High Power RF Interference to Cellular Radio and Telephone Switch Equipment Part I – Technical Note 104

It Worked Fine When I Got It!

Many of the “interference” problems plaguing consumers using cellular telephones, or trouble-shooters called in to fix cellular problems, are traceable to events originating, not within the phone system, but at cell or switching sites located near broadcast or other high power radio frequency (RF) installations several miles away. An increasing number of switches are being located at these same electromagnetically crowded cell sites, or in other areas with high ambient RF levels generated by a large concentration of radio transmitters.

Interference Symptoms

The symptoms of these RF interference problems might first manifest themselves during operations, trouble-shooting or repair situations. For example, through:

• Test equipment malfunctions
• Unusual power supply regulator problems
• Transient, undesirable audio superimposed on signals
• A high rate of unexplainable hand-offs, dropped calls, or
• Other problems traceable to a particular cell site or switch.

How do these problems occur? Aren’t there regulations governing these installations? What can the telecommunication / EMC specialist do to isolate and correct the basic equipment problem, and to understand a little more about the evolution of the problem at the crowded cell site or switch contributing to the deficiency?

Site Deficiencies

As cellular operators expand their systems in MSA’s (Metropolitan Statistical Areas) and build new systems in RSA’s (Rural Statistical Areas), they are encountering increasing difficulty in obtaining authority to build new towers to meet increasing cellular service demands. Frequently, the only way to locate a cell site in the right geographical area is to share an existing tower already in use by one or more transmitters. Alternatively, the cellular operator may be required to locate a new tower in an existing antenna “farm.”

In either of these situations, there is a distinct possibility that high RF signal levels will be present in the immediate vicinity of cellular equipment - signals that can cause interference problems. There are even instances where ambient field intensity levels approach OSHA RF safety limits for radiation hazards to personnel.

Practical Interference Solutions

What is the solution to these problems? It isn’t as straightforward as one might think, since the electromagnetic environment (EME) at the site may be continually changing. For example, the transmitters might not always be transmitting during test measurement periods, and even after the new cellular tower or piggy-back installation has been completed, other transmitters might find their way to an incompatible site cohabitation. But there are a few basic problem prevention or resolution efforts which might be taken - assuming that you have no influence or legal basis for controlling the offending site electromagnetic environment.

One technique is to essentially enclose the cell site equipment and/or switch in a “box” designed to block as many outside RF signals as possible from entering the sensitive equipments, as well as confining cellular signals. As a practical matter, a perfectly shielded container is neither practical nor cost efficient. There must always be penetrations of the shielded box to accommodate antenna transmission lines, main power hookup, telephone lines, doors, grounding cables, air conditioners, water and sewer pipes, etc., (See Figure 1). The trick is to restrict RF passage through these penetrations so that unwanted signals which do get through are sufficiently small to prevent cellular or switch operational problems.

Site Selection

Control of RF penetration problems begins with site selection. A careful study, including field intensity measurements where needed, should be made to identify and document existing hazards, and to anticipate future problems. This can be difficult, especially if site acquisition people or land mobile engineers are unfamiliar with electro-magnetic effects. Sometimes FCC data is unavailable for the site, or if available, is out of date.

Shielding

Most shielding needs can be met by innovative applications of advanced architectural RF shielding materials . Fiber attenuation composites, coating suspensions, and more conventional techniques can yield attenuations of 40dB or more in existing buildings. While not to “CIA/NSA” standards, such attenuations will frequently be adequate for preventing undesirable penetration by electromagnetically destructive RF signals, and potential radiation hazards to personnel.

Other RF Protection

Shielding by itself won’t prevent all RF penetration problems. Other techniques might be necessary, such as the use of filters for power lines and possibly RF transmission lines. That is why it is important to accomplish a comprehensive engineering analysis from the outset of the project; to measure and determine the scope of potential RF generated problems and define various fix alternatives for ensuring complete system/environment compatibility.\

Steps For Abatement

Actual abatement of induced RF interference can be addressed during system acquisition or upgrade by (1) avoidance or (2) design improvements based on a complete collection and analysis of data gathered during the preliminary site selection process. Additional diagnostic measurements will be necessary when using an existing site.

Preliminary Analysis

A database study should be incorporated into initial or site improvement analyses, using proven electromagnetic parametric and geographic data and models such as are maintained by consulting firms specializing in this area. They maintain the FCC and other databases and typically support area site selection and RF interference analysis such as:

• Defining regions where high-power fields exceed established radiation hazard criteria and provide information for inclusion in environmental impact statements.
• Defining constraints due to EMC considerations to support the site-selection process.
• Comparing the coverage of equipment located at one site with the coverage at another candidate site to ensure an optimum location.
• Identifying systems in the environment that represent potential sources or victims of interference.

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 lay-outs to ensure that existing installations and future site expansion will remain RF interference resistant.

Test Measurements

Test measurements must be built into the overall design process to include baseline measurements at the suspected “threat” frequencies after the new equipment and shielding installations are in place. These data carry forward a baseline to support future preventative maintenance, which should be done on a regular basis, and problem trouble-shooting. No matter how well the equipment installation and site selection process is planned and executed, there will be the invariable, “unauthorized” cable wall penetration, air conditioner replacement, or other change which can corrupt the shield or other EMI preventive procedure.

Conclusion

It is never too early to plan for effective RF interference abatement. Shielding and other RF interference preventive actions must be taken into account at the very beginning of building design, equipment selection/upgrade or site selection process. A competent consulting organization or engineer, qualified and experienced in RF compatibility engineering, should be retained early in the design/planning process. A thorough study and analysis of the electromagnetic environment of any potential cell site will prevent expensive surprises in the final product - compatibility between the cell or switch site and the attendant environments.