Tower maintenance

Conventional wisdom holds that you start at the bottom and work your way to the top. There are very few jobs in broadcasting where one can start at the top; but tower maintenance is the one where it is best to start at the top and work down. At one time it was not unusual for engineers to climb their towers to change lamps. Fortunately today, with OSHA and more tower climbers being available, the highest most engineers go is to check the current loop position on a DA.

Beacons and lightning rods are often forgotten until a lamp fails or a bad strike takes out some equipment. Lightning rods are a very good investment in handling lightning, and a few dollars spent here can often save a lot of down time. If it is possible, beacons and other items at the tower top should be checked regularly, and results should be entered in the maintenance log. Particular attention should be paid to glass fittings and seals to keep moisture out. Some engineers keep a lamp life tally and change lamps before failure if total hours are high when a tower is climbed for another purpose. These same people have been known to measure and record voltages at the lamp sockets.

An AM tower’s base insulator is a crucial part of the structure and must be checked for cracks and other structural wear.

The tower log should also include outages and details and the quarterly FCC mandated tower light inspections. I've visited many stations where there was no record of this vitally important tower light inspection for a very long time. Even when tower light inspections were done, the important check of the beacon flasher cut-in sensitivity was often omitted. A combination inspection/log keeps all tower data in a convenient location, and space can be provided for noting FAA notification and restoration of service after a reportable failure.

Paint should be examined for discoloring and chipping. Rust indications coming through paint should be investigated for internal rusting. Bolts must be checked for presence and tightness. It's easy to pass over a loose bolt that has lost its nut when paint has obscured the loose head. Many engineers like to insert bolts upwards so that a bolt that looses its nut will fall out and leave a gaping hole that advertises the lost bolt.

All items attached to the tower must be checked for tightness and bolts must be replaced as necessary. Devices that carry RF or rely on metal-to-metal contact on the tower should be examined and contact surfaces should be cleaned and secured as needed.

Not just the tower

It is extremely important to check guy insulators at the tower attachment points. Any cracked or broken insulators can cause changes in operating parameters. If operating logs show such indications, and other faults have not shown up, these might be good places to look. In most AM towers, top guy wires shorting to the tower are the most likely to affect base operating resistance. Johnnyballs should be examined using field glasses if possible, and guy wires must be shaken while operating parameters are checked for indications of undesired contact.

Guy wire greasing seems to be somewhat neglected these days except for tall towers. It is worth considering this operation when performing tower maintenance. Over the years, guy wires do rust and are not often examined for tensile strength. How often does the average station check the guy tensions? How often is the tower itself checked for perpendicularity?

Turnbuckles must be checked for security and the presence of locking wires. While looking at guy anchors and turnbuckles, anchorbase conditions must be examined. Over the years, a leaning tower or overtightened guy can loosen guy anchors. Concrete also deteriorates over the years, and anchors must be examined for cracking and failure to hold.

Guy anchors and guy wires must be kept clear of growth.

The tower structure itself must be checked for loose and missing bolts (especially in the case of AM towers) for clean and good electrical contact between leg sections. Welds should be checked on welded towers. Every tower should have good continuity between leg sections to minimize lightning damage. If electrical bonding has been used, each bond must be present or restored so that I2R losses do not increase.

Down to the base-ics

The heart of an AM tower is probably the base insulator. The weight of the tower, guys and insulators, lighting and any other antennas and transmission lines all rest on the base insulator. Weep holes in the base insulator of series-fed AM towers must be kept clean and open, and any cracks need to be inspected by a tower specialist. In some parts of the country, birds like to nest in base insulators, and I've heard what may be apocryphal stories of base insulators cracking because of excessive nesting materials and resulting in uneven heating in very hot areas.

Replacing a base insulator is not an easy job, and it's very impressive to see a tower jacked up while the old insulator is removed and a new one installed. If rigid FM lines are involved, the operation becomes much more complicated. Great care is required to ensure that no lines are stretched or broken.

The tower footing has to be examined and the state of the concrete verified together with the four-inch copper strap that should be mounted on the four sides of the base between the lower part of the insulator and the ground system.

Replacing an AM base insulator is further complicated when an FM antenna shares the vertical structure. It may be necessary to move the additional RF feed lines temporarily while the work is done.

A grounded tower is simpler to work with, but the need for secure foundations is just as great. The tower base is connected to the ground system by the usual four-inch copper straps, and the slant wire, if used, is located at a convenient place up the tower.

A grounded folded unipole antenna must have its rather complicated installation examined very carefully. The connections from the skirt cables to the tower are vital and must be very secure. The same caveat goes for the shorting stub from the skirt wires to the tower as well as all the ring connections at the base of the skirt. The tower lighting neutral grounding connection is especially important when using a folded unipole.

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