In-building systems present challenges and surprises
LAS VEGAS–RF Distribution Systems–the new term for in-building systems to propagate signals in areas where radio coverage is poor at best–can present a variety of challenges and surprises, according to a panel discussion at IWCE 2006.
“Buildings are often not what they seem,” said George Potter, an engineering manager at ComTech Communications. “Reinforced concrete will often pass RF. Clear glass sometimes doesn’t.”
Potter has worked on numerous in-building projects and often has been surprised at what he’s found when taking RF measurements at a job site. So-called “clear” glass in modern buildings is coated with metal oxides to reflect sunlight and/or keep in heat. Those coatings can interfere with the ability to get signals in and out of a site, he said.
On the other hand, buildings constructed with lots of steel may actually need less antennas and other equipment than similar structures without them. “Metal pans used on each floor act as waveguides,” Potter said. In addition, metal sheets used by builders to improve the strength of building floors also can provide unexpected challenges—and benefits, according to Potter. “This [building technique] isolates each floor, so you need to have an antenna on each one. It also means you don’t have interference between antennas,” he said.
Potter’s mantra for avoiding unpleasant surprises is to measure early and often. “Measure before you bid on a job. Measure before system installation. And measure before the system is turned on,” he said. “You want to make sure you don’t bite people once you turn it on.” In-building RF distribution systems not only can increase desired signals but also have the potential to deliver and amplify unwanted energies that could cause other problems. “They will bite you,” he said.
Broadband distribution systems that incorporate digital signal processors (DSPs) to filter through and amplify signals also can cause problems. “DSPs may have significant delays,” Potter said. Such delays can interfere with radio and cell phone coverage, so installers should consult the system manufacturer with information on the type of radio system to be used and other site characteristics. “Plan for the worst,” said Potter. “It doesn’t take days to take quick measurements, only an hour or two.”
In some cases, signal-boosting technology can be used creatively to avoid interference, feedback, and blockage issues. Jack Daniel Company has used RF-over-fiber technology in downtown Los Angeles to provide lower-floor building coverage for the city government, using an antenna on one high-rise, then using RF-over-fiber to move signals to a ground floor of a smaller office building. “We put a [signal booster] three blocks away using underground fiber,” said owner Jack Daniel. The top floors of the building effectively block another antenna on the high-rise next door, but placing a signal booster directly on top of the smaller building would have caused interference across the area, he said.