It is my hope that hams who have never put up a tower with a concrete foundation, or even those who have, will learn something and perhaps gain motivation to take on a project of this size. Certainly I have learned new skills, including rebar form construction, and it's fun to pass that along to others. Ham radio is a fantastic opportunity to learn new skills and it would be unfortunate to avoid those opportunities by hiring someone to do it or going without.
Eventually I'll get back to antennas and operating. Since every antenna needs a support structure this long sidetrack is, in my opinion, well worth the time. I hope that readers will agree.
With that out of the way let's plant that tower in the ground. All the work up to this point will finally show tangible results.
Alternatives to positioning and securing the tower base
The base section and anchor stubs for the majority of towers of this type (typical for ham use) must hang rigidly suspended over the foundation excavation for the pouring of the concrete. Since magical levitation isn't an option we need a practical approach. There are a few objectives:
- Position the tower, laterally and vertically, per the manufacturer's specification.
- Support the weight of the tower without sag, movement or risk of tipping or other failure mode.
- Cheap and effective, but not necessarily pretty.
- Can be built and the tower placed by muscle alone.
- Withstand the force of concrete pouring from a height and lateral flow.
- Rapid adjustment of tower position, lateral or vertical angle, if and when concrete or other force shifts it.
- Does not impede pouring of the concrete, whether by chute or wheelbarrow.
This is a long list! I have seen many systems, both good and bad, devised by hams over the years. Professionals have their own favourites. Here are the ones I am most familiar with:
- Chairs: Concrete blocks -- they must be concrete -- are positioned at the bottom of the hole. The base section and stubs sit directly on those blocks. The base section must be guyed top and bottom to prevent movement and collapse of the chairs while the concrete is poured. From asking around this is option chosen by many hams with Trylon towers.
- Side bracket: This method is only possible when the tower is next to a rigid structure. Enough tower is built (usually one or two) sections to allow bracketing to the adjacent structure. In this way the tower is suspended over the hole and held rigid. It is guyed or otherwise secured just above ground level. After the concrete sets the bracket and guys are removed.
- Scaffold; Two or three lengths of lumber, steel or other rigid and strong material pierce the base section near ground level so that the horizontal or diagonal cross members between legs rest on them. These are in turn supported at either end so that the tower is correctly positioned. The scaffold should be clamped to the tower for best results. Top guys may be required.
- Cradle: A steel or lumber cradle is built on the base section. The cradle is wider than the tower and rigidly attached. It is then carried or lifted by muscle or machine over the hole and supported at all corners. Top guys are not required.
In my situation a side bracket is not an option and chairs require at least 3 people for a tower of this size. Even a scaffold is difficult to install with fewer than two people. This is impractical for the Trylon Titan series since all the cross members are diagonals that would require 3 supports rather than just two.
A cradle is more elaborate but it only requires one person to do all the work. Also, since I've never built a cradle before that's what I decided to proceed with. I came up with a design that would work for the Trylon and bought the lumber I needed. Pretty much all the hardware was acquired from my junk box.
|Picture from an earlier article included for convenience|
The cradle was built onto the #12 section with it lying on the ground and the stubs on the bottom legs resting on the hole casing. The construction of the cradle is captured below in a mosaic of views. The large picture on the left shows the tower immediately following lifting onto the skids and centred. The rear stub is not attached since it would strike the casing when the tower is levered upright due to its large turning radius. Draw it on a piece of paper and you'll see why that is so.
The remaining picture shows details of the cradle design and truing technique (next section). The front of the cradle consists of two lengths of 2x6 lumber, the inner one cut to fit inside the tower face and the outer one long enough to rest on the skids (2x4 nailed to two sides of the casing). The inner one butts up against the lowest diagonal on two sides, clamping the tower face between them. Scrap angle steel provides a hard surface for the diagonals to rest on to ensure they don't cut into the lumber.
The skids are worth a further look. These lengths of 2x4 on opposite sides of the casing serve multiple purposes. First, the keep the casing square by preventing any twisting. Second, the cradle is above the top of the foundation so that it cannot touch the wet concrete. Third, the wider surface is better for placement of shims to true the tower (next section).
The rear of the cradle is a single length of 2x6 lumber clamped to the tower leg with copper-plated steel strapping (it's what I had on hand). The choice of 2x6 lumber was determined by measuring the distance from the leg bottom to the supporting diagonals and adding the height of the 2x4 skids so that the manufacturer spec of 1" between the leg bottoms and concrete surface is met.
