For many years the FCC has required licensees with antenna towers near AM stations to protect the radiation patterns of those stations. Currently, the FCC rules apply to any tower within 3.0 kilometers of directional AM arrays, and 1.0 kilometer of non-directional towers. Affected licensees need to take measures like AM field intensity measurements and detuning to ensure minimal interactions. This may significantly impact cellular and PCS carriers, public safety, and tower companies.

Now, in an effort to apply method of moments (NEC) mathematical modeling to AM patterns, it has proposed changes in the traditional way of managing AM protection. It has proposed a more administratively convenient approach that sets frequency dependant guidelines for both spacing and height of licensee towers. It has also expanded the reach of the rules to “structures” holding antennas – buildings or water tanks, for example. It further permits the impact of these antenna structures to be gauged by NEC modeling, rather than traditional measurements.

Unfortunately, the result, like most government mash-ups, is problematical for both the AM stations and the nearby licensees! Certain structures that can seriously impact AM patterns and previously required to be studied will fall out of any regulation at all.

Antennas such as those on buildings, will now need to be studied. Where previously, height was not a criteria, many structures will need to be studied to determine their electrical height – often quite different from physical height. It will now be necessary for the AM station to hunt down reradiation from the unregulated antenna structures and negotiate or litigate solutions. Since all antenna structures will have some degree of impact on AM patterns, it also raises the specter of AM owners holding antenna owners for ransom. One member of the Washington legal community already has told us that he sees a fertile field in the potential confusion!

One potential solution is to colocate communication antennas on AM towers. AM collocation has emerged as a cost-effective way to counter tower zoning challenges.

The proposal is in FCC MM Docket 93-177 and comments are due on 23 July 2007. Check our Learning Center for a powerpoint discussion of key aspects of the proposal.

About The Author

LBA Group

LBA Group, Inc. has 50 years of experience in providing RF asset solutions and risk management for industrial and telecommunications infrastructure assets. The group is comprised of LBA Technology, a leading manufacturer and integrator of radio frequency systems, lightning protection, and EMC equipment for broadcast, industrial and government users worldwide; the professional engineering consultancy Lawrence Behr Associates, and LBA University, providing on-site and online professional training. The companies are based in Greenville, N.C., USA.

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