Author: Lawrence Behr Associates, Inc.
Company: Lawrence Behr Associates, Inc.
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Many of the "interference" problems plaguing communicators using
cellular, PCS, microwave, satellite or other systems are traceable to
events originating not within the communicator's facility, but from
proximity to broadcast or other high power RF installations. An
increasing number of communications facilities also are being located
at electromagnetically crowded sites, with high ambient RF levels
generated by a large concentration of lower power radio transmitters. It
has been projected that some 100,000 new transmitting and receiving
installations will be required in the next few years to satisfy demand for
new wireless services. Pressure on site availability is forcing the
collocation of many services, and electromagnetic interference (EMI)
problems are ubiquitous. Unless properly anticipated and dealt with,
these costly problems can seriously hinder wireless system
deployment and operation. THE RF THREAT Although EMI may manifest itself in spectacular fashion, such as crashing whole systems, its touch is usually more subtle. Victims often endure significant system degradation before realizing that a problem is at hand. This often occurs during operational, troubleshooting or repair events. Typical signs of EMI problems include: |
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Many wireless facilities have traditionally been constructed at well-located sites, frequently high-power FM and TV broadcast antennas. It is not uncommon for FM signals to exceed 100,000W or for TV signals to exceed 5,000,000W.
In such situations, there is a real threat that the high RF signal levels will cause brute force interference problems throughout the wireless transmitting or support equipment. The potential for receiver overload, desensitization, harmonic reception and intermodulation interference are considerable. There are even instances where ambient field intensity levels approach established limits for radiation hazards to personnel, even within shelters and buildings.
Sites located near AM broadcast stations that have effective radiated powers up to 250,000W may be subjected to intense electrical and magnetic fields, which couple into equipment through coaxial cables, telco lines, and grounding and power lines. Fundamental and harmonic frequencies of AM stations and their modulation components overlay common audio, data, T-carrier and clock spectra. The potential for system disturbance is enormous. Towers commonly associated with wireless sites actually "concentrate" the AM broadcast signal to local values far in excess of free-space predictions.
Even at sites not immediately adjacent to broadcasters, it is commonly misjudged what significant EMI potentials exist. Although wireless is not generally thought of as "high power," the concentration of channels at a site may rival the potency of a TV station. For example, 32 PCS channels at FCC maximum power may radiate about 50,000W. A group of eight paging channels may aggregate more than 25,000W. A wireless cable installation with 32 channels may contribute more than 200,000W in the 2GHz band.
At these power densities, not only must the RF engineer be concerned with EMI mentalities, such as intermodulation and desensitization, but also personnel RF hazards and direct equipment signal ingress. Perhaps recognizing this potential bath of RF, the FCC now requires every PCS transmitting installation to be studied and certified free of RF hazards.
EMI from intentional RF emitters is by no means the whole of the problem. On the other end of the scale, as Pogo once said, "We have met the enemy, and he is us." The mass conversion of radio site equipment to digital architectures has led to a site environment rich in incidentally radiated signals. As digital signals tend to be square waves, with rich harmonic spectra, and as clock speeds converge on the VHF range, wireless operators are increasingly finding that they are generating EMI energy combs right through their own-and their neighbor's-receive allocations. This problem is further exacerbated as receive antennas are lowered. Even digital equipment that meets FCC Class "A" specifications can interfere with receivers several hundred feet away, if not appropriately shielded.
PRACTICAL EMI MANAGEMENT
Managing EMI containment and resolution challenges isn't as straightforward as you might think. Action
should start with a clear corporate policy to effect electromagnetic compatibility procedures in the site
development process (often a critically missing factor). Just as loss of revenue from an interference-riddled
system is a corporate concern, so also must be the pro-active programming for an interference-free system.
One of the most frustrating aspects is to attempt to define, much less maintain, a steady-state RF situation at any site. In fact, the electromagnetic environment at even the best controlled site is continually changing. Most wireless sources are by nature "bursty" and unpredictable in their operating cycle. Even broadcast stations change modes of operation and observe irregular operating schedules.
New transmitters, operating channels and emission modes may be established on or near the site and be incompatible with the other existing systems. Identification of new entrants is further complicated by the FCC's market licensing policies; consequently, no record may exist of all exact transmitter locations.
Space does not permit an exhaustive discussion of resolution techniques for such better-known problems as intermodulation and desensitization. However, there are some fundamental approaches that can help contain or resolve lesser-appreciated equipment EMI ingress challenges:
Generally, many potential troublemakers can be identified early in the site selection process so that appropriate protection may be designed efficiently into the new system. A careful study, including field intensity measurements where needed, should be made to identify and document existing hazards, and to anticipate future problems. (Existing diagnostic measures may be needed for preparing an existing site.) Such studies should employ proven electromagnetic parametric and geographic data and models such as those maintained by consulting companies specializing in this area.
| A careful study, including field intensity measurements where needed, should be made to identify and document existing hazards |  |
Most wireless industry shielding needs can be met by innovative applications of advanced architectural shielding materials. Fiber attenuation composites, coating suspensions and more traditional techniques can yield attenuations of 40dB or more in existing buildings. While not to "military secret" standards, such attenuations will frequently be adequate for preventing undesirable penetration by electromagnetically destructive RF signals, and potential radiation hazards to personnel in the site equipment buildings.
In addition to creating the shielded "box," other techniques are necessary to properly treat the penetrations, such as the use of filters for power lines and RF transmission lines, as well as blocking the offending signals from door, HVAC and other openings. Careful determination of how best to meet this challenge should be exercised for each site in question. Importantly, accomplishment of a comprehensive engineering analysis from the outset of the project will measure and determine the scope of potential RF-generated problems and define various fix alternatives for ensuring successful system/environment compatibility.
Based on these and similar supporting data, shielding and other RF protection techniques can be selected to complement and influence architectural and building construction planning. In some cases, it may be necessary to modify standard electronics layouts to ensure that existing installations and future site expansion will remain RF interference resistant.
Site operators must recognize that shielding "systems" combat RF ingress to their equipment, and
this system is as integral to the site's effective operations as the HVAC or power supply systems.
Further, they must recognize that their shielding system must be cared for. Too often, the
out-of-sight, out-of-mind attitude regarding shielding components allows for future violation of the
shielding system's effectiveness.
CONCLUSION
When dealing with EMI, it's a pay-me-now or pay-me-later world, and the latter option almost always is
more costly. It is never too early to plan for effective RF interference abatement. Early analysis of the RF
environment, shielding and other RF interference preventive actions must be taken into account at the
beginning of the site selection, building design or equipment selection/upgrade process. It is important to
realize that abatement is a life cycle requirement for a wireless site, demanding vigilant attention to
ever-changing site EMI conditions that can degrade RF or switch system performance.
A competent consultant, qualified and experienced in EMI compatibility engineering, should be retained
early in the design/planning process and maintained through the operational phase. An analysis of the
electromagnetic environment of any potential wireless site will prevent expensive surprises in the final
product-and ensure compatibility between the site and the attendant environments.
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 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.