FCC Moves Forward on AM MoM Modeling

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Washington - Sep 24, 2008 - The Federal Communications Commission has issued a second report and order and a second further notice of proposed rulemaking for MM docket 93-177. In the first report and order, the FCC simplified the requirements for traditional proof of performance measurements for AM directional systems. The second report and order allows the use of computer modeling techniques to verify AM directional antenna performance.

The FCC's decision was largely based on input from the AM Directional Antenna Performance Verification Coalition, a joint partnership of 24 broadcast licensees and 10 broadcast consulting firms. The rule allows moment method modeling for a proof, but such a proof still requires field strength measurements at reference locations. According to the FCC, "The reference measurements will provide a general indication -- the only indication external to the antenna -- that the antenna continues to function properly."

The second further notice of proposed rulemaking seeks additional comment on new rules regarding tower construction near AM stations that would not depend upon the service for which a tower is used. This is a follow-up to the suggestion from the AM Directional Antenna Performance Verification Coalition. The Coalition proposal would allow licensees to verify antenna performance based on measurements of internal parameters (current or voltage and phase measured at specific locations on antenna elements), which would then be used in moment method programs in conjunction with a physical model of the directional antenna, represented with varying degrees of complexity as a series of wires, to compute the contribution of each antenna element to the directional pattern. The procedures recommended by the Coalition include the calibration of the model with measured impedances for each tower in the array. To predict the resulting directional pattern accurately, the internal array parameters that the program uses in its calculations must be carefully measured.

To this end, the Coalition’s proposal describes in detail the proper construction of the antenna sampling system that provides input to the modeling software. The Coalition recommended that the Commission permit but not require moment method proofs for antenna arrays using simple series-fed elements. AM stations using directional arrays consisting of top-loaded or sectionalized elements or folded unipoles would not be eligible to use moment method calculations in place of field strength measurements.

Comments are due 30 days after the date of publication in the Federal Register. Reply comments are sue 60 days after the date of publication in the Federal Register.

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