Sep 1, 2000 12:00 PM, Kirk Harnack
Tower maintenance may not rank up there with your top 10 engineering duties: It's costly and time-consuming. Tower work is usually contracted, so engineers must rely on the honesty and integrity of the tower inspector. Moreover, we do not often realize immediate benefits from tower inspections, so they may be viewed as just another expense.
Certainly, however, good tower maintenance saves broadcasters from more costly repairs and lost airtime. Maintenance can even save the tower itself from early failure. The number of problems discovered and repaired on tower checkups often reveals the value of regular tower inspections.
How often should a tower be inspected? Inspecting towers every 12 to 24 months is considered wise. It is common for insurance policies to dictate regular inspections at 12-month intervals.
It is also good engineering practice to get a tower checked after a significant storm, especially if damage on the ground indicates that there may be damage overhead. Look for changes in VSWR on transmitting antennas, an increase in nitrogen or dry-air usage and post-storm tower lamp failures. Also, listen to the tower for coax lines slapping in the wind. These conditions warrant a closer look upstairs.
The tower inspection Each tower site is unique and presents its own set of challenges during a thorough inspection. A station can get more for its inspection dollar by making the job easier for the tower climber.
Make sure the site is accessible, keys are available and work properly, and other site users are notified for power-reduction coordination, if necessary. Often overlooked, guy anchors are critical to the tower's integrity. Clear the area around each guy anchor prior to inspections.
Look for the following items during a tower inspection.
Tower plumb. A tower typically exhibits one or more physical changes during its first year. Guy wire stretching and even slight shifts in the ground will cause slack in the guys and variations in the tower's plumb. Some tower-erection firms recommend that a new tower be checked for plumb and guy tension one year after construction and that subsequent inspections be scheduled if any significant corrections are needed.
Guy tension. Tower plumb and guy tension are closely related. It is possible, however, for a tower to be plumb while some or all guys are too taught or too slack. Tests to determine proper guy tension include the use of a shunt-type dynamometer, the transit intercept method, the vibration method and the tension dynamometer method. The tower manufacturer will often have a preferred method for determining guy tension. Ask your tower inspector to use that method and show you the results.
Proper paint. Tower paint near the bottom of a tower is easy to see, but the care that goes into painting the lowest orange and white bands does not always make it to the top. Have the tower inspector check for differences in paint quality and alert you to any deficiencies.
Bolts and hardware. Missing or loose hardware is common, especially at higher points on the tower. Some tower crews have been known to skimp on hardware and fasteners where they will not be noticed right away. Coax, conduit and other vertical runs need the same or even greater attachment frequency up high where the wind is greater. It is important to check for loose or missing tower bolts, lock washers, guy attachment hardware, and other critical fasteners and parts.
Guy termination. Guy termination failures, rusted hardware, improperly installed preforms and ice damage to preforms all carry responsibility for tower collapses. These problems are relatively easy to spot and correct during routine inspections.
Turnbuckles should be inspected not only for corrosion but also for the presence of safety wires. Make sure adjacent turnbuckles are not rubbing and chafing against each other or other guy wires or hardware.
Guy anchor points are often choked with weeds and vines. Clear these and other obstacles out of the way. Commercial vegetation killers will knock down weeds quickly. A longer-lasting choice, if suitable for use in your area, is a "soil sterilizer." These keep vegetation under control for up to three years. The guy anchor itself should be checked for rust, cracks or bends in the steel. It is also helpful to carefully dig away a few inches of soil to inspect the anchor below the normal soil level. Corrosion, especially that caused by galvanic action, can cause severe weakness here.
Transmission line and antenna integrity. Supporting the transmission lines and antennas is the sole function of most towers. Check all lines and antennas one foot at a time for leaks, dents, lightning damage, bullet holes and damage from fallen ice.
Remember to check for the presence of basic hardware and accessories. Coax runs should have proper hoisting grips installed at recommended intervals to hold their weight. Proper coax grounding kits are often overlooked at installation, so check for these and install them if they are missing. Rigid transmission lines require copious amounts of hardware along their runs. Make sure you and your tower inspector know what hanger parts must be present and what condition is acceptable.
What is legal and what is prudent for tower grounding are usually two different things. Without specifying a particular grounding scheme, a typical tower is likely to have just two or three solid copper wires leading from the tower base to a couple of ground rods. Although this scheme may satisfy the electrical code, it is not prudent for broadcasters interested in protecting their tower and transmitter assets.
Safety Before completing a periodic tower inspection, examine those items relating to safety - both employee and public safety.
Most towers are fenced in some way, thanks to the FCC Rules, insurance-policy requirements or even local jurisdictional ordinances. For AM towers, the Rules require "an effective, locked fence." Keeping fences in good shape is important.
Towers are felled each year by trucks, tractors and backhoes running into guy anchors or snagging the lowest guy wire. These items need protection from such vehicles. Fencing, concrete barriers and bright markings should be used wherever vehicles or heavy equipment could be operated nearby.