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.

Saturday, July 19, 2014

Notes on Gin Pole Mechanics

If you've ever erected a tower you are likely familiar with gin poles. The alternative, raising tower sections is to do this by brute force. I've done this many times, but only with a reliable partner. This is not recommended practice even if it is common among amateurs. Other alternatives are typically expensive and inconvenient: cranes and helicopters (sky hooks are yet to be invented).

Rather than borrow or manufacture a tower-specific gin pole to raise my DMX tower I chose to fabricate a simple gin pole that, while a bit unconventional, is up to the task. My reasons were to avoid the need for a welder (and the associated expense of parts and labour) and simply for the joy of it. Now that I have one it is available for other projects.

DMX gin pole recommended
by the manufacturer
Before we look at the gin pole itself it would be helpful to understand what we are asking it to accomplish. It is vitally important that it is up to the demanding task of safely and effectively lifting heavy and bulky tower sections. Failure can result in serious injury or death, or at least damage to property.

Forces on the gin pole

If the tower section weighs 25 kg what is the force on the gin pole when it is lifted off the ground? The correct answer is not 25 kg.

When the tower section is in the air and not moving, and both sides of the rope are vertical, there is an equal downward force on the side of the rope being pulled. This doubles the force on the gin pole to 50 kg.

An additional downward force is required to move the section upward. The greater the force the faster the section rises. This force must also be supported by the gin pole. Another consideration is acceleration: the force needed to change the upward velocity of the tower section: F = ma, according to Newton.

If you pull on the rope hard enough (large accelerating force) you can lift yourself off the ground. Less amusing is that the acceleration can break even a strong gin pole. Never jerk hard on the rope; use gradual, fluid movements when lifting tower sections.

Vectors, not scalars

As a physicist would tell you: force is a vector, not a scalar. What this means is that calculation of the net force requires taking into account the directions of the contributing forces. You can't simply add together the magnitudes of X and Y. When you have that section hanging in the air the scalar net force is 25 + 25 = 50 kg, but that is only a valid sum if both forces are in the same direction.

Since you cannot easily stand directly underneath the gin pole when pulling on the rope the net force will be in a direction other than straight down. This not only stresses the gin pole it requires more scalar force (how hard you pull on the rope).

When you pull on the rope the gin pole will bend towards you. This creates bending stress along the length of the gin pole and lateral stress on the bottom attachments between the tower and gin pole. Since the tower section is hanging vertically the additional force causing the stress must be coming from you pulling on the rope. That is, in addition to the force that is equal to the weight of the tower section you must pull harder (additional force) so that the force causing the aforementioned stress, when added to the weight of the tower section, equals the pulling force. Not only are you working harder that extra effort is creating a serious safety risk.

If at any point on the gin pole the stress exceeds its yield strength it will fold or split. This is something we do not want to occur.

Let's assume that you are standing where the angle between the pulley and the rope in your hand is 30°. Just as in the guy station/wire calculation there is a lateral force component additional to the 25 kg vertical (downward) force. The scalar value is tan 30° x 25 = 14.4 kg. The tension of the rope in your hand is therefore SQRT(25² + 14.4²) = 28.9 kg. When you pull on the rope to raise the section (as discussed above) this value is even higher, as is the lateral force.

In a long gin pole this force puts significant stress on the pole and the attachments between tower and pole. Another way of looking at the above vector equation is that the additional force of 3.9 kg (28.9 - 25) multiplies 3.7x to a lateral force at the top of the gin pole of 14.4 kg. That is, if you stand a little way from the gin pole and you pull hard you can easy gain enough mechanical leverage to destroy the gin pole, and all the mayhem that entails.

Another area of concern is where the bottom of the gin pole attaches to the tower (typically at two points). The top attachment acts as a fulcrum for the lateral force of 14.4 kg at the pulley. If the two attachment points are on the tower are 1 meter apart and the gin pole is 4 meters long the lateral force on the bottom attachment point is 4 x 14.4 = 58kg. This is in addition to the vertical force of 25 + 25 + lifting force + acceleration force.

Bottom line: stand as close to the tower as possible when manually lifting tower sections with a gin pole, and wear a hard hat. Gin poles can and do break. Don't turn yourself or your friends into statistics.


