guy wires are attached, goes down the tower. Now you can tight the guy wires up to the anchors and attach them tighly.