With everything done I manhandled the ridiculously heavy assembly until the front cradle was just past the edge of the rebar cage. There is little clearance so I had to get this right. I next adjusted the cradle's lateral position by eye so that the ends of the stubs were square to the hole. Otherwise one of the stubs will crash into the rebar cage when the assembly is rotated upward. That would be very bad.
Pushing up on the top tower leg I kicked a milk crate under the diagonal closest to my feet. That makes it easier to lift by giving lots of finger room underneath the tower. But here I experienced something unexpected: it was remarkably easy to lift the tower. The weight of the stubs and much of the cradle counterbalanced a significant portion of the 170 lb #12 section.
As it turned out I could have lifted the section with just one arm, rather than doing a power lift. The step in the operation I was most worried about turned out to be the easiest. I think I had a silly grin on my face as I lightly pushed the tower upright in a few seconds, even holding it up halfway while I ensured the position of the stubs was where they needed to be. What was to be a simple test of the cradle before I got on the phone to call up a friend to drive out to my place turned into the actual lifting. Lots can go wrong in tower work so it was pleasant to have this turn out so well.
The last task is to attach that last stub. I loosened both straps and inserted bolts into the two holes not covered by the straps. One at a time I lifted one side of each strap to insert the other bolts. This reduced the risk of the cradle slipping out. With the straps back in place but loose I dropped into the hole to attach the lock washers and nuts and tightened them.
Truing the tower
When I installed the casing for the tower foundation I got it almost but not quite level. With more fussing around with shaving dirt here and there I could have gotten it near perfect. But there is no need since even with the best of intentions there will be a problem elsewhere. In particular with the positioning system for the tower base over the hole. Such was the case with my cradle.
Truing the tower is the process of having it exactly vertical. With the cradle in place I had only to adjust it with shims. Although the cradle front and rear both used 2x6 lumber butting against the bottomost diagonal cross member the rear end used the bottom (vertical) of the L-shaped diagonal and the front used the top (horizontal). That is enough error for about the thickness of a lumber stud (less than 2").
As a result the rear leg sits higher on the skids than the front. Since it cannot be lowered the shims must go on the front. You can see them in the photo mosaic above.
For measurement I used a long carpenter's level. On towers with straight sections it is a simple matter to place the level along the vertical legs and ensure they are indeed vertical. On a tapered tower, which is typical for self-supporting towers, the legs are not vertical when the tower is vertical. The tower manufacturer often recommends taping a spacer at a prescribed position on the level as compensation for the taper.
My preferred techique is instead to aim for an equal error on all 3 legs. There is no spacer to make or attach (and lose). I learned this from another ham so far in the past I can't remember who it was. It's the method I always use and it has never failed me. However, there are things to watch out for:
- Not all levels are properly calibrated. For this reason always use the same edge of the level to measure all the legs. It's a surprisingly common problem.
- Wipe the level and tower legs since it takes only a small amount of dirt to skew the level.
- Tower legs are not as perfectly straight as you might expect. This is especially true on towers with bent sheet metal legs such as the Trylon. When the level is against the tower leg you should not see any gap between them. If there is a gap move the level to a better position. The leg vertex is a better choice than one of the sides even though it is more difficult to set the level properly on it.
|Secured, vertical and ready for concrete|
With the tower in position and trued I dropped into the hole and wired each leg to the rebar cage in two directions. I then removed the temporary lumber spacers between the rebar cage and the hole walls. The rebar shifted a small amount which I adjusted by tightening the wires. Although the rebar cage weighs as much as the base section plus stubs the tower does not shift since it has the advantage of leverage.
Affix temporary guy wires to the top of the base section so that the tower can be adjusted back to vertical during the concrete work if tilts off vertical. Although the cradle was solid enough that I didn't expect to use them (and I didn't) I had them ready just in case.
Now, finally, it is time to call the concrete company.
In contrast to the step to this point the pouring of concrete goes fast. Perhaps too fast if you don't call a pause at sensible intervals. You usually have time since (here at least) for a given quantity of concrete you have the truck at your disposal for a set period of time. Only if you go beyond that will extra charges apply. That is unlikely to happen unless you are transported the concrete by wheelbarrow. (Someone did take a few pictures during the pour but I don't have them so there are none included here.)
