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.

Thursday, June 20, 2013

Floating Tower Base

I had a couple of hours last evening, just enough time to build and install the floating base for the opportunistically-guyed Site-C tower. It is floating in the sense that it is entirely above the frost line. The conventional base for a Golden Nugget tower is to secure the base plate with 3 x 1-meter steel studs that are driven into the ground. The floating design -- which is entirely my own -- is worth a few words.

My requirements for the base include:
  • Do not interfere with the septic system tile bed under the tower.
  • Support the dead weight of the tower and antennas (and ice), plus the vertical force due to guy tension from preload and wind load.
  • Secure the base from lateral forces due to wind and from accidental or deliberate interference.
  • Easy to remove and restore the lawn when the tower is dismantled.
  • Survivability of at least 5 years.
The base is quite simple. The only material is two 8'-long 4x4 pressure-treated lumber rated for burial, plus 8 galvanized steel spikes. These can be purchased at almost any large building supply store. Galvanized lag bolts and washers will be used to secure the tower base plate to the base.

Each piece of lumber is cut into 3 sections: 3' and 2 x 2.5'. The longer sections form the bottom of the base, and the 4 shorter ones are secured across those. The depth of the base is about 6", leaving approximately 1" above grade (2 x 3.5" = 7"). There is just enough space between the upper sections to allow water to drain, so that is does not pool for long between the base plate and base.

The tricky part of the installation is getting everything level while not leaving any air gaps beneath any part of the base. The picture shows the base centred, levelled and ready for final hammering and packing of soil. The unrestored tower base plate is included in the picture for perspective.

The base gets its strength by coupling to the soil, both downward and laterally. This is fine for wire antennas. If you plan to install a yagi or similar rotatable antenna it is important to add diagonal strength to the base so that it better resists twisting. Unlike a concrete base there is not enough mass or overburden to resist much uplift force so I'll be out of luck if a tornado strikes. These are happily rare in the Ottawa valley. Frost heave during spring thaw is modest in my experience here (~1 cm), which I judge to be negligible in this application.

Once installed and the sod replaced it makes for a tidy installation. Even the local waterfowl seem unperturbed by its presence. The trees that will serve as guy anchors bracket the picture. Notice that the base and lawn are not on the same plane. The base is level, the lawn is not. Never assume that the ground is level. Measure!

The last test is to ensure that after all that surveying, calculating and construction work that the base is actually in the correct position. I did this by placing a pad of paper on the base, marking the point where the base plate will be centred, and then sighting along a straight edge to the 3 anchor points. With lines drawn it is a simple matter to use a protractor to measure the angles between guys.

The result is quite good considering that I intended to not adjust for minor asymmetries. About 1° of the 3° error is because I hadn't taken into account that the southwest tree bends a bit towards the south and the anchor point is above head height. At a distance of ~9 meters from the base a 1° lateral offset is equal to ~16 cm (6").

With all the ground prepared I now have some finishing work to do on the tower, after which it will be ready to be raised.

Tuesday, June 18, 2013

Opportunistic Tower Guying

You have likely heard the old saw that goes: no matter how long the legs, a 3-legged stool never wobbles. While true, sitting on such a stool can be uncomfortable. If the leg difference is large enough you will tend to slide off. Worse, if the legs are not symmetrically placed -- an equilateral triangle -- it could tip over and dump you onto the floor.

The analogy is only fair yet the situation is not unlike the opportunistic guying of a tower -- the use of pre-existing objects to anchor the guy lines. Since that is what I am going to do for the Site-C tower let's use it as an example.

As noted in an earlier article, the tower will be anchored by two mature trees with large trunks (spruce and tamarack) and a steel pipe coupled into the house frame. Although this configuration is not an equilateral triangle -- one where the interior angles are all 60° -- it can work fine if one goes about it in the right way.
Golden Nugget Guy Station

The ideal 3-guy configuration has the guy lines 120° apart (3 x 120° = 360°). You should not deviate much from this since a wider angle between any two guy lines increases instability -- at 180° it behaves like a 2-guy configuration. A small increase -- say, a maximum of 130° to 135° -- is okay, and can be compensated somewhat by increasing the guy line preload (tension under static load, without wind or other weather affects). We don't want too high a preload since it increases the stress, such as on the tower welds. This is a good reason to use the manufacturer's guy station hardware rather than connect the guy lines to the tower legs or cross members.

I surveyed my yard, knowing that the configuration would not be an equilateral triangle, but would be close to an isosceles triangle. That is, the house anchor would be farther from the tower than the trees but would be an almost equal distance to both trees. This will work.

