Thursday, June 27, 2013

Tower Up

The weather was finally conducive to tower work today, without rain, high winds or lightning. I also had no business or other responsibilities. Sunny, warm and calm air -- perfection.

Since I had prepared in advance it was only a matter of actually doing the work. However it did not go as I would have preferred. The reason is that I wanted to do this job without helpers. One-man tower erections are a little different than the sort most hams are familiar with. I enjoy challenges.

My first plan, if only it had worked out, was to lift the entire tower (8.8 meters high and 40 kg weight) in one go. Using trigonometry and careful placement of ropes my plan was to manoeuver it into a vertical orientation by using "lift ropes" to both cradle and rotate the top end of the tower. For small towers and masts this can be easily accomplished with 3 people.

Read the full article in June 2013 QST
By my lonesome it did not work out. I have no problem lifting the 40 kg weight over my head. The problem is that you must simultaneously move the tower laterally, against considerable forces, so that the lift ropes rotate upward. I can do one or the other (lift or push), not both at once.

To get an idea of the forces involved please read the article in June 2013 QST, page 67, "Field Day Towers - Doing It Right". Figure 1 gets the idea across. The difference in my configuration is to fix the end of the lift rope (actually 2 of them, 120° apart) and move the bottom. The vector analysis is otherwise identical.

I am not so stubborn that I would contest the laws of physics (and risk my safety). I know vector calculus all too well! But if it had worked ...

On to plan number 2. I separated the 3 tower sections and planted the base section and base plate on the floating base. With everything properly machined and leveled it stood up on its own. I then loosely bolted it to the floating base, allowing some play. Lift ropes were attached to this 10' section to act as temporary guys. I tensioned the ropes and made sure the section was close to vertical. So far so good.

I brought out my climbing equipment, leaned the second section against the first and secured myself at the top of the, so far, very short tower. I grabbed the 12.1 kg second section and lifted it up. The weight isn't much but lifting it up until you are holding it near the bottom, while simultaneously keeping it vertical, is difficult. I know enough to avoid disasters so I rapidly assessed the odds of successfully mating the section without tipping the section. The odds didn't look good.

Tipping is dangerous since the centre of gravity is well above your head. This technique works well with two experienced tower workers, even for tower sections 2 or 3 times the weight of this one. I couldn't do it on my own, or at least the odds were against me. I put the section down again and moved onto plan number 3.

Plan 3, my ultimate fallback choice, was guaranteed to work. It involved cobbling together a gin pole from bits and pieces I had at hand. I made one from the 10' mast that came with the tower and a length of aluminum angle stock that (conveniently) had a pulley affixed to one end. From my bag of pipe clamps I chose one that easily secured the mast to the aluminum angle stock. The round mast nestled perfectly inside the angle stock. The pulley was rusty but servicable.

U-bolts and hose clamps secured the improvised gin pole to the tower. A thick slab of wood bridging and tied to the horizontal braces served as the bottom support for the gin pole. Don't rely on only clamps to support lifting forces!

Once in place it took about 10 minutes to lift the second section into position, lock the rope, climb the tower and bolt the section in place.

The third (top) section went nearly as smoothly. First, however, I installed the UV-resistant guy ropes to the top of the second section, up about 18 feet. The spruce tree gave me a good fight since the rope needed  to get up and over several healthy, large branches. I used 4 x 4' nesting fibreglass mast section to move the most stubborn bits of foliage. Finally the guy broke free and I proceeded to tension this permanent set of guys and adjust the tower into perfect vertical position. The tower feels stable and safe all the way to the top.

As you can see in the picture (taken immediately following site clean up) the lower lift ropes are still in place as temporary guys. They will briefly remain until I install top guys, just as added insurance. They're bright yellow polypropylene so hopefully no one will run into one. The tower won't care, but they might. By the way, do not use polypropylene rope as permanent structural supports for towers and antennas. It rapidly degrades in sunlight (not UV-resistant).

The tower erection did capture the attention of my neighbours. Some I had already informed while others were surprised. Many of them are long-term residents and will remember my big tower from the distant past. It can't be seen from the front of the house, except perhaps by those looking out their second-story windows. It'll be more visible once the mast is in place for the low-band delta loops.

More about antennas later, once the tower is fully secured. I have a plan for temporary (summer) antennas and, perhaps, an experiment or two. The delta loops will likely not be installed until early fall. Amateur radio will not monopolize my enjoyment of summer.

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