EMI and Effective RF Shielding
In a wireless system Electromagnetic Interference (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 ratios 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 the 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,
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.
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