If you enjoy mathematics please note that there always exists a point within a triangular area from which lines to the triangle vertices are separated by exactly 120°, if you know how to find it! The constraint is that none of the triangle's interior angles can equal or exceed 120°. Get some paper, a protractor and a pencil and experiment. Keep in mind that there is an additional constraint, where the tower cannot be closer to a ground anchor than ~60% the height of the guy station to ensure that the guy is effective by keeping the angle between tower and guy line at least 30°. The angles between the tower and all the guys do not have to be equal, provided you adjust preload accordingly (I'll leave that as an exercise to the reader).

Since the triangle from my survey is not exactly isosceles the tower would have to be displaced slightly off the centre line of the triangle (line from the house anchor to the bisection point of the line between the trees) and a bit closer to one tree than the other. I may ignore this since the offset is only about 5°, thus keeping the tower centred in the yard. This leaves the most room for low-band delta loops.

Another mathematical item: for small offsets (in the above, almost-isosceles case) you can get close to 120° guying by shifting the measured tower centre over to the long, bisecting line of the triangle (in my case, from the house-anchor vertex) on a line parallel to short (bisected) side of the triangle (line from tree to tree).

The house anchor could have been placed for better symmetry but at the cost of strength and reliability. You can see the newly-installed pipe in the attached picture (complete with temporary varmint plug). I won't discuss how I've coupled it to the frame to avoid giving readers any ill-formed ideas about how to go about it. As built it easily withstands over 100 kg of force applied in any direction and should be good for much more. The pipe clamps are there to prevent the guy from slipping sideways. The guy will exit the photo towards the upper left, orthogonal to the pipe, towards the south.

One final note on opportunistic anchoring regards height of the anchor above ground. All the anchor points in my design are above ground level. This requires additional measuring and calculation. It can actually help if the anchor (e.g. tree) is close to the tower since the angle between guy and tower increases as the anchor point on the tree is raised. Doing so does not affect the angles between guys, which remains at a nominal 120°.

When I surveyed my yard I took special note of levels and anchor points on the trees and house. My yard was artificially raised for the installation of the septic system, which the trees predate. They are just outside this area and therefore their ground is about 1 meter below the level of the tower base. The anchor points on the trunks are higher yet. Their position was selected to be a compromise between getting lower on the trunk for maximum stability versus higher to use a thick branch as a natural retainer for the guy and to reduce interference from foliage along the guy line.

The triangular area (as described above) must not use the anchor points as the triangle vertices. Instead we must project the guy to a virtual anchor point on the plane of the tower base. It is that intersection point that is the vertex to be used in our calculations.

I hope to be in a position to raise the tower within the next week once I build the tower base and measure out the guy lines. I will have more to say about tower raising at that time.

Monday, June 10, 2013

Preparing The Site-C Tower

As time has allowed I have been making decisions on siting, constructing and erecting the tower at (what I am calling) Site C. I think I now have everything I need in regard to hardware. The tower is, as earlier mentioned, a secondhand (actually third-hand) Golden Nugget 30' TV tower.

What antennas the tower will sport I will discuss later, once the tower is in the air. For the most part those will be low band antennas. However that will be delayed while I experiment with other, temporary antennas. That's part of the fun of doing all this: just playing around with stuff and see how it does.

Shopping for tower hardware isn't easy unless you patronize actual tower supply firms, of which there is little in the way of retail in this area. Part of the fun of building towers and antennas is to gather up what's needed from what's available. You just have to be careful that you are indeed picking the right hardware, for which experience is a handy guide.

My partial list of purchases:
  • Various U-bolts, for the tower and for anchoring guys -- Ottawa Fastener Supply
  • Class 5 hardware for the tower -- Ottawa Fastener Supply
  • Pipe clamps -- Canadian Tire (automotive section)
  • Guying hardware -- Ottawa Fastener Supply
  • UV-resistant rope -- Home Depot
  • Tower base -- Home Depot
There's more I have to buy once I get to erecting antennas, including wire and supports. These are easy purchases once I reach that point. Some specialized hardware such as antenna insulators I purchased from amateur radio outlets.

The first thing I needed to do with the tower was to remove old hardware. As is all too typical the hardware on the tower is terribly rusted. There are any number of ways to remove this hardware. My choice is the one of greatest expediency -- an example is in the adjacent picture -- in keeping with the legend of Alexander the Great cutting the Gordian Knot.

Those bolt cutters have been put to good use over the years. Many times I have walked down the length of every element of an old yagi with cutters in hand rapidly dispatching the rusted clamps holding together the tapered aluminum tubes. Hardware is cheap while my time and patience are in short supply.

