Gremlins In Your Cell Site? – Technical Note 103

The static and dropped calls that are haunting your customers may be coming from your cell site.

It’s late at night, and instead of sleeping you are wrestling with the myriad of inexplicable problems that plague one of your cell sites. Test equipment that worked on your bench goes haywire in the building, power supplies don’t regulate like they should, weird audio creeps in and out of your signals, and sometimes just walking in front of a rack causes switches to flip and relays to chatter, as if possessed. Could it be that mischievous little imps have invaded your equipment? No, it’s not a nightmare come true, only high-power RF interference, an albeit invisible, but controllable, menace.

Many interference problems that haunt consumers using cellular phones, or engineers and technicians charged with fixing cellular problems, are traceable to events originating not in the phone system, but at cell or switching sites located near high-power radio frequency (RF) installations several miles away, such as AM, FM and shortwave radio, TV, microwave links and more.

As cellular operators expand their systems in metropolitan statistical areas (MSAs) and build new systems in rural service areas (RSAs), they are discovering increasing difficulty in obtaining authority to build new towers where needed. Frequently, the only way to locate a cell site in a particular area is to share an existing tower that is already being used by one or more other transmitters. Alternatively, the cellular operator may be required to locate a new tower in an existing antenna "farm".

In either case, there is a distinct possibility that high RF signal levels will be present in the immediate vicinity of the cellular equipment. In some locations, ambient field intensity levels may even exceed OSHA RF safety limits and pose personnel hazards.

What can a cellular operator do to isolate and correct the basic equipment problem and to understand more about the evolution of the problem at the crowded site contributing to this situation?

The practical solution

The answer is essentially to enclose the cell-site equipment and/or switch in a “box” that blocks all RF signals, keeping all other signals out of the box and confining the cellular signals inside the box.

As a practical matter, a perfect shielding box is neither achievable nor economically desirable. There must always be penetrations of the shield for antenna transmission lines, main power, telephone lines, doors, grounding cables, air conditioners, water and sewer pipes. The trick is to restrict passage through these penetrations so that unwanted RF signals that enter the shielded area are small enough to cause no harm to practical equipment installations.

In the real world, most shielding needs can be met by innovative applications of advanced RF shielding materials. Using such things as fiber attenuation composites, coating suspensions and more conventional techniques, attenuations of 40dB or more can be readily obtained in existing buildings. While not to CIA specifications, such attenuations frequently will be adequate to prevent RF ingress into your equipment.

In addition to protecting your investment in expensive equipment, reducing maintenance costs and increasing reliability and customer satisfaction, architectural shielding can protect your employees from potentially hazardous exposure to high RF signal levels, and may make the difference in the viability of a cell site.

Control of RF ingress problems begins with site selection. A careful study, including field intensity measurements where needed, should be made to identify existing and future hazards. This can be tricky, because the “world” of high power RF is usually not familiar to site acquisition people, or to land mobile engineers. Even the acquisition of meaningful FCC information may be difficult.

Actual abatement proceeds from 1) avoidance or 2) proper design based on data taken in the site selection process. Of course, at existing sites, diagnostic measurements must be used, along with a database study to characterize the actual and future environment. From these data, shielding techniques are selected and, with the architectural or building designer, integrated into construction planning. In many cases, it may be necessary to modify standard electronics layouts to ensure that not only the present installations, but future expansion will remain RF-free.

After equipment and shielding installations, test measurements must be made at the specific “threat’’ frequencies to establish that the design objective has been reached. These data also establish a baseline for future maintenance checks, which should be done regularly. Checks are recommended annually, or whenever unexplained equipment problems pop up. No matter how well the shielding is planned and executed, there will be the invariable, unauthorized cable wall penetration, air conditioner replacement, or other change that can corrupt the shield.

Conclusion

It is never too early to plan for effective RF interference abatement. Shielding should be taken into account at the very beginning of building design. A competent consulting engineer, with experience in RF compatibility engineering, should be retained early in the process of planning a new cell site, to study the electromagnetic environment of any potential site and prevent surprises in the site selection and acquisition process, as well as provide expert guidance in achieving compatibility between the cell site and that environment. RF interference is not a malicious gremlin sent to haunt you, but a real-world problem that can be controlled through good engineering and management.