The major components to be selected in any gin pole include:
  • Pole: More than anything this determines the strength and weight of the gin pole. A steel cylinder (tube or pipe) provides the optimum balance between strength and weight. Wall thickness and heat treatment, if any, determines yield strength. The longer the pole the greater the stress on it, so the stronger it must be. It must be long enough to securely attach to the tower and project high enough above it to, at a minimum, place the pulley above the lifted section's centre of gravity.
  • Pulley: The pulley should be labelled with its working load rating. A pulley without a rating should be avoided. For light duty tower work the rating should be a minimum of 100 kg but even so avoid a pulley that is rating less than 150 to 200 kg. I recommend avoiding one with a centre axle that is secured to the housing in rivet style. The axle should have wide flanges and/or clips outside the housing body that cannot easy break or wear through. Get a decent match with the rope diameter you'll be using.
  • Tower attachment brackets: As with the other components the brackets must support all the loads on the gin pole. Since the brackets are at stress points the pole can break at the brackets if they are improperly joined, allowing the pole to fracture or bend. The brackets must also ensure that the gin pole cannot slip off the tower structural members on which it rests, that the tower at the attachment can deal with the additional load, and do not require more than two hands (!) to position and secure the gin pole.
  • Rope: Match the rope diameter and the pulley, but ensure that the rope's working load strength is at least twice the forces involved (4x the weight of the heaviest tower section). If the rope is too small for the pulley there is a chance that the rope can get trapped between the wheel and housing. You don't ever want that to happen. I can assure you from experience it is difficult and dangerous to free a trapped rope from a pulley that is holding a tower section in mid-air and is out of hand's reach above you.
In summary, the objectives in selection of gin pole components include:
  • Light enough to be safe and easy enough to be maneouvered  by one person on the tower, with optional assistance by ground crew. I generally do this sort of tower work with at least one other person, but I decided to design the equipment and process in this instance to permit me to raise the tower on my own as a challenge to myself.
  • Withstand all calculated forces with a large safety margin.
  • Easy to construct, or at least fall within a comfortable budget.
With all the forgoing in mind I fabricated a gin pole for the DMX tower. It is pictured at right.
  • I used material on hand to keep the cost low since I do not know if it'll get used again, other than to eventually take this tower down. The 7.5' 18 gauge fence rail used to be the replacement bottom section of my version 2.0 antenna mast for the inverted vee. A 4' length of heavy gauge angle aluminum is attached at the top with muffler clamps. The pulley is fastened at the top, on the side facing the tower. Total length is 10.5' (the photograph perspective altered the apparent length ratio between the pole and angle stock).
  • A close-up of the pulley fastened to the angle stock is shown above. It is rated at 200 kg (440 lb) and was bought at Canadian Tire (of all places) for $7. It comfortably accommodates ⅜" and ½" rope without binding. The pulley mounting has just enough free pivoting to respond to the load. It should turn so that the rope enters and exits in the plane of the wheel as the load shifts. Its motion does not allow the rope to be trapped between housing and pole or to spin the pulley and twist together the two sides of the rope.
  • 5/16" Grade 5 hardware is used to fasten the pulley and the two tower brackets. I used ⅜" nuts as spacers for the pulley mount, and allow room for the brackets to comfortably grab and rest on the tower's X-braces. The rectangular pieces are galvanized steel, thick gauge construction brackets that hold the X-braces between the pole and brackets.
  • I did not weigh the gin pole but it is very light. I can manipulate it on the tower with just one arm.
  • There is an important difference between this gin pole and the design Wade (Delhi) recommends (see earlier picture). The "official" gin pole uses two vertical rods at each bracket which surround the tower leg and rest on the top of an X-brace on both sides of the leg. Mine uses one vertical rod at each bracket and these fit the bottom of X-braces on the left side of a tower face. This is important since the distance between X-braces on a DMX tower is not constant, varying between 36.5" and 37.75" on those I measured. As a result typically only the top bracket on both gin pole designs rests on an X-brace (the brackets are 36.5" inches apart), and therefore the bottom brackets can move sideways. On the official design the tower leg prevents this motion. On mine the free bracket must be tied to the tower so that it doesn't pivot. I use a $2 heavy-duty rubber tie-down which is easy to attach and remove with one hand.
  • Another difference is the total height of the gin pole. The official design is 15' tall, for the purpose of picking up a tower section from the top. This is not strictly necessary since it is only mandatory to pick up a tower section from above its centre of gravity. This allows for a shorter and lighter device, and one that is stronger for the same steel gauge. That is why I can get away with using 18 gauge steel rather than the recommended 16 gauge. However it causes the section to stray further from the vertical, requiring the section to be manually rotated when dropped into the lower section.