Next, I came to the need for a conduit suitable for routing transmission lines and other cables through the wall of the house into the basement, which is where the shack is to be located. In most cases this is not a difficult decision, there is only the need to decide on the inside diameter of the plastic pipe to support the quantity and size of cables, with enough room for future expansion.

In my case the choice was more involved. The conduit will serve a purpose above and beyond that of merely routing cables. Because of its planned location I am able to kill two birds with one stone by employing the conduit to secure the tower. How this will be done I will come to in a future article. For now I will say that it needs to be very strong and able to be coupled to the house's frame.

Any plumbing supply outlet has suitable pipe. That trip wasn't necessary since my garage is stacked with lengths of aluminum and steel from my previous station. I settled on a old mast, a standard 22-foot length of 1.5" Schedule-40 galvanized pipe. I no longer need it to support stacked yagis so I lopped off less than a meter of it to serve as the conduit.

With an inner diameter of ~4 cm it can hold perhaps a half-dozen runs of RG-213. It is also strong enough to serve as an anchor. The prepared pipe is seen in the adjacent photo, hack-sawed, filed and leaning against the rough doorway to the as-yet unpainted shack.

One final aspect of the structure I'll mention here is the tower base. The standard (manufacture's) method of securing the base of a Golden Nugget tower to the ground is with three 3-foot long steel posts that are driven through the base plate into the soil beneath. This provides good lateral strength, sufficient resistance to uplift forces and plants the base below the frost line (some protection against motion due to freeze-thaw cycles). This option is not permissible at Site C since it is directly over the tile bed of my septic system.

The base I have come up with is a "floating" design, where the base does not reach below the frost line.The lumber you see in the picture is suitably rated for this application, and should provide many years of service. My objectives are to distribute the tower weight over an area larger than the base itself (to protect the tile bed), maximize the coupling to ground (for lateral stability) and to get it down far enough for a reasonable amount of protection against uplift forces and human tampering.

Now all I need is time enough to put it all together and have a tower raising. I also need to put a couple of finish paint coats on the shack walls and do related finishing work. I don't relish doing this in summer, but I may be persuaded by the several days of rain that are forecast this week.

Thursday, June 6, 2013

40 Years

My how time flies.

Since I have a few moments between tasks and the bands are effectively dead with this no-antenna antenna of mine I figured a brief note about the following item is called for.

A few months ago I got a small brown envelope from the ARRL.  Inside was the pin pictured above. That was nice but a little odd since there was nothing else found within. The mystery cleared when a letter arrived under separate cover a few days later. I imagine they plan that these would ordinarily arrive in the reverse order.

40 years! Actually it's 41 years since I became a ham as a young teenager in Winnipeg. QST and the ARRL came later when I had enough pocket change from doing summer jobs to afford a few luxuries. Life membership came several years later in what for the time was a major investment for a poor student.

As I recall more folks at this end of the country tended to join CARF at that time. On the prairie many hams viewed CARF as a suspicious "eastern" organization and were more likely to look south for leadership. I'll skip all the rancour that sometimes broke out between the organizations, and their membership, some of which hit close to home. Times change.

It was nice to have QST arriving like clockwork every month during my long period of inactivity, keeping me minimally in touch with ham happenings though I didn't always take the time read it.

I even got my picture in QST one time. It was (I think) May 1979 when I made the CW Sweepstakes top-ten (low power) in 1978 in my final SS as a VE4. My station wasn't much, so the rare prefix had a lot to do with my achievement. Sweepstakes was never the same as a VE3.

Ok, that's more than enough woolgathering. I still have a tower to put up. Once the rain finally stops.

Monday, June 3, 2013

Tower Bolts

Here's a question that more hams (and OTA TV enthusiasts, for that matter) ought to ask themselves: are you smarter than the engineers and technicians that designed and manufactured your tower?

Most would, correctly, say 'no'. Yet too many go on to behave as if they do believe they are smarter, or at least smart enough to tamper with the tower. This is a bad idea. The bolts that hold together the sections of a tower are a good study since it is one of the common items that people will substitute with inappropriate hardware.

