Technical Note 105: 
High Power RF Interference to Cellular Radio and Telephone Switch Equipment Part II

Pattern Interference from Communications
Antenna Systems to AM Broadcasting Stations

As land mobile communication systems have sprouted towers across the landscape, a seemingly unlikely conflict has arisen between radio operators and old-fashioned AM broadcast stations.

Unfortunately, typical two-way communications, microwave, paging, and cellular towers are just the right height to reradiate signals transmitted from AM broadcast stations. In effect, each tower operates like a "mini" AM station, rebroadcasting interfering signals. Unlike the signals from VHF/UHF antennas that are affected little by metallic objects several feet or more away, AM broadcast antenna signals can be affected seriously by structures as far as two miles away, particularly if the AM has a multitower, directional antenna system.

The rapid increase in the number of communications towers, coupled with a profusion of AM stations in many metropolitan areas, has produced significant conflicts between the two facilities. System planners often have ignored this interference problem, which the FCC requires them to "adjust" away on paper and prove out in the field, ascertaining the system is EMI free from then on!

Many AM station radiation patterns are controlled to within 0.5dB or less of a design pattern, which is much more precise than the control on most communications antennas. Thus, correcting distortions caused by the proximity of communication towers to AM antennas can be extremely expensive. Engineering costs ranging from $25,000 to $100,000 to tune an AM antenna system are not unusual today. Because sole responsibility for correction is on the communications licensee, there is an obvious need for AM awareness and enlightened design to avoid major FCC intervention and fiscal liability.

FCC Protection Policy
The FCC has made the protection of AM stations mandatory for all licensees and permittees planning to construct or to modify a tower within two miles of a directional AM array, or within one-half mile of a non-directional AM tower. Significantly, the FCC includes both new construction as well as modification of existing towers. Under certain circumstances, even moving or changing the configuration of transmission lines and antenna can significantly effect a nearby AM station. So, the FCC policy is broadly applicable, and must be considered whenever any tower or antenna work is done. In the case of common carriers, some very specific procedures are required by the FCC to ensure that proper steps be taken to protect the AM licensee. Since the detailed procedures required are beyond the scope of this article, the advice of qualified consulting engineers should be sought in these circumstances.

Last In Cleans Up Policy
Two important points should be noted. Because the FCC is operating under a "last in" policy, there are no grandfathering benefits. Even though a communication facility had been constructed for some years and interference is not detected until much later, the communication licensee remains fully responsible for its cleanup. On the other hand, if an AM facility is constructed or significantly modified after the communication facility is built, it is the responsibility of the AM licensee to adapt the new construction to the presence of the existing communication facility. Nevertheless, there are circumstances where the communication operator could later be liable for changes or additions to his facility. Thus, close coordination between the communication operation and the AM station is always advisable.

Solving the Problem
To avoid interference with AM stations, the location of all AM facilities should be determined in relation to all existing or planned communication tower facilities. The relative electrical and physical parameters of all these facilities are then analyzed to identify any expected interactions. This often involves the use of a sophisticated computer program that models the complete structural environment of the involved AM and communication towers.

The extent of the predicted interaction determines whether it will be necessary to treat the communication tower to make it non-radiating at AM frequencies. Where reradiation is a predicted factor, it is necessary to design appropriate isolation or detuning systems for the tower that will permit its communications transmission function while appearing "transparent" to AM broadcast signals.

Tower Detuning
The preferred technique to detune most towers uses a folded unipole system such as the DetunipoleTM manufactured by LBA Technology, Inc., Greenville, North Carolina.

As noted above, the FCC requires a series of precise actions on the part of many licensees to protect AM stations during construction and to verify that the proper tower isolation has been carried out. These are critical steps, requiring competent execution. Improperly done, it may be impossible, after construction, to prove that an AM antenna system has been restored to proper operation. This is particularly true in an environment with multiple tall structures. Without adequate baseline data, it is extremely difficult for the communication licensee to make a distinction between the influence of his antenna tower and those of other nearby structures. Obviously, do-it-yourself efforts for AM detuning lead to much grief and despair.

Solution = New Problem
Even after careful design, installation, adjustment, and operational verification of the detuning apparatus has been completed, the cellular operator’s problems may not be over.

One phenomenon has frequently been exhibited by a tower properly detuned by a folded unipole. That is, downward radiation at the frequency for which the tower is detuned is actually increased above that radiated in its detuned status. In other words, the communications operator’s diligence in alleviating interference to the radiation pattern of a neighboring AM broadcast station may create interference from the AM station to itself, where none existed before.

Interference Yardstick
Experience has shown that planewave field intensities exceeding 1 V/m are adequate to cause interference problems in computer and communications equipment. It should be noted that the FCC uses a value of 1 V/m for AM and 0.562 V/m for FM "blanketing contours," within which there is an expectancy of RF interference to third-party equipment. Thus the communications operator can create RF field intensities at someone else’s frequency, of interference-producing levels immediately below his own detuned tower in his own equipment building!

Back to Square One
RF interference to communications equipment was the subject of the first of this two-part series published in the July-September issue of NARTE News. The earlier article stressed that RF interference abatement must be considered at the very beginning of the planning process for a new communications site or for significant modifications to an existing site.

To accomplish this, particularly when AM broadcast facilities are part of the environment, a qualified broadcast technical consulting firm should be engaged to carry out the necessary research, design, detuning, and verification of communications antenna systems. The importance of such professional advice for this specialized technology from the beginning of the project cannot be over emphasized.

Reprinted from Narte News, July/September and October/December 1991 - Prepared by the late George Grills, P.E., former Vice President of Consulting at Lawrence Behr Associates, Inc.