Tower erection is dangerous. You can be killed or seriously injured by nothing more than a moment's inattention. Erecting a tower on your own without requisite experience is foolish. Don't do it. Get experience working with others on tower projects before going solo.

That said, here are a few points on safety relevant to this project.
  • Have a plan for emergencies. If something breaks and stuff falls are you standing in a safe place? If a snag occurs up in the air do you have a way to secure everything while you effect a solution? Is a partner nearby to watch over you, and help if necessary? Are you carrying a phone in case you can't move and need to call for help? These are uncomfortable thoughts but you must think about them before you begin. This goes for any tower work, no matter how many people are involved.
  • If you have someone helping you do you trust them? If you tell them "do this" or "don't do that" will they listen and obey? Of equal importance, will you take direction when someone points out a flaw in your plan or execution? Learn to listen to what others are telling you. Stubbornness and pride can kill.
  • Wear a certified hard hat and sturdy shoes when you are underneath any heavy equipment, tools, dangling tower sections, etc. These many not save you from injury but that injury will be reduced, and you get to live a little longer.
  • Test your equipment before starting. Look over those forces described earlier and then, with the gin pole near the ground, lift some test weights a short distance, and try it at various rope angles. Do this even with borrowed or bought equipment, not just for (as in my case) homemade equipment. For one test, since I weigh 55 kg, I could subject the gin pole to 2x+ the weight of a tower section by grabbing both ropes and lifting myself off the ground.
  • If you must climb the tower while a tower section is in the air keep your body and hands away from the gin pole and climb on a side of the tower that is not under the tower section.
  • Plan, don't improvise. Know exactly what will happen, what to do when the wrong thing happens, and lay out all the needed tools and parts where you'll need them. Clean up afterwards.
  • Use a rope cleat on or near the tower to easily secure a load with one hand and no knots. I typically bolt a cleat at shoulder height on my own towers.
The anti-climax: lots of planning and preparation makes for short work

Be safe out there
To close off this article I'll post one more picture. Above is the third section (DMX-4) dangling in the air, ready for me to climb up and bolt it into place. The lift rope is secured to a cleat and then tied off to the tower for redundancy. I did have to climb the tower once during lifting when the section got caught on a temporary rope guy. My homemade equipment performed well.

That tower section is now attached, the bolts torqued to spec, and the first set of guys attached to the "ugly" guy station. The guys are not yet at final tension but they have been adjusted to bring the tower into good vertical alignment. There are now 3 more sections (half the tower) to go, plus one more guy station.

Lifting and securing the tower section took only 30 minutes, including the time taken to unsnag it from the guy rope. It took longer, 90 minutes, to attach and tension the guys and plumb the tower.

Wednesday, July 9, 2014

Overhauling a BBMB Mast Bearing

The majority of rotatable yagis mounted on a tower require two plates (platforms) at or near the top of the tower to mount the rotator and a mast bearing. The bearing fits over the mast and transfers (lateral) wind forces on the mast and antenna to the tower while allowing the mast to freely rotate and greatly reducing bending moment at the rotator.

Since the bearing acts as a fulcrum to the mast's lever it is important that it be correctly placed when the mast extends far above the top of the tower to support stacked yagis. This is accomplished by moving the rotator plate lower in the tower. Either way the bearing is subject to extensive abuse from wind loads and precipitation. Eventually every mast bearing (sometimes called a thrust bearing around here) will require maintenance or replacement.

