Wednesday, May 28, 2014

Buying a Used Tower

A few hours before the start of WPX CW this past weekend a tower arrived on my doorstep. Back in January I did say that a taller and stronger tower are part of my plans for 2014. To that end I have been shopping the used tower market for the past couple of months. Below is a picture of the tower loosely assembled in my back yard.


This is a used Delhi (now Wade Communications) DMX-52. Tower weight is ~120 kg and it stands 14.1 meters (6 x 8-foot sections, with 5 x 4¼" overlap between sections). The tower is typically erected as a free-standing tower but it can also be bracketed or guyed. I will be guying it since I don't want the cost and inconvenience of planting it in concrete. It may only be in use for 1 or 2 years.

You are most likely unfamiliar with this tower if you are not Canadian. They can be found from one coast to the other in both ham and non-ham applications, both residential and commercial. Like many towers it is uncommon to find them far outside their countries of origin. For example, while Rohn towers are very common in the US they are a rare sight in Canada. The same is true of DMX towers in the US. This is despite the extensive commerce between the two countries.

Coincidentally this tower model was my very first tower, back in 1974 when I was in high school. It served me well for several years until I graduated from university and moved out of my parents' home. I didn't have a tower again until 1985. During those intervening years about the only times I got on the air were as part of multi-single and multi-multi contest teams at other hams' stations.

Since all of us like to save money I thought that it would be helpful to say a little about the right way of buying a used tower. Even if you buy new you may find something useful here as well. This is not a comprehensive list, just some points that some might benefit from.

There are no bad towers

There is an old saying: there are no bad dogs, only bad dog owners. The same is true of towers. That doesn't stop many hams from declaring a tower to be "bad" when in reality they are responsible for ignoring the engineering limits and installation and use instructions, only to have something unfortunate happen.

All they're doing is shifting blame for their own poor choices and behaviour. Please don't be one of those hams. Choose the right tower for the task at hand.

Availability vs. need

When looking for a used tower you are at the mercy of the market. Often you can't get exactly what you want. Don't use that as an excuse for buying a tower that is not adequate to your needs. In the long run that can cost you dearly. Know your local market and the specifications for types of towers that come on the market so that you'll know what to pursue and what to pass over. Around here those are mostly DMX and Golden Nugget towers by Wade Communications. The latter is what I put up last year to support a high-bands dipole and delta loop for 40.

The residential OTA (over-the-air) television market is much larger than the ham market so don't only focus on buying from hams. Many non-hams will give away unwanted towers if you'll only take them down. This is less true in Ottawa this year than last year, probably due to increasing numbers of people "cutting the cord" to sate their television appetite. If you've never done tower work before don't start your education by offering to take down someone's unwanted tower for free. At least not without experienced assistance.

Consider buying two imperfect towers if the prices are very attractive and you can mix and match pieces to make up one good tower. You can endear yourself to your ham friends by giving away the pieces you don't need. Your family will appreciate that you don't clutter the yard with unwanted tower sections that slowly surrender to entropy.

Nothing is forever

No tower lasts forever. Every product has a limited lifetime. When you buy a used tower you are buying a tower that has less life in it than a new one. That's one reason you pay a lower price. Always keep this fact firmly in mind: a used tower will not last as long as a new tower. Worse, there is no easy way to estimate the remaining service life.

Never buy a used tower sight unseen, even though that may restrict you to the local market. Too many towers are misused, overused and abused by their previous owners. This goes for both hams and non-hams. The dreadful things I've seen done to perfectly good towers over the years continues to amaze and disgust me. While you may not be able to estimate the service life of a used tower you should be able to identify abuse and neglect. But to do so you or an acquaintance who knows towers needs to be able to inspect it firsthand.

What you can't see can hurt you

Stresses from handling, normal use and even poor storage conditions can result in micro-fractures that are impossible to see in a visual inspection. Aluminum in particular is prone to invisible flaws since stresses below the metal's yield point can weaken it.

An example of fatigued aluminum can be seen in the tower I just bought. Aluminum rivets attach the cross braces to the tower legs and to each other. The action of the harder metal (steel) against the aluminum rivets is hidden from view. That is until a rivet breaks. This problem commonly afflicts DMX towers of a certain age, especially when they are overloaded.

It is a simple matter to drill out the remnants of a broken rivet and replace it with a bolt. But first take note of any bends or breaks in the steel braces themselves that would indicate a more serious problem. If there is a problem you need to closely inspect the rest of the section, and even other sections since it is rare that excess stress on the tower causes just one problem. That is, when you can see one rivet that is broken due to fatigue or steel abrasion you can be sure that others are not far behind.

The one broken rivet in the tower I purchased (picture at right) has now been fixed. It appears to be an isolated failure and therefore of no undue concern. However I will still periodically inspect the area after the tower is raised.

In a tower made of formed sheet metal such as the DMX pretty well every part and surface is visible. In a tower with tubular legs (e.g. Rohn or Golden Nugget) there are areas that cannot be easily inspected. There are many ways for water to get inside the legs and gradually, over many years, to find any flaw in the steel coating through which rust can get a toehold.

The problem of inspecting inside tubular legs gnaws at me and I therefore tend to distrust the integrity of used tubular towers. One quick though imperfect test is to look for rust stains on the inside bottom of tubular legs. These are caused by loose rust transported downward by moisture. You need to establish how serious the problem is and reject the section if there is any doubt. One weak point can bring down the entire tower.

