Author: Lawrence Behr Associates, Inc.

Company: Lawrence Behr Associates, Inc.

HIGH POWER RF INTERFERENCE TO CELLULAR RADIO EQUIPMENT

Do you have trouble getting test equipment to function properly? Do you have unusual problems with power supply regulators? Do you have unwanted audio superimposed on your signals? Do you have a high rate of unexplainable hand-offs, dropped calls or other artifacts occurring from a particular cell site?

The phenomena described above are but a few of the symptoms of unwanted RF signal ingress into cellular telephone systems. These typically occur at cell or switching sites near broadcast or other high power RF installations, even several miles away.

As cellular operators expand their systems in MSA's and build new systems in RSA's, 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 which 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 - signals that can cause the kinds of problems outlined in the beginning of this application note. In some locations, ambient field intensity levels may even exceed OSHA limits and pose personnel hazards!

What is the solution to these problems? The answer is essentially to "enclose" the cellular equipment in a "box" which 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, etc. The trick is to restrict passage through these penetrations so that unwanted RF signals which enter the shielded area are sufficiently small to cause no harm to practical equipment installations. In the real world, most shielding needs can be met by innovative applications of advanced architectural shielding materials. Using such things as fiber attenuation composites, coating suspensions, and more conventional techniques, attenuations of 40 dB or more can readily be obtained in existing buildings. While not to "CIA" specifications, such attenuations will frequently be adequate to ensure no 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 just 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 upon 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 this 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 lay-outs 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 in order to establish that the design objective has been reached. This data also establishes a baseline for future maintenance checks, which should be done regularly. An interval we recommend is annually, or if unexplained equipment problems pop up. No matter how well built, there will be the invariable, unauthorized coaxial cable wall penetration, air conditioner replacement, or other change which can corrupt the shield.

Lawrence Behr Associates, Inc. has broad consulting engineering experience in solving unwanted signal ingress problems for cellular operators. Our engineers have the expertise in architectural shielding to control effectively all problem signals ranging from low frequency through UHF. We have successfully shielded cell sites and switches from high level RF interference arising from LORAN, AM and FM broadcast radio, VHF and UHF broadcast television, and VHF and UHF communications transmitters.

Our consulting services include site selection compatibility surveys to determine potential sources of high level RF signal interference, on-site signal measurements, internal and external conventional and architectural shielding of existing buildings, "high tech" shielding for new buildings to blend with and provide the finished look of attractive office decor, and verification and certification of shielding effectiveness.