Many tower manufacturers sell mast bearings that mate to pre-drilled holes on the tower plate. There is a multitude of industrial bearings and matching flange housings that are cheaper and capable of being employed as thrust bearings but for the following shortcomings:
  • Weather - The bearings, while nominally sealed, are open to the elements.
  • Mast adjustment - There is no provision for a mast size other than the one that exactly fits the bearing's inner diameter.
  • Vertical force - Although the mast bearing is not designed for vertical forces it can deal with a modest amount of it. A typical industrial bearing has less ability in this respect since that is not its intended use.
Although the purpose-built thrust bearing tend to be expensive -- this is a low-volume specialty market -- they are almost always the superior choice. That doesn't mean we must always buy new, if you are willing to put in some effort.

The BBMB mast (or thrust) bearing made by Wade Communications (Delhi) for their DMX towers is typically sold by Canadian retailers for $85 to $90, plus tax and shipping. It is made from 2 pieces of cast aluminum that are held together by the ball bearings in the mated half-races, as shown in the diagram at right (taken from the spec sheet linked to above). There is a hole in the outer surface through which the bearings are inserted at the factory. A steel slug is sealed into place by pressing the aluminum of the cast body over the edge of the slug. This product is designed to be replaced not repaired.

Replace is just what many hams with DMX towers will do when the bearing succumbs to the effects of age. Yet it is possible to repair these sealed bearings if the internal wear is modest and it has not been abused by excessive loads or poor installation. Often it only requires cleaning and greasing the bearings and races. That is, if we can get inside to do it.

I know many hams who have repaired these bearings and I have done a few myself. A web search failed to turn up any instructions from hams who have elected to tell others how it's done. Since I was going to repair an old BBMB anyway it seemed to be an ideal opportunity to fill this gap. Although this procedure is specific to the BBMB it may also prove helpful to owners of other types of towers and bearings. It only takes an hour, or perhaps two hours if you're doing it for the first time.

Opening the bearing

The only difficult task in the overhaul process is the removal of the slug that seals the bearings inside the race. Until this comes out the bearings cannot be removed and the halves of the bearing cannot be separated.

To remove the slug it is necessary to grind or file away the aluminum from the casting that is holding the slug onto the housing. From what I've seen of these bearings there are 3 equally-spaces arcs of aluminum pressed in over the edge of the slug. Unfortunately the area is quite small and I couldn't take a picture (with my camera phone) that gets in close enough to show this. Look for it on your own BBMB and you'll see it.

A Dremel tool can be used or a small cylindrical grinder that fits an electric drill. Alternatively, if you have a small, curved file you can manually remove the aluminum. This is not precision work so don't worry if you remove more of the housing material than necessary. Just try not to enlarge the circular slot in which the slug rests.

If there is still resistance to the slug coming out you can drill a small hole in the steel plug and use a sharp, strong tool to lever it out. In this instance I did it with a steel awl with a hardened tip.

Don't lose the slug! You'll need it later.


You will need a small container for this next step. While holding the outer half of the bearing over the container slowly rotate the square flange. The bearings will, one by one, roll over the now open hole and drop into the container. You may need to shake or tap the housing if the balls stick to the grease residue.

This time around I counted the ball bearings: there were 29. There is room for more in the race so don't quote me on this figure.

You'll know when the last ball bearing is out when the halves of the bearing slip apart in your hands.


Pour a grease solvent into the container holding the ball bearings until they're submerged. I used the cheapest available: paint thinner. For this application there is no value in spending money on higher-quality solvent. Drop the slug into the container so that it also gets cleaned.

Dip a paper towel or disposable rag in the solvent and use it to clean the dirt and grease from both halves of the bearing race and other surfaces that are in close contact to the other half of the bearing. Some vigourous rubbing may be required.

When the parts are clean to your satisfaction wipe and dry the surfaces and ball bearings with a clean paper tower or rag. Letting them air dry may leave some residue -- this is a disadvantage of cheap solvents.


When you look at the cleaned races you'll almost certainly notice that the races are indented, one indentation per ball bearing. This is to be expected with steel balls and an aluminum race. The same thing occurs in Hy-Gain rotators, though to a lesser degree.

