Technical Note 110: 
When RF Interference Strikes

In a wireless system EMI often does not cause problems in providing quality reception but it does cause problems in providing quality service. Typically a wireless operator will raise or lower tower height, add radios or change system configurations in an attempt to eliminate service problems. These approaches do not work when EMI strikes. As the following three instances demonstrate, well-designed shielding systems provided cost-effective and lasting solutions.

GREMLINS IN A CELL SITE
A cellular provider in the Midwest encountered a hair-pulling problem. From time to time and for no apparent reason, a key cell site would go off-line. Each time it was a hard crash requiring a maintenance visit and at least an hour of downtime. The troublesome crashes continued frequently and jeopardized the customer's overall system quality perception. After investigating possible causes it was discovered that a 2-way radio repair shop was located in the same business complex. A correlation was established between tests on 47MHz State Forestry Department radios and the cellular system crashes. RF signals from those radios were strong enough to bring the cellular system to its knees when they overloaded the DC power system controllers. The solution was to install an RF shielding system around the cellular equipment room - completely eliminating the problem.

THE BUSY SIGNALS THAT WEREN'T
One Southeastern cellular system had numerous customer complaints of frequent busy signals along an important thoroughfare. Before the cellular provider added expensive channels to service the perceived demand, it accomplished an EMI audit of the site. On observation it was noted that the site transceiver status panel showed several channels were available even while a technician's handset was unable to engage a channel. When a channel was finally secured, three others were forced to drop. Tests revealed that EMI was present in the form of several volts of RF from nearby AM and FM broadcast stations. An architectural shielding system was designed and installed at a fraction of the cost of the planned new channels. After shielding, false busy signals were eliminated, dropped calls were reduced and customer satisfaction improved dramatically.

A BLOCKAGE IN THE PIPELINE
One cellular system in the Northeast was experiencing a problem using sophisticated test equipment that it attributed to EMI. A site survey revealed numerous high-power FM, TV and communications systems on the same hilltop that were responsible for intense RF fields at the cell site. An appropriately effective EMI shielding system was designed and installed. Not only did problems with test equipment disappear but later analysis showed that blocked and dropped cellular phone calls decreased by 20%. Unnoticed, the EMI also had been corrupting cell-site control systems.