Most hams that do have towers have self-supporting towers. These are usually the only kind that fit in a suburban or urban lot where most live. House bracketed towers are also fairly common for those with wire antenna or small yagis; these towers typically don't handle much wind load, and there is increased risk to the house.
For hams in rural areas a guyed tower is a relatively inexpensive way to attain height, at the cost of more labour, more excavating and construction and the consumption of land area. They also require more maintenance and require yagis to be trammed up the tower. However one you approach 100' (30 meters) height a self-supporting tower can become prohibitively expensive.
If you have never worked with a guyed tower this article may be of interest. My 150' LR20 tower is slated to be raised this fall and I have been doing the required preparatory work. The initial steps were to choose a location per my site plan, probe the ground and finally survey and stake the base and anchors. It's a bit daunting to stand back and look over the staked ground; the tower consumes an acre of land.
Probing the ground
|Handmade probe next to a 6' drill bit I also tried|
Glacial debris can be easy or difficult to remove. Much of it consists of boulders of hard rock such as granite in a matrix of eroded softer rock and clays. The rocks and boulders have to be removed whole since they are too hard to break with hand tools. The mix of soft rock and clay sounds deceptively easy to deal with but in reality is almost imprevious to shovels and so must first be broken up by hammering a large cold chisel or mechanical breaker into it.
The purpose of probing the ground is to (hopefully) find spots for the base and anchor where there is more soil and less of the harder stuff. Ground composition can vary a surprising amount over a short distance.
I made my own probes out of sharpened thin steel rods I had lying around. The steel is soft so some care is needed when pushing them into the ground to avoid ruining them too quickly. But they're cheap and easy to make.
The conundrum is that probing the ground here is like dealing with Olber's Paradox. Sooner or later (often sooner) the probe will always strike a stone or stone-hard clays. When the probe hits a stone you withdraw the probe and try again a few inches away. You'll hit another stone, either shallow or deep. Sometimes I was lucky and went down the full length of the probe. But that only means the probe isn't long enough to hit rock!
When I chatted about this with the local tower pros one of them jokingly suggested moving the tower a few feet to avoid shallow obstacles. That doesn't really work since when you move the base or an anchor the other three points must also be moved. You are almost guaranteed to hit shallow rock at one of those locations. The effort isn't worth it.
So I tossed the probes aside and put the tower where I wanted it. Yes, the probes do show rock but I expected it. However I don't expect it to be bedrock based on my probing the ground and the hand excavation for the Trylon tower (which is complete as I write this, and is a topic for a later article). Thus a backhoe should
Before we do the surveying of the tower area we need a set of specifications. All tower manufacturers provide this, and if not the geometry and engineering requirements can be calculated. But for this exercise we'll rely on the L & R documentation from long ago. I have been told that, in general, engineering requirements have been strengthened over the years so some recommend exceeding these older specifications.
L & R follow the common 80% heuristic: the guy anchors should be placed 80% of the tower height away from the base. In my case the nominal 150' tower has the anchors 120' distant. If you plug the numbers into a calculator -- α = atan(D / H) -- the angle between the tower and top guys is a little less than 39°. Due to the 14 section splices (overlap) the true height is closer to 143' and the angle is almost exactly 40°. The distance D is from the centre of the base to where the anchor rod pierces the ground. The anchor itself several feet further out.
I did not include in this article the large table in the spec sheet that includes the values for the diagram on the left. For my tower they are: A=120'; B=208'; C=180'. My tower's anchor code is 6228. For my tower V=9' and H=12'. The back of the anchor is 7'-4" behind where the anchor rod pierces the ground. We are now ready to take to the field.
Survey and staking
My tools were the above set of numbers, 200' and 16' tape measures, an armload of wood stakes, a steel mallet and good footwear. I spent about 1 hour on this job, including checking and even rechecking measurements. Despite the many things that must be done it really isn't difficult.
It's straightforward when you go about it methodically. The adjacent diagram describes the process steps by number. You may want to refer to my site plan article for the placement of this tower in the field east of the house.
