Broadcast Writer

By Vicki W. Kipp
Society of Broadcast Engineers (SBE) Chapter 24 Newsletter 
July 1, 2001

The truism “take care of your things and they will last a long time” applies to towers as well as personal possessions. Taking care of your tower is a multi-step process that involves first performing an inspection of the tower, then completing the needed repairs.

For several important reasons, you should examine your tower regularly. First, you want to resolve problems that could cause injury to site visitors, site personnel, and tower technicians. Inspection and maintenance can prevent problems that might lead to tower collapse, as well as prevent damage to tower appurtenances. A conscientious tower inspection and maintenance program will enable your tower to last as long as possible.

INSPECTION

Once every twelve months is the recommended frequency for inspecting your tower. In fact, many tower insurance policies require an annual inspection. The inspection should at a minimum meet – and ideally should exceed – the ANSI/EIA/TIA 222F standard checklist for inspection.

When you construct a new guyed tower, it should be inspected one year later for physical changes experienced during its first year. Slight shifts in the earth and stretching of the guy wire will decrease guy wire tension and change the plumb (straightness) of the tower.

Several specific occurrences necessitate a fresh inspection of the tower. Addition of a dish, antenna, or other load to your tower should prompt you to analyze and re-inspect tower loading.

If the Voltage Standing Wave Ratio (VSWR) reading for a transmit antenna changes, if you begin using more nitrogen or dry air in your transmission lines, or if you hear tower coaxial lines slapping in the wind, it is time to do a tower inspection.

If the tower is exposed to a damaging weather event such as a strong windstorm, a tornado, or a heavy ice storm, the tower should be inspected for damage.

TOWER GROUNDS

If the tower has an entrance gate to block trespassers, that should be inspected first. While you’re at the entrance, check to make sure all appropriate signage is displayed. For example, you should have an FCC RF License sign designating the tower owner and the FCC registration number. Perhaps you have a “No Trespassing” sign. Some towers have hazard signs such as “Watch for Falling Ice” or “Electrical Hazard.”

FENCING

Inspect the fencing around each of the guy anchors and the base of the towers. If a fence is sagging, damaged, or plagued by an overgrowth of vegetation, note the deficiency so it can be corrected. For an AM tower, the owner is required to keep an effective, locked fence around the antenna to protect people from touching the electrically hot antenna/tower. You may wish to mark the guy anchor and surrounding fence with bright paint to alert heavy vehicles to its presence.

FOUNDATION

Check the tower foundation for damage. If a foundation is unable to handle the tower load or if the soil underneath the foundation is weakening, the foundation can be reinforced by widening the area where the foundation bears against the soil.

AM INSULATOR

For an AM tower that sits on an insulator, the insulator should be inspected for damage and cleaned to remove any conductive coating. Also, keep the tower base free from plant growth, as it can become a fire hazard when the AM antenna transmits at high power.

STEEL

The steel tower itself should be an integral concern during any tower inspection. Steel towers are usually galvanized to resist rust, but are still susceptible to rusting. Any visible rust should be scrubbed off the tower structural members with a wire brush, and then these areas should be coated with spot paint or liquid galvanizing. If the tower is painted orange, white, blue, or black, then the galvanizing (once dried) should be painted over with the same color.

Internal rusting can be a huge problem for monopoles and towers with hollow legs. A tower structure may look stable from the outside, but actually be rotting from the inside out. Hollow tower elements have drain holes – called weep holes – which allow moisture to exit. These drain holes can become clogged by debris, trapping moisture inside and causing corrosion.

STRUCTURE

To assure that a tower is likely to remain vertical and that it is safe to climb on, you should visually inspect the structure before ascending it. Using a pair of binoculars, scan up and down the tower looking for missing bolts.

This search will be easier if the riggers put the bolts in upside down when assembling the tower. The benefit of installing the bolts upside down in the tower is that gravity will cause the upside-down bolt to fall to the ground if the nut comes off. If this happens, an open hole with sky showing through it will clue you in to the missing bolt. If the bolts were installed right side up, it will be more difficult to notice if any of the bolts have lost their nuts.

The plumb of a tower needs to be observed. If a tower isn’t plumb, it could be because some guy wires are too taunt or too slack. It is possible for a tower to have improper guy wire tension and still be plumb.

Simple observation may show you if the guy wires sag, and whether or not they sag consistently at the tower and anchor ends. A measurement of the tension of each guy wire needs to be taken. Using the shunt-type dynamometer, transit intercept method, vibration method, or the tension dynamometer method, determine the numerical tension value for each guy wire. Compare your results with the original measurements for the tower.

If the values are different, adjust the tension of the guy wires back to the original value. If guy wire tension is incorrect, the tower may not be able to withstand as high of wind loads as it was designed for.

COW FACTOR

An industry veteran advises that you check each guy wire near where it meets the anchor for wear and tear, particularly if any farm animals are pastured in the tower field. The reason for this is that some animals like to rub their backs against the guy wire. Although the wire does not wear down immediately, it can be substantially affected by if animals continue this behavior for eight to ten years. Although this phenomenon may be referred to as the “cow factor” in Wisconsin, it can also apply to wildlife wherever a tower happens to be located.

TURNBUCKLES

On a tower, a turnbuckle is a piece of hardware near the anchor (Figure 1) that connects to guy wire at both ends such that when it is turned, it takes in or releases both wires uniformly. While inspecting the turnbuckle for corrosion, make sure that the turnbuckles aren’t chafing other hardware. All turnbuckles attached to an anchor should have a comparable number of threads for take up and let out. Spray used and exposed turnbuckle threads with cold Galvanizing to protect them from rust. While you’re looking at the turnbuckle, take a moment to examine the safety wire. If you should find that the safety wire has become welded to the turnbuckle, this is a sign of a grounding system problem. Since you’re at the anchor, take a look at the preform and preform grip – the dead-end grip that holds the guy wire. Inspect the thimble – a ridged component that keeps the end of a guy wire from closing and maintains a uniform curve – and shackles also.