If the indentations are shallow and smooth you can get more service life out of the bearing. If the aluminum is so badly galled or abraded that there are sharp edges or deep pits in the race it may be time to purchase a new thrust bearing.

Pitted balls can be replaced. Take one to a motor or bearing shop and buy replacements of the same size (I didn't measure them). It is (far) more likely that some balls bounce away during disassembly and you can't find them. It is also not unknown that the slug falls out of the housing on its own and steel balls rain down from the top of the tower from time to time. Yes, that really does happen with this bearing. At least that saves you the trouble of removing the slug!


Assuming that the bearing passes your inspection you will need a few items to put it back together.
  • Grease - I use the same white lithium grease that I also use for rotators and protecting hardware. Use a liberal amount on the races and on the top and bottom rims of the lower housing (the one with the flange).
  • Slug - If the slug is damaged during disassembly you may need to replace it. If it's just bent you can hammer it flat again. If you do need a replacement keep in mind that it is smaller that any coin I have. The smallest in my collection is a pre-Euro 1 peseta coin (Spain) and it is too large. The slug should be steel but I think that any reasonably thick metal will suffice. Just be sure whatever you use to replace the slug does not penetrate into the hole since that will interfere with the balls' movement.
  • Hose clamp - A 3½" hose clamp (stainless preferred) to secure the slug after reassembly.
After reassembly test that the bearing freely turns, only showing resistance due to the grease. As the excess grease is pushed aside by the balls the resistance will decrease. Wipe off any grease that is pushed out at the seams between the bearing halves.

Also test that the bearing turns reasonably well with a modest amount of vertical force. Do this by placing the bearing on a flat workbench and turning it while pressing down with your hand. If there is metal-to-metal contact you'll feel it. A small amount of contact is not a problem.

If the resistance is significant you should disassemble the bearing and sand the surfaces of the bearing housing that show signs of contact. You can see in the cutaway view at the top of this post where this contact is likely to occur. Do not sand or file the races! Be sure that all metal filings are removed before greasing and assembly.

You can see that the bearing is not designed to turn under high vertical force by the fact that the the housing only behaves as a bushing to this force, with only grease to prevent binding. The balls offer little support against vertical forces.

To finalize reassembly insert the slug into the bearing hole and tighten the hose clamp around the housing. You may need a shim to hold the slug securely since it sits slightly below the housing surface. The final result should look like the one in the adjacent picture, which is the one I just finished overhauling.


You will likely need new hardware to bolt the flange to the tower plate and screws and nuts for the mast adjustment. The stock hardware supplied with the bearing is not stainless and readily rusts. Use a bolt cutter if necessary to remove old hardware; the old hardware isn't worth saving.

To mount the flange to the tower plate use 5/16" bolts that are at least ¾" long. You'll need a lock washer as well. The mast adjusting bolts are ¼" and should be at least 1" long, or longer if the mast diameter is small.  If you want to avoid excess length of adjustment screws protruding from the housing you can choose screws that are just long enough to secure the mast you're using. Two nuts are needed on each bolt: one on the inside to set the distance to the mast and one on the outside to lock the screw. High strength hardware isn't required, so use coated (and greased) or stainless hardware to inhibit rust.

Sunday, July 6, 2014

Impatience Breeds Action

With the solar flux pushing 200 and my tower projects taking longer than anticipated I grew impatient. Since I now have a short, perfectly-usable tower bracketed to the house it was time to get some use out of it.

It only took 90 minutes to fit together my 4-band trap dipole (what I call a TH1vn) with a steel mast, lift it into place and hook up the already-prepared run of coax and coax choke to it. QSOs rapidly followed on 20 through 10 meters.

While I can't work anything on 6 meters with this antenna I was able to copy EA8 and W6 among a host of others. I'm tempted to piece together a small yagi from the antenna parts in the garage and point it east.

I have another plan for this tower but for now this will keep me occasionally active until the larger tower is complete.

Friday, July 4, 2014

Guy Stations - the Good, the Bad and the Ugly

After a brief hiatus I have returned to the building of towers. This brings me to guying since the DMX-52 tower I am putting up will be guyed rather than free-standing, which would typical for this class of tower. In particular I want to discuss guy stations (or guy brackets): what they are, what they do and why they are needed.