In step 2 I unreeled the long tape measure (and short one) to mark out the anchor at the other end of the 208' baseline (B), again ensuring ample room behind the anchor. Walking back for step 3 I staked the midpoint between the anchors (104'). I returned to the first anchor to ensure the three stakes were in a line. Of course they were not quite aligned so I moved the first stake to correct the error. I chose the closest stake but moving any one of them would have worked as well to do this.
Returning to the baseline centre I estimated a right angle and measured 60' southward (C - A) in step 4 to mark the tower base. Don't worry about how accurate you are in making a right angle since no matter what you do there is sure to be an error. Just do your best and we'll correct it later. Measure a further 120' (A) in step 5 to complete the centreline and stake the final anchor. Sight it so that the stakes for the baseline centre, base and third anchor are in a line.
|Baseline after step 8, amid maple leaves; the far anchor stake is aligned/hidden behind the centre stake|
Move the stake for the top anchor right or left, keeping it parallel to the baseline, until the 208' mark on the tape measure coincides with the stake. Move the stake approximately parallel to the baseline and take care to keep the tape measure straight. The latter will require some walking back to lift it over the ground cover.
Now do the same for line from the bottom left anchor to the top anchor. A few iterations will get these distance equal and 208'. Try to get it to within a few inches if you can, although realistically an error of 0.5% (1') is adequate.
The base stake will also need to move left or right (again, parallel to the baseline) so that the top anchor, base and baseline centre stakes are aligned. This is step 7. Remeasure the distance from the base stake to the baseline centre stake and top anchor stake to confirm the result. Adjust as needed.
|Centreline, looking north from top (south) anchor stakes, perfectly aligned after step 7|
You may have noticed that the distance from the baseline centre to the base is exactly half of the anchor-to-base distance. This is because sin 30° = 0.5. This and other measurements can be similarly calculated from first principles if you don't trust the manufacturer's spec sheet. All you need is a diagram and a little trigonometry.
On the other hand those round numbers in the tower spec are just that: round numbers. If you like (and I did, out of interest) you can generate your own A, B, C, H and V values from first principles and get greater accuracy. That isn't necessary, but the point is you can, provided you do not compromise the tower's basic engineering requirements.
For example, if you want a larger angle between the tower and top guy you will have to calculate your own specifications. You might want to do this to allow room for long-boom side-mounted yagis. The tower footprint will grow since the anchors will be further from the base. However, do not decrease the guy angle since that will reduce the tower's capacity and can introduce a serious safety issue. You need the land to put up a tower this size. There are no shortcuts.
Steps 9 and 10 are the markers for the excavation. In step 9 the back of the anchor is staked, and in step 10 the corners of the base are staked. For the anchors, which in my case are 8' long, the ends of the anchors should be staked for the use of the backhoe operator. Set them back about 6" or 1' or as the operator prefers.
The stakes are 4' either side of the one marking the back of the anchor and must be orthogonal to the anchor rod and guy wires. This can be done (similar to step 6) by adjusting the stakes at the anchor ends until the distances from the stakes to the stake marking where the anchor rod pierces the ground are equal.
You may choose to hand dig the hole for the base, especially if the soil is to be the concrete form for the base, since you can be more accurate with a shovel than a backhoe. Not all backhoe operators are into surgical precision. The base excavation is easier than for a self-supporting tower since the volume of soil to be removed is much less.
For the anchors you should pick up the phone and hire a backhoe. The volume of soil to be removed is unreasonable without mechanical assistance. The anchors go deep so you are more likely to strike rock and so need a backhoe with a breaker attachment. Despite the expense this is the right way to go about it. When the concrete is set almost all the soil goes back into the excavation. It will need to be compacted, for which you may want a tractor to drive over it as the hole is filled or, better, bring back that backhoe. Again, it is unreasonable to compact the soil without mechanical assistance.
You'll notice in the picture above of the baseline there is a small pile of dirt. That is a test hole to probe the top foot of ground and check for buried rock. Yes, there is rock or at least plenty of stones down there.
I received a load of re-bar this week and I am now in the process of building the re-bar cages for the Trylon and LR20 towers. Once I have those done and the anchor rods in hand it will be time to do excavation for the guyed tower. Open pits are a safety hazard so I am delaying excavation until I am ready to proceed.