FCC Docket MM93-177 sets out to adjust FCC rules regarding the adjustment and measurement of AM directional antenna arrays. It also seeks to modify the manner in which the reradiation impact of nearby structures is considered. Today, all towers within a fixed distance of AM antenna systems must be evaluated. Under the new proposals, only those structures exceeding a three-dimensional test must be considered. The test consists of a frequency dependent height, along with a frequency dependent spacing distance. These limits are different for directional and non-directional antenna systems.
Understandably, some confusion has arisen among AM broadcasters and tower owner/users over the practical impact of operations complying with the three-dimensional test.
Using NEC (Numerical Electromagnetic Code) modeling of some typical antenna towers at typical distances from a hypothetical AM antenna site, LBA engineers Kathryn Tesh and Marvin Brewer have provided some interesting results as a point of reference in considering these impacts. These results suggest that using the FCC proposed method will not necessarily protect either AM stations or tower users against unwanted pattern impacts.
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