ANCHORS

Prior to inspection, the ground surrounding each guy anchor should be cleared of weeds. In fact, it is best to keep this area clear of vegetation at all times. Plants hold water in against the anchor, leading to corrosion. Applying commercial vegetation killer or soil sterilizer is an effective long-term method to eradicate weeds.

Examine the concrete anchor for cracking or chipping. Check for sinking or upward movement of the anchors or tower by using the line where the concrete anchor meets the earth as a reference. Next, dig away a few inches of soil to inspect the anchor below ground for corrosion.

PAINTING

If the tower is painted international orange and maritime white, compare the tower’s current shades to an FCC color tolerance chart to see if repainting is needed. Make sure to check the paint condition at the top of the tower where it is more affected by environmental forces.

GROUNDING

When lightning strikes a tower, grounding components give limited control of where energy will go by guiding the electrical surge to pass through a conductor to the ground. Lightning strikes can take a toll on tower grounding components. Thus, it is necessary to inspect the ground halo and grounding connections at all locations including the tower legs, anchor heads, site buildings, and the top, bottom, and building entrance for each transmission line run.

To keep your grounding system functioning efficiently, inspect all grounding connects to make sure that they are still intact and properly positioned. Bent ground wire is easily straightened. Check for corrosion, breaks, or deterioration. Corroded ground wire can be repaired by wire brushing corrosion off and then spraying the clean copper with a zinc chromate primer.

ON TO THE TOWER

Before climbing on the tower, you should check the climbing pegs to make sure that they are inserted firmly, and will support your weight. Likewise, you should make sure that the ladder is free of rust and securely attached to the tower.

ELEVATOR

Being exposed to the elements can cause tower elevators to have control circuit problems. Before taking a ride in the elevator, verify that the hoisting equipment is working, the control circuits function reliably without operator intervention, and the emergency braking system runs freely.

TRANSMISSION LINE

Transmission line allows the signal to travel from the transmitter, up the tower, to the transmit antenna. Every inch of line needs to be visually checked for dents, leaks, bullet holes, lightning damage, ice damage, and hot spots. Many transmission lines are pressurized by gas to keep contaminants and moisture out. If there is a change in logged usage of gas or compressor operation, the transmission line may have a leak.

When inspecting transmission lines, check the line pressure. Leaks are not always easily found by inspecting the line, but can sometimes be located through TDR testing.

Faulty connections or line damage cause transmission line hot spots. Flanges and elbows are especially vulnerable to having problems that cause hot spots. Hot spots become apparent before problems begin affecting VSWR. If transmission line problems go undetected, they can result in arc-over or burnout of the transmission line.

VERTICAL RUNS

When inspecting vertical runs of transmission line, the key is to ensure that proper hardware placed at adequate intervals is used to keep the transmission line securely attached to the transmission line ladder. As a general rule, there should be a hanger installed for every three feet of transmission line. When used correctly, hardware such as hoisting grips, cable supports and spring hangers prevent suspended sections of transmission line from stretching. At higher tower altitudes, lines may need a greater attachment frequency due to high winds.

LINE SWEEPING

Using test instruments such as a Time Domain Reflectometer (TDR) or a Frequency Domain Reflectometer (FDR), transmission lines can be tested for faults such as impedance discontinuity and poor connections. TDRs and FDRs identify the Distance To Fault (DTF). The TDR is used to test coaxial cable, while the FDR is used to test waveguide. The TDR computes conductor opens or shorts by transmitting pulsed DC signals into the transmission line and then analyzing the reflected pulses. The FDR transmits a swept frequency RF signal into the line to check for transmission line and antenna problems.

LIGHTS

Start by visually confirming that all of the tower lights are functioning. Check that the photo control senses daylight levels to successfully turn the tower beacons and sidelights on at night and off at dawn. Verify that your high intensity white strobe lights do not exceed the FAA peak light intensity of 270,000 candelas during the day, 20,000 candelas at twilight, and 2,000 candelas at night.

Ensure that the flash rate control for the beacons is timed for the proper number of flashes per minute. Simulate a tower light failure to demonstrate that the automated alarm system detects the failure and sends an alarm. Physically inspect the safety chains, fasteners, vent holes, light lenses, liners, fixture gaskets, junction boxes, conduit runs, and visible wiring for damage.

ICE BRIDGE

An ice bridge (Figure 2) is a metal grate mounted over an antenna, microwave dish, or transmission line with the purpose of shielding these items from falling ice and debris. Verify that the ice bridge cover is intact and positioned correctly, and that the hangers are holding the bridge in place tightly.

ANTENNA

The antenna, probably the most important piece of hardware attached to the tower, needs to be inspected for weather damage. Antenna mounts Figure 2. Ice Bridge protects the microwave dish below it. should be inspected. The weatherproofing that covers all antenna connections should be inspected and reapplied if necessary.

MAINTENANCE

Upon completion of a tower inspection, document what repairs are needed. Plan for completion of those repairs. Next month, we’ll continue our discussion of the tower industry.

Information for this article came from the following sources: BE Radio, John Battison “Care of Transmission Lines”; Broadcast Engineering, BE Radio, Kirk Harnack “Tower Maintenance”; Broadcast Engineering, Don Markley “Preparing Towers for Winter”; ComTrain Basic Tower Technology, Winton W. Wilcox; Wireless Review, Betsy Harter, “Ready for Inspection”. Ice Bridge

Figure 1. For a guyed tower, each anchor needs to be inspected.              

Figure 2. Ice Bridge protects the microwave dish below it.