Galvanizing

Pretty much all towers are made from galvanized steel. This is typically a zinc-based coating put there by hot dipping (best) or electroplating (not as good).

Depending on local climate the coating will wear off sooner or later. Once the base metal starts to rust it is difficult to stay ahead of the problem. If you've ever tried to paint a standing tower you'll understand. It's a messy and difficult job. In fact painting a tower on the ground is tedious work due to the intricate structure.

Reject a tower that has more than small patches of rust, or where just one spot has eaten deep into the steel. A badly-rusted tower is too expensive even if it is freely offered. It is well past its service life. Even if you repair the visible rust it still indicates that the coating is wearing thin and rust will soon appear elsewhere.
 
At right is a picture of the top section of my newly-acquired tower. The rust you see is due to attached clamps, not within the tower itself. This is nothing more than a cosmetic issue. Learn to distinguish between base metal rust and rust that "drips" onto the tower.

Hardware

Towers are engineered structures where every piece of it is designed or selected to a set of specifications. Violate those specifications and the tower will not perform as advertised. One of the most common violations I have seen is the use of improper hardware. In particular the bolts which connect the tower sections. This is a topic I talked about last year.

Tower bolts have a couple of properties that are especially important: shape and strength. In the picture at right are a few bolts from DMX towers and related equipment.

At the top is one of the bolts that came with the used tower I just bought. There is some corrosion but it is usable with some cleaning. Below it is a new bolt of the same type. These bolts have a ½" shank (outer thread diameter) and are Grade 5 strength. Although only the bolt head carries the Grade 5 marking (3 bars on the head) the nut and lock washer are the same strength.

None of the hardware on the used tower came with lock washers. What happened to them I don't know. The fellow who sold me the tower is a construction professional and came across as very honest and careful. But he does not know towers especially well. He is not the original owner and so I expect that the lock washers were not present when he acquired it. They must be replaced. I purchased new Grade 5 lock washers for $0.09 apiece (and $0.05 for the sections which use ⅜" bolts).

The bolts are not standard. Notice the bevelled shank just below the head. The bolt holes in the tower sections are slightly larger than ½" so the threaded part of the bolts fit loosely within the holes. When the bolts are tightened the bevel creates a small dimple in the tower leg underneath the bolt head and forces the other tower leg into alignment.

http://www.wadeantenna.com/Wade/DMX%20Instructions-English-RevE.pdf

A standard bolt does not do this and so cannot properly secure and align the sections. Get replacement bolts from the manufacturer whenever specialized bolts are missing or badly corroded. They will cost more but the price of an improperly built tower can be far, far higher. If you don't want this expense you should reject a used tower that does not come with a complete set of bolts that are in good condition.

The bottom bolt in the above picture is also for a DMX tower but has a standard shape. It comes as part of guy station kits. These longer bolts are used in place of the lower of the pair of bolts at the section joint where the guy station is mounted. The other, bevelled bolt should be torqued first since only it can align the sections.

Bolt holes

Improperly secured tower sections and excessive load are often visible in the bolt holes of the disassembled tower. Bolts that are insufficiently torqued and slip due to weight pressure or tension due to high wind loads will show up in wear and stress marks in the bolt holes.

Some wear is normal so don't expect the attachment points to be pristine. One common problem is wear around the hole due to rotation of the bolt head during installation and removal, which scrapes off the steel coating and even the steel underneath. The bolt head should be held still and the nut turned during both installation and removal. The lock washer under the nut prevents abrasion of the tower when the bolts are tightened and loosened. Excess metal loss due to improper bolt handling is a problem.

Galling and elongation of bolt holes are a clear sign of abuse, inadvertent or otherwise. All the bolt holes on the tower I bought show normal wear except for the bottom holes of the bottom section. This is where the tower is normally attached to the base (engineering stubs in concrete).

This wear does not appear to be due to movement caused by insufficiently torqued bolts. The visual evidence coupled with the seller's story of the tower condition he found when he took it down seems to indicate that a non-standard mounting method was employed that involved shaving material from the holes. This is not good.

Since I need to build a custom, ground-surface mount for this tower this damage is something I can work with. If it were to be installed with a traditional concrete base as a free-standing tower or off the ground with larger sections underneath I would be more concerned. I must ensure that the tower is capable of supporting the combined static and live load of the tower itself, antennas and the climber (that is, me).

Modifications

Beware of modified towers! Too many people imagine that they're smart enough to cut, attach or pierce the structure to suit a unique application. They are almost always wrong. Unless there is good evidence that all modifications meet a high engineering standard a modified tower should always be rejected.

Next steps

Apart from minor repairs to the tower I need to construct a base and a gin pole, and buy or build the guying hardware. I plan to use the same guying system as for the current 30' Golden Nugget tower. In fact it will replace that tower at that same spot (Site C).

When all is ready I will completely dismantle all antennas and support structures. Everything will be rebuilt from the ground up. This will not prove as large a task as it might seem. In any case I have less interest in operating during the summer so it will not be an inconvenience to be off the air for a few weeks. Other activities get priority this time of year.

Once I complete the dismantling I'll say more about how I will go about building the next iteration of my miniature antenna farm. By summer's end I expect to have a moderately competitive QRP DX and contest station.

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