Wednesday, July 30, 2014

Tower Up, Lessons Learned

My objective was to get the DMX-52 tower up by the end of July and I just managed to accomplish that feat. It is standing, guyed and ready to be adorned with antennas. The adjacent picture shows the tower near completion. In it you can see the as-yet untensioned upper guys and the temporary rope guys to steady the tower as it was topped off.

This is of course nothing remarkable. I expect that most of you reading this have a tower already (as I have had in the past), so it's nothing of surpassing interest. What makes this particular project interesting are the following:
  • All steps of tower erection were done by myself, with no helpers.
  • There is no concrete involved, neither for the base nor for the guys.
  • Specialized tools were improvised, mostly with parts I had on hand.
The experiment was interesting and educational despite the fact that as an experienced (amateur) tower worker there were no surprises and no big doubts to overcome. It was just that when I was younger it was so easy to solicit helpers since we were all energetic and eager to help each other out with building our stations. After being QRT for 20 years I found that many hams have resorted to hiring professional help due their advancing age and the general aging of the entire ham population.

To conclude this project I will list some of the lessons learned since they may be of use to others out there.

Gin pole performance

My homemade gin pole performed quite well at its assigned task, provided I paid close attention to its limitations. As should perhaps be expected of its construction from readily-available parts (such as those found in my garage) and put together with fasteners (no welding) it had some shortcomings.

  • The brackets that grab the X-braces were a bit too long on the upper side. I intended those to grab the inside of the adjacent tower leg for redundancy and, possibly, added strength. In practice it never quite touched the tower leg. Instead it complicated the attachment and removal of the gin pole due to that extra length getting caught in the X-braces.
  • Although the angle aluminum attached to the top of the steel pipe was plenty strong the clamps that hold it to the pipe were a problem. First, since the back of the angle aluminum isn't round the muffler clamp could rotate a bit under load. You may be able to see the abrasion marks the saddles made as they shifted from side to side. This is a concerning even though the coupling didn't slip. More aggravating was the projecting U-bolt that eagerly hooked on to tower sections as they slid past. Next time I'll purchase a pipe of the requisite length.
I don't know if this gin pole will get used again, but I will keep it intact for as long as I don't need the parts for another project. It'll have lots of company hanging on the garage wall until it gets called back into use or disassembled.

Through a thicket of trees

Since I have no concrete base to make this a free-standing tower the tower has to be guyed. Although this adds complexity it is cheaper, easier to erase when/if I sell the property, and increases the load capacity of this light-duty tower.

Two of my guy anchors are mature trees that just happen to be perfectly positioned on my property (the third anchor is the house frame). A consequence of this opportunistic guying is that the guy wires have to pass through the tree. That makes for an interesting challenge.

The adjacent picture was taken from beside the southwest tree anchor. In it (if you look carefully) you can see the two steel guys with the still-attached temporary rope guy. The bottom guy wire is ⅛" aircraft cable and the upper guy wire is 3/16" aircraft cable.

The lower wire is unimpeded by the tree branches. This is not the case with the upper guy. I faced a choice between cutting off all the branches in the line-of-site to the tower guy station or route the guy wire through the tangle of branches. While it may be difficult to pick out in the picture the rope guy deflects around a branch. That will not do for the permanent guy.

The way I went about it (after failing at the "brute force" technique) was to tape the guy to a heavy socket wrench and drop it through a 20' long aluminum yagi boom. You can see it propped up by the other tree in the first picture of this article. This is the same boom that formerly supported my multi-band inverted vee. I then threaded that combination through the branches until the end of the boom was in the target "window" among the branches. I then pulled the boom back, exposing the guy wire, and proceeded to attach it to the anchor. After pulling it taut I removed one small branch that strayed too close to the guy.

The other tree, a spruce, was not so difficult but it did require some acrobatics to slip the guy over a large branch that could not be readily manipulated from the ground with that 20' boom.

As I mentioned, the upper guy wires are 3/16" cable. I chose it even though ⅛" cable is sufficiently strong (2,000 lb breaking strength) so as to minimize the risk of wear on the guy by the branches under the influence of wind and ice.

Upper guy station

As earlier discussed the DMX guy stations attach at the joins between sections. It is also possible in most cases to place the guy station at the top of the top section by drilling out the rivets holding the top plate (for the mast bearing) and using those bolt holes. That does not work for the DMX-1T top section since neither stock guy station fits that position.

I had originally planned to place the GS123 guy station at the join between the top two sections, which is 8' below the top. With mast installed this would work out to 11' below the main antenna load. Although this is the simplest approach I was not comfortable with that large distance between guy station and antenna load for a light-duty top section.

The manufacturer suggests setting the GS123 adjustment bolts to the smallest size that will fit over the top of the DMX-1T, finding the position where it naturally comes to rest on the tapered section and mounting it there by drilling ⅜" bolt holes in the tower legs.

At first I laughed at that idea. As I said earlier, it is wise to reject a used tower that has been modified. Drilling large holes in tower legs can qualify. However, in this case these holes are no different than the holes that are used to bolt together tower sections. The critical need is that the hole be filled with a properly torqued bolt to compensate for the weakness created by the hole. Left open a hole this size will weaken the tower.

I eventually decided this really was the best thing to do. But it's trickier than it sounds.

Getting the position right is the first problem. Even a slight deviation from the precise position where the GS123 fits snugly will stress the tower section when the bolts are torqued. A bit too high and the legs will be pulled outward; too low and the legs will be squeezed together. I spent some time experimenting with the parts and came up with what worked out to be a near ideal position: bolt hole centres 47.5" from the bottom of the legs.

Now the top guy station is 4' below the top plate, 2' below the rotator plate and 7' below where the antenna will be mounted. The height of the mast is the highest I can go (15 meters above grade) and stay exempt under city and federal regulations.

All dressed up and no place to go

The tower is up but I still do not have a suitable tri-band yagi. Since doing an analysis of the various possibilities earlier this summer I have not come across a reasonably nearby used yagi that suits my needs for performance and wind load.

I have a mast bearing and I have a rotator so those can be mounted on the tower. I will also further adjust the guy tension to find the best balance between rigidity and tree movement. Then it's back to antenna shopping.

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