Find out the required concrete strength from the tower specification. It is typically not an ultimate strength but that achieved after 28 days of curing in "normal" conditions. The Trylon requires 3,000 psi so I ordered 6 yd³ of 3,500 psi as a safety margin. Actually it was 4.6 m³ of 25 Mpa concrete since most business in Canada is conducted in metric. The truck of ready-mix arrives the same afternoon, after a wait because the driver got lost for a while (even locals occasionally have trouble finding this isolated QTH).
My preparation for the truck was quite simple. I called a friend to lend a pair of experienced eyes to the proceedings. This is very helpful since everything happens fast. Second, I cleared a path so the truck could easily back up right to the tower.
This brings up an important point: do you know where all your buried services are located? The truck is very, very heavy and can do a lot of damage. In the city the driver will often ask is there are any buried services he's being asked to drive over, and may insist you sign a waiver absolving the company if there is damage. Out in the country it can be different; our driver didn't ask. However I knew exactly what was where. Make certain you know.
Talk to the driver. Explain what you're trying to achieve. I told him I needed good flow to fill the flare at the bottom of the hole and to fully encase the chairs and rebar. A rented concrete vibrator can help, if you are willing to go to the bother and expense. I never have. You can achieve a lot by pumping a shovel up and down in the concrete and the driver adding some water to the mix. The small amount of added water slows the curing but does not weaken its ultimate strength.
Concrete is heavy and it is falling from a substantial height. Make sure the chute is positioned to miss the tower structure and rebar. The driver for my pour was careful and helpful. Concrete splashes so don't wear your best clothes! Cleaning your clothes is far easier when you wait for the stains to dry.
Adjusting the true of the tower during the pour can be a problem if you are unprepared. If you've attached those temporary guys that I mentioned earlier you're part way there. You have to do it quickly when necessary since, as I've said thing move fast during the pour. It can help to have a tensioner for the guys that is quick and easy to adjust. Look at the adjacent picture (and try not to laugh).
The guy is tensioned by adding or removing stones from the plastic crate that is attached to the rope guy mid span. This is far quicker than untying and tying knots or a turnbuckle. Silly but very effective. For the other guys (not shown) one is the #11 section (150 lb), which is moved back and forth to change the tension, and the other is a plank over the guy to which stone or bricks are added and removed. Again, silly and yet effective. Your friends may smirk and question your sanity but only until they see how fast you can true the tower.
The screw and chute have to be cleaned after the pour. Pick a good spot in advance for the driver to do this. They usually ask, but not always. It can be messy.
If you want a smooth finish mist the concrete and go to work with a trowel. Don't delay or it'll be too late. Do not clean the concrete on the tower, cradle, casing or elsewhere on the assembly until it dries. Should you try it when the concrete is liquid you'll only damage the surface of the foundation.
Shades of Cool Hand Luke
Digging a hole and filling it in again has been used as an instrument of torture. You may remember the scene in the movie Cool Hand Luke where the sadistic warden does this to Luke. In our case it isn't torture although there may be mixed feelings as you watch the hole fill with concrete. A lot of work went into the planning, digging and the rebar cage.
Relax and don't fret! Once the tower is standing proud and your yagis are singing and dancing on the bands you'll forget all about it. That is, until the next tower project.
Waiting and preparing
Keep the exposed concrete damp. Experts agree that hydration speeds curing and can increase the ultimate strength. If the temperature approaches or dips below freezing it's a good idea to cover the concrete with plastic or other covering that inhibits evaporation and ice formation.
The casing can be removed as soon as the next day. I like to leave it be for two days. Since I had to be away I did it the third day. For those who like a tidy look to the concrete you can use a cold chisel to remove ridges and other protrusions. Do not use a file since aggregate in the concrete (such as sand) can quickly ruin the file . Once you're happy with it you should restore the lawn around the tower to appease spouse and neighbours. Appearances matter.
After removing the casing I applied a fast-drying concrete repair compound to fill the voids I missed on the inside of the stubs (note the wet spot in the picture above). The cradle made it difficult to do properly while the concrete was being poured. You can use the compound to build a shallow slope so that water flows away from the legs. Pooling water over the years can initiate corrosion.
Now is a good time to call your friends to arrange a tower raising party. Borrow or build a gin pole and do other tasks to prepare. You'll want everything in place so that no time is wasted when the time comes.
I would wait at least 7 days before constructing the tower. There is really no need to delay longer since by then the concrete is sufficiently cured for all but the most severe weather with a heavy antenna load. Concrete curing is a long-term process with no definite date when it stops strengthening.
As I write these words several days of snow are forecast. There will be a delay.