The 30' secondhand Golden Nugget tower I recently brought home is a good case in point. Take a look at the adjacent photograph of 3 sets of bolts and nuts.
  • The one at the top appears to be one of the originals that was supplied new with the tower. It's a bit rusty with age and a few years of burial but still quite usable. High-strength carbon steel bolts are often like that, rusting more slowly than their inexpensive brethren. I don't know what class of strengthening (and hardening) it has since the markings on the bolt head are mostly illegible and perhaps proprietary.
  • The middle set is stainless steel, which was bought and installed by the tower's previous owner. Notice a couple of things here. First, there is no lock washer. Second, the shank diameter is ¼" rather than the ⅜" of the original bolt.
  • The bottom set I just purchased. The set is class (or grade) 5 with a ⅜" shank and 2" long, which is about ¼" longer than the original. The important differences are the longer section of unthreaded shank and coarser threads. I still need matching lock washers.
In that earlier article I mentioned that I was not entirely comfortable with the quality of the tower. While I didn't say why at the time -- only mentioning the rust -- the major points were that the tower sections were not linearly aligned and the overlap where they mate was compressed enough that separating sections was difficult. On further inspection the sections look fine. It's those stainless steel bolts and their application that were at fault.

Towers are not piston engines and do not need the same machining precision for their bolt holes. Even so there is no accident to their design. There is enough play in the tower leg bolt holes to allow the installer to drive the bolts home while dangling up in the air even when all the legs are not simultaneously aligned, but not so much that the there is significant play once the bolts are in place. Leaving ⅛" of play is too much. Not only did the sections slip and slide over time due to weather the threads on the bolts were galled by the weight and motion of the tower legs against the threads.

Stainless steel is also the wrong material. Avoiding rust is nice but never at the cost of tower integrity. The least-expensive stainless steel hardware tends to be harder than carbon steel bolts but is not of the required strength.

In the past I have gone to the source and purchased replacement bolts from the manufacturer. As far as I know they all do this since it is a common need in the field. This time I bought standard bolts of the approximately correct type to save time (and probably money) with the confidence that they will work out fine for this light duty tower. As a general rule I don't recommend this.

To do the subject justice I will delve into a little more detail. Look again at the picture with the 3 bolt and nut sets. Notice the shank wear just under the head of the original bolt. That is caused by the weight of the tower sections pressing down on the bolt shank. Yet the threads are not damaged, just rusty. The opposite is seen on the middle, stainless steel bolt where the shank is pristine and the threads are galled just inside of where the nut was positioned.

What causes the different wear pattern? In the former case the shank is properly sized so there is no room for play even if the nut is loose. Thus there is no wear on the threads at the inside of the legs. The shank shows compression wear due the tower weight, but the bolt is strong enough to withstand the wear.

The stainless bolts are harder but because they are undersized the bolt will get twisted up and down and (slightly) side to side by action of the wind. Hard or not, the threads will gall and, being harder than the tower, the tower leg bolt holes will get worn, and that wear is visible on closer inspection. No amount of bolt tightening will help since if you torque the nuts too much the tubular legs will yield inward and loosen the nut.

It is the purpose of the lock washer to hold the nut in place with the nut properly torqued. The leg doesn't distort and the lock washer, by cutting into both the nut and tower leg face keeps the torque steady. In this application the correct lock washer must be selected, which is one with multiple locking flanges on the interior of the washer, up against the bolt shank where the nut can work on it. The lock washer should be the same hardness as the nut or it won't cut. Split ring and exterior lock washers must not be used since neither can properly accomplish the job.

This is where the coarser thread of the bolts I purchased have an effect. With the coarser thread there is a greater change in torque for an equal angle rotation of the nut. So if the nut rotates by the effect of weather or improper installation there is greater risk that the lock washer will cease to operate, promoting further loosening of the nut. Although the difference is modest, and probably not a problem, I will periodically check the hardware once the tower is up. Of course I would do that anyway. It is something I do before working on someone else's tower, especially one of uncertain pedigree. I've encountered many towers with several or many loose bolts!

You should gather from all this that the tower is supported by the bolt shank, in particular the outside end without threads, not by compression of the tower legs by torquing the nut. This is also true of towers like the DMX series and Trylon with formed sheet metal legs. The tower rests on the bolt and the compression holds everything in the correct position.

As a further example, the adjacent picture shows two sets of class-5 ½" bolts, nuts and lock washers, where one bolt has a custom tapered shoulder for DMX towers and the other with a standard, fully-threaded shank. In this case you must use the custom bolt to properly align and mate the sections, so order them if you need replacements. I've seen many hams use the standard variety, but they cannot hold the tower sections in proper position and will almost certainly loosen over time.

The bolt, nut and lock washer each play their unique role. This is only possible if they are all of the correct type. If you're at all uncertain and you need new hardware buy new from the manufacturer or their dealer. An improperly installed tower can kill.