The Otay Water District in southern California has 39 water tanks to serve its 200,000 customers, and 13 of them have wireless communication antennas attached.

Due to environmental and safety issues and concerns about damage to its tanks from welding and epoxy, the district now requires all new antennas mounted to its tanks to be installed using a magnetic mounting system or a nondestructive alternative. The district also upholds those standards when any antenna is modified or replaced and when a tank is painted.

“It costs half a million dollars to paint a water tank inside and out,” says Brandon DiPietro, inspector supervisor for the district. He notes that welding can damage a tank’s interior coating and that hiring a diver to repair that damage is costly.

The use of epoxy to attach antennas is also an issue. “They have to grind through the exterior paint,” says DiPietro. “You have expansion and contraction issues, and eventually the epoxy mount needs to be replaced.” Antennas mounted with epoxy have been known to fall off towers, according to Tai Irish, senior project manager at Goodman Networks, a telecommunications network services company. He described one such incident in which “Everything on the tower was on the ground. The epoxy failed.”

Goodman Networks is the vendor for AT&T Mobility in San Diego. When that company wanted to expand its coverage by attaching eight antennas to an Otay Water District tank in Jamul, Calif., Irish was charged with finding a system that met all the specifications. He recommended the Magnemount Antenna System from Metal & Cable Corp.

Noninvasive solution

The Magnemount system is a permanent, noninvasive technology for securing antennas to steel surfaces. It is designed to accommodate the curvature of water tanks. Because the system relies on magnets to attach the materials, no welding or epoxy coating is needed.

DiPietro noted that a layer of factory-installed Mylar film between the magnet and the steel tank keeps stray voltage from getting into the tank and damaging the coating system. Installation typically takes a day or two, rather than four or five days using epoxy. Antennas are attached to 12-foot masts made of anodized extruded aluminum using 300 grade stainless steel U-bolts so that rust is not a concern. 

The Magnemount system is available in five basic designs. The one selected for the Otay district project was the Side Tank Mount (STM) — all eight antennas were attached to the side of the tank near the top of the steel structure.

“Wind was the governing factor in this case,” says Al Di Donato, a structural engineer and owner of Di Donato Associates. “Antennas are like sails in the wind. We have wind gusts here up to 80 miles per hour. They can shake the entire tank.” Taking that into consideration, Di Donato recommended the contractors install three magnetic plates for each antenna rather than the customary two plates.

Handling the weight

Every 24-inch-square plate in the mounting system is secured with 24 magnets. Each magnet provides 100 pounds of vertical pull and 33 pounds of shear strength. Using three plates for each antenna provided 7,200 pounds of vertical pull and 2,400 pounds of shear strength, far more than needed to hold the 8-foot panel antennas and the remote radio units (RRUs) AT&T added to each antenna. The RRUs added 110 pounds of weight to each mast.

Bob Sabb, construction manager for AT&T in San Diego was not concerned about the added weight. “You can load these things up with weight, and we did,” he says. “It was a substantial load.” Di Donato, who hadn’t worked with the Magnemount system before, notes that the third plate may have been a bit of overkill but, “I put in a safety factor. I am confident it will hold.”

The installation proceeded without difficulty, according to Dennis Ferquez, construction supervisor for Aliantel, the firm hired to install the equipment. Using a man-lift, two workers completed the job in two days. “It was a lot quicker than using epoxy and a lot less work,” notes Ferquez. He also found that the system provides a quick and easy way to remove the antennas temporarily for maintenance or for tank painting.

Irish observes, “There was no prep involved; no grinding down, no painting.” DiPietro says the time saved by using the magnetic system instead of epoxy also saved money. “Epoxy would have taken much longer; three to five days to prep, apply and let it cure and then test it,” he says. “It just took two days to install the Magnemount system. At a cost of $5,000 to $7,000 a day, it’s a big savings. It’s cost-effective.” 

Sabb was impressed with the clean and efficient installation and especially with the system’s noninvasive character: “It doesn’t damage the water tank like welding can. You reduce the risk of damage dramatically. All it takes is one bad welder and I buy a water tank. And I am not interested in buying a water tank.”

ABOUT THE AUTHOR

David Klein is president of Metal & Cable Corp., a supplier of communication antenna components and antenna mounting systems for steel structures, including water tanks. He can be reached at david@metal-cable.com.