In brief, a guy station is the structure that attaches to, or is integrated with, the tower to terminate the upper end of a set of guys. Guyed towers have one or more of these at regular intervals specified by the tower manufacturer to support the tower under a set of static and dynamic loads.

In this article I will not delve into the mechanics of guyed towers and guys, which is a large topic of its own. I will assume that the tower guying is being done according to engineering requirements for the structure.

Why Guy Stations?

Why not just attach the guy wires to the tower directly and save the cost and trouble of using guy stations? This is a good question. Certainly that is what I did with my Golden Nugget tower.

Should I have done so when I knew full well that it was a bad idea? I judged that in that particular case it would be acceptable. There is no bright line between good and bad, just a continuous band of gray shading. It is unlikely that I'd advise anyone else to do what I did since I have no control over others' practices and situations. Prudence dictates using the proper guying hardware when you cannot easily or reliably do the stress calculations.

Let's take a short, mainly qualitative look at the forces on a tower at the point where the guy wires are attached. Keep in mind this is an example directed to a conventional guyed tower, not the tower I am currently erecting.

In the standard configuration the tension in each guy wire (pre-load) is 10% of the guy wire's breaking strength. In the case of ¼" EHS guy wire this is 600 lb. Since the tower is laterally stabilized by 3 or more guys with equal tension there is a vector transformation of that tension into a vertical (downward) compression of the tower. With 30° guying (angle between the tower and guy wire) the compressive force on the tower is ~500 lb (cos 30° x 600), multiplied by the number of guy wires at the guy station, or ~1,500 lb for the typical 3 guy wires, at each guy station.

There is also a normal stress on the tower at the guy station (horizontally outward in the direction of the guy wire) of sin 30° x 600, or ~300 lb per guy wire. Since each of the 3 guy wires points in a different direction there is a large force that is essentially trying to pull the tower apart!

Dynamic loads (ice, wind, vibration, etc.) can increase the various forces and stresses. For example ice on the guys wires increases the normal stress and tower compression. Wind has a more complex affect since the tension increases in the windward guy(s) and decreases in the leeward guy(s), and not necessarily in exact proportion.

Towers that are specifically engineered for guying often integrate a high-strength set of cross braces, typically located at the centre of each section. It is designed to withstand the normal stress of static and dynamic guying loads, thus ensuring the tower is not torn apart. Guying hardware attaches directly to these braces. On other towers, especially those typically those that are tapered for free-standing applications, a separate guy station (bracket) must be installed to strengthen the tower to withstand the guying stress.

That is all that a guy station really is: an integrated or supplemental structural component to increase lateral strength of a tower where guy wires are attached. Now we can look at some examples, including those that I will be using on the DMX-52.

GS123 at the top of the DMX-2 tower section
The Good

A good guy station (or bracket) is one that is built by the tower manufacturer for that application. This is almost always the most expensive option but also the best. Provided that the tower is built and loaded within the rated height and capacity, and the manufacturer's guy stations and guys are similarly installed per specification, you can be assured of a safe and reliable tower.

Add-on guy stations are most often of two types: attached at the joint between sections, or; attached mid-section, usually resting on a horizontal cross brace. Wade (Delhi) guy stations are of the former type. Rohn guy stations are of the latter type.

The Wade guy station (in my case, the GS123) attaches to the lower of the set of bolts that bind the sections. As should be obvious (and can be seen in the picture) there can be no cross braces where the sections join so the guy station must supply the lateral strength on its own.

Rohn guy bracket
Rohn and Trylon guy stations grip the tower from the outside. In both cases the lateral members are heavy-gauge steel to withstand the guying stresses and ensure that when there is lateral force or motion that the tower moved as a single unit, not allowing the legs to move relative to one another. Unlike the Wade guy station the Rohn guy bracket complements existing tower cross braces, and in fact rests upon them.

Whether by gripping the legs, abutting the cross braces or bolted to the legs the guy station effectively transfers compressive force (parallel shear) to the tower legs without slipping on the legs or "hinging" the legs.

