Thursday, July 4, 2013

Opportunistic Tower Guying - Follow-up

Over the past week I have been gradually working on completion of tower guying. The tower is small so this is nothing elaborate. The challenges are due to the location and type of guy anchors. Since I am now done -- and the tower is ready for antennas -- it is perhaps worth a few final comments on opportunistic tower guying.

Trees Move

Southwest anchor - tamarack tree
It is worth the effort to get an estimate of tree motion due to wind. Even a large trunk can move around. Indeed, this should be obvious since the roots don't move and the the upper part of the tree does move. Movement is therefore some function of height (and wind).

We had some thunderstorms recently that provided the data I needed. The spruce tree to the southeast was surprisingly immobile at my selected anchor height even in a strong storm surge. However the tamarack to the southwest noticably moved even at head height. This is despite the small difference in trunk circumference at my chosen anchor positions: 125 cm for the spruce versus 105 cm for the tamarack.

First, I moved the anchor point lower on the tamarack. The position cannot be arbitrary since there must be some means to prevent the guy from slipping upward under tension and motion. Luckily there was another small branch stump that met the requirement. It's small enough that I will provide additional restraint.
Southeast anchor - spruce tree

I made wire rope trunk cradles from ¼" wire rope (aircraft cable) I had in my junk box. The cradles are wider than the trunk and held fast by guy tension. The wire size was selected to minimize stress on the tree, not because I needed several tons of breaking strength. The picture shows the tamarack cradle, complete with thimble and (smaller) guy wire attached and under tension. The thimble was a perfect place to stow the extra rope of the lower rope guy, in addition to its primary job of terminating the guy wire.

Wire or Rope

One of the problems with guyed towers is that guys are typically made of steel and steel conducts. Antenna currents will be induced on steel guy wires so it is important to minimize interactions. Interactions can result in distorted antenna patterns, especially reduction of front-to-back depth, and impedance/match changes. Rope and kevlar do not conduct, but are not as robust as steel.

Plastic rope is susceptible to ultraviolet (UV) degradation, depending on which material is selected. It also stretches. The stretching was attractive since, as noted above, trees move. Kevlar and steel barely stretch at all. Even selecting UV-resistant (not UV-impervious!) rope such as dacron (polyester) is not sufficient since there are additional concerns: cutting; foliage abrasion; and weathering.

In the end I decided to go with steel for the main, upper guys and rope (polyester) for the lower guys. Steel is cheaper than rope or kevlar, though not so much once you include insulators, clips and thimbles. Abrasion from foliage is a concern -- which favours steel -- because both guys from the two tree anchors pass through foliage. The problem worsens when the wind blows.

Non-resonant steel guys
To minimize interaction it is necessary to break steel guys into non-resonant lengths. Each section of the guy should be non-resonant on every band of all antennas within about 2 wavelengths. I had a bunch of ¼" guy insulators in my junk box, and I purchased the additional clips I needed. I used bolt cutters to cut up the cable -- ordinary wire cutters are inadequate since the wire is hardened steel.

You can see how I broke up the upper guys in the adjacent picture. The top section is kept as short as possible so that the tower itself is not made electrically longer. Tower resonance is a separate problem that will impact the vertically-polarized low-band antennas I am planning. I will address this in a later article.

The second section of each guy is 26'-0". This is non-resonant on all pre-WARC bands, but is resonant on 17 meters. I deliberately did this since there is some possibility I could put the guys to some use in this way, without impacting the antennas since none for 17 meters is planned for this tower.

The final and third section of each guy is short enough or far enough away that nothing needed to be done to avoid resonance. In each case the cable was cut to make up the remaining length.


The guy tension is not according to the text book. A good rule of thumb is to select the wire to meet the installation's strength requirement, then pre-load the guy to 10% of breaking strength. This only works if the anchors are fixed and the angle between guy and tower is within a fairly narrow range. I could not follow this prescription for the following reasons:
  • Two of the anchors are trees, which move. Steel does not appreciably stretch so if you follow the textbook it is likely that something will break. What breaks, in my case, is likely to be the tower welds.
  • The angles between guy and tower are all different. This requires different tension in each guy to keep the tower in position.
The tree trunk cradles I described above handle a large proportion of the movement, by means of the cradle changing its shape as tension changes and the rest coming from the guy itself, and by opposite tension changes in the other guys. The rope guys simply stretch as required.

This only works if the pre-load tension is low. The trick is to find a compromise between tower rigidity and room to accommodate increased tension when the wind blows. I adjusted the tension by hand, so the pre-load is likely no more than 20 kg. This was an iterative process to get the tension about right and the tower vertical. At that low tension the tower does move, as I intended, yet is comfortable for climbing and working at the top. The tower can't be shifted much by hand since the steel guys rapidly increase in tension with small lateral deflections.

Safety and Reliability

No antenna or tower installation should be attempted without addressing safety and reliability. These are too often skipped or, perhaps unwisely, done by brute force by going large and expensive. There are a few points in regard to this tower that I will cover here, and not the subject area as a whole.

I do not climb unsafe towers. It is therefore no surprise that I want this tower to be safe to climb and work on. I believe I've achieved that. As mentioned earlier, it is reasonably stable and vertical, without too many encumbrances along its length. On this last point, I plan to cover up the ends of the wire guys adjacent to the tower to avoid contact with their razor-sharp edges.

Although I don't anticipate vandalism or other mischief it is worth some effort to protect the tower. It is impossible to protect against a deliberate attack, but it is possible to not create opportunities for interlopers. Kids in particular are curious and enjoy a bit of mischief. Steel guys help in this regard. Unless one comes along with the correct wrench and sockets (and arm strength) and high-quality bot cutters, the tower won't come down. The rope guys are easily cut with a knife but in most cases are out of reach without a ladder.

North anchor, with post to raise the guy wire
The north guy anchor is (or was) a problem. The anchor is set low in house frame and, since this anchor is far from the tower, the guys were chest height near the back deck. This is an accident waiting to happen! My solution can be seen in the picture. I used a pressure-treated post to raise the effective anchor point above head height. It is not enough to just run the guy wire through the top of the post. I used ¼" wire so that it would remain strong when bent at entry and exit points (and harder to cut). The post must also be stabilized since otherwise the guy tension, especially as the tower rocks in the wind, would bend the post forward and down. The main protection against this is the lower stay which holds the post in position. The redundant protection is use of clips at the entry and exit points to keep the guy wire fixed with respect to the post.

Redundancy is your friend. Guy hardware is cheap so don't scrimp: 2 clips at every guy termination; two set of guys, each of which alone will hold up the tower; oversize cable and insulators; etc. Rust, accident, weather and manufacturing defects do occur. Be ready for it.

Next Steps

Before putting up any antennas on the tower I would like to do a wind test. With the hot and humid weather this week there is very likely to be a good test very soon!

With that out of the way I plan to put up a temporary antenna for the summer. Come fall I will erect permanent low-band antennas. I will have more to say on these in future articles.

One of the benefits of a small tower rather than a mast it the ability to experiment with antennas without having to lower and raise the mast for each antenna or, worse, for each adjustment.

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