Prevent antenna failures

In January, this column discussed the growth of the antenna complex on the Sears Tower in Chicago. As noted in that article, the installation was successful, but problems occurred, and they were taken care of by an excellent group of contractors.

Minimizing disruption

Helicopters were used to place the two towers on the cylinders at the Sears Tower. The towers were fabricated in pieces weighing 10,000lbs or less to conform to the lifting ability of the helicopter. Then, they were picked up from a parking lot a couple of blocks from the building. The work was done early on Sunday mornings because at that time of day, the wind was calm, allowing the pieces to be swung into place with the least degree of difficulty.

It was important for the equipment to be as close as practical to the building where it was being placed. That minimized the actual time that the helicopter had to fly around carrying the load. It also minimized the amount of property that was flown over while carrying the load.

If the helicopter had gotten into trouble for some reason, it would have dropped the load. No one wants to see 10,000lbs of tower fall onto the roof of an office building. Even more so, no one wants to pay for the damage that would result from such a drop.

Other than physical damage, life safety issues for all but the workers on the project were monitored by the city. The top floors of all buildings that were under the path of the helicopter were vacated, and all streets being flown over were blocked off.

Structural analysis

Antenna manufacturers depend on mechanical engineers to do the structural analysis and design of their products. The electrical engineers determine how they want the antenna made for the desired electrical performance. That design must then be coordinated with the mechanical engineers to ensure that the completed structure will survive the anticipated loads in accordance with the existing version of the applicable standards. The standards obviously include the current edition of ANSI/EIA/TIA-222 as well as the local building department requirements.

Some large antenna companies have mechanical engineers on staff, while others hire engineering consultants to do the needed design and testing work. In the case of the two antennas going on the Sears Tower, the work was farmed out to area firms.

Supporting the structure

The supporting structure of a TV antenna can be either a lattice tower or a metal cylinder. In either case, the vertical portion of the antenna has to be connected to a ring or plate at the base, which in turn will be bolted to the tower. Many older antennas used a “bury” section to help with the mechanical transition from tower to antenna. That has largely been replaced by the use of a plate or ring with a bolt circle specified by the mechanical engineers. The ring or plate is welded to the vertical section.

The material used in welds and the amount of that material is carefully specified by mechanical engineers. For the type of project where unusual loads are involved, such as holding an antenna up by one end, the welds should be tested by an independent laboratory. The testing should include determining the depth of the welds and their quality. That testing usually includes X-ray analysis as well as the use of several different magnetic methods of evaluation.

Look at it this way: Although you may be hanging an antenna out in the country where there will be minimal damage from a structural failure, you still want to avoid that failure. Even if no one is hurt, a structural failure can bring the tower down, resulting in a lot of downtime and costs for the station.

For an antenna on a building in a major city, a structural failure could kill or injure a large number of people and open all involved to financial losses that simply are too huge to contemplate. That's why it's important to make sure that such a failure doesn't occur.

Final check

On the Sears Tower, the antennas were delivered by truck and placed on the roof of an adjacent parking garage. The final preparation for the lifts was done there, and the hardware was checked one last time. The installation company didn't think the final system complied with the original design specifications. In particular, the welds didn't seem to be as massive as anticipated. A new testing firm reviewed the steel work and determined that there were several problems with both the quantity and quality of the welds. Repairing the welds wasn't complicated because the problem was caught before the antennas were placed on the building.

Avoid harm

If you're involved with a project of this type, don't be hesitant to question any part of either the electrical or mechanical equipment construction. An old practice used to be to confirm the antenna input impedance before it was taken off the truck. That still isn't a bad idea. However, the function here isn't just to avoid problems. The overriding consideration is to avoid harm to people and damage to the station. Don't be afraid to question anything that you are not fully comfortable with. I assure you that the station management isn't going to complain that you were too careful.

Don L. Markley is president of D.L. Markley and Associates.

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