The hardware that attaches the guy wire to the bracket must be high strength and able to accommodate a range of guy angles. The better designs allow resistance-free pivoting (see the Rohn bracket above). Many commercial towers utilize a shackle that freely hinges in the vertical plane. The Wade guy station utilizes flanges with a fixed angle, and achieves freedom of movement from the pivoting of the thimble (supplied with the guy station). This imparts a shear differential between the bottom and top of the flange. While not ideal I have yet to hear of this being a problem in the field.

I purchased a new GS123 for this tower. I wanted the best option for the guy station positioned near the top of the tower, close to the antenna load (between the DMX-1T top section and DMX-2). This will suit for the loads I am planning. The GS123 cannot be placed higher without modifying the tower.

The Bad

The worst option is to wrap the guy wires around the tower legs. The reasons are simple enough:
  • The guy wire must rest on a cross brace which may be inadequate to handle the stress of static and dynamic loads. The cross brace may also be inadequate to resist the normal stress caused by the tower legs being pulled outward by the loads.
  • Abrasive active of the guy wire on the tower leg and cross braces will first remove the protective coating on all three, then by abrasion and rust will cut through each strand of the guy wire. Failure will occur more quickly for towers formed from sheet metal than from rods and tubes.
Despite these serious problems this is how I had guyed the Golden Nugget tower. But I knew it would not be for long -- just one year -- and the static and dynamic loads would be far less than in a conventional guyed tower.

I also used polyester rope at the lower Golden Nugget guy station. Rope is a poor choice since it has a very high modulus of elasticity (stretches easily in response to increased tension) and is degraded by ultraviolet radiation from the sun. Polyester is rated "UV resistant", but don't let this fool you. After one year there is evidence of UV damage on this rope. Dacron is a superior choice if you must do this. I recall a VK who, years ago, used heavy hemp rope (around 1" or 2" diameter) to support a couple high towers. It seemed to work for him, so who knows.

The picture above shows that I am using this style of "bad" guy station to temporarily support the bottom two DMX sections. Once the third section is lifted into place it will use a more appropriate guy station at the top. When that is done the ropes will be removed.

The Ugly

I did not want to purchase two new guy stations for this tower. Rather than use a GS456 guy station at the top of the DMX-4 section I improvised a guy station. Yes, it is ugly. When I say "ugly" I mean a guy station that is up to the assigned task but looks as if it is not.

The guy station I designed and built is shown in the adjacent picture. The total price is $6, with all parts purchased new:
  • 5' of coated twisted-link, 5/32" chain, cut into 3 equal lengths
  • Four ¼ x 1¼" Grade 5 bolts and nuts, and 8 ¼" flat washers
The ends of each chain are bolted together, but not tightened. Each chain is looped around a tower leg and the other end is loosely bolted to a link on the same chain. Flat washers sandwich the chain links, acting as pressure plates.

The objective is to keep the common tie point centred within the tower and the tension in the chains high enough that some effort is required to slide the final link over its connecting bolt. As the bolts are then tightened the twisted links at each connection will shift to accommodate the pressure. When this happens the chains becomes taut. The improvised guy station is now rigid, a key requirement of a "good" guy station.

The guy station is completed by inserting guy wire thimbles into links on the outside of each tower leg. These are not yet there in the picture since I had run out of thimbles of the correct size.

Final tensioning is to be done after the tower section above this one is bolted in place. Otherwise the upper section might not easily fit into the lower one. Also, the grip of the chain is strong enough that it cannot be slid down into its resting position on the cross braces when it is tensioned.

Since the ugly guy station rests on cross braces to support the vertical load it is not the same quality as a good guy station. For this reason I am using it for the bottom guy station where the loads will be lower than at the top guy station.

The Plan

I have now been off the air since dismantling the antennas and towers in early June. Unfortunately this means I am now missing some good conditions due to the high solar flux. But all I've had time for since returning home is construction of the guy stations. I may put up a temporary antenna on the house-bracketed tower just so that I don't completely miss out.

Next up is the construction of a gin pole and lifting the section with the ugly guy station into place. It will be guyed at low tension until after the next section (the fourth) is in place. Once these guys are at working tension and the tower is plumbed the rest of the tower can be raised.

If all goes well the tower should be up later this month (July).