(a) Proponents of construction or significant modification of a tower which is within one wavelength of a nondirectional AM station, and is taller than 60 electrical degrees at the AM frequency, must notify the AM station at least 30 days in advance of the commencement of construction. The proponent shall examine the potential impact of the construction or modification as described in paragraph (c) of this section. If the construction or modification would distort the radiation pattern by more than 2 dB, the proponent shall be responsible for the installation and maintenance of any detuning apparatus necessary to restore proper operation of the nondirectional antenna.
(b) Proponents of construction or significant modification of a tower which is within the lesser of 10 wavelengths or 3 kilometers of a directional AM station, and is taller than 36 electrical degrees at the AM frequency, must notify the AM station at least 30 days in advance of the commencement of construction. The proponent shall examine the potential impact of the construction or modification as described in paragraph (c) of this section. If the construction or modification would result in radiation in excess of the AM station's licensed standard pattern or augmented standard pattern values, the proponent shall be responsible for the installation and maintenance of any detuning apparatus necessary to restore proper operation of the directional antenna.
(c) Proponents of construction or significant modification of a tower within the distances defined in paragraphs (a) and (b) of this section of an AM station shall examine the potential effects thereof using a moment method analysis. The moment method analysis shall consist of a model of the AM antenna together with the potential re-radiating tower in a lossless environment. The model shall employ the methodology specified in Sec. 73.151(c) of this chapter, except that the AM antenna elements may be modeled as a series of thin wires driven to produce the required radiation pattern, without any requirement for measurement of tower impedances.
(d) A significant modification of a tower in the immediate vicinity of an AM station is defined as follows:
(1) Any change that would alter the tower's physical height by 5 electrical degrees or more at the AM frequency; or
(2) The addition or replacement of one or more antennas or transmission lines on a tower that has been detuned or base-insulated.
(e) The addition or modification of an antenna or antenna-supporting structure on a building shall be considered a construction or modification subject to the analysis and notice requirements of this subpart if and only if the height of the antenna-supporting structure alone exceeds the thresholds in paragraphs (a) and (b) of this section.
(f) With respect to an AM station that was authorized pursuant to a directional proof of performance based on field strength measurements, the proponent of the tower construction or modification may, in lieu of the study described in paragraph (c) of this section, demonstrate through measurements taken before and after construction that field strength values at the monitoring points do not exceed the licensed values. In the event that the pre-construction monitoring point values exceed the licensed values, the proponent may demonstrate that post-construction monitoring point values do not exceed the pre-construction values. Alternatively, the AM station may file for authority to increase the relevant monitoring-point value after performing a partial proof of performance in accordance with Sec. 73.154 to establish that the licensed radiation limit on the applicable radial is not exceeded.
(g) Tower construction or modification that falls outside the criteria described in the preceding paragraphs is presumed to have no significant effect on an AM station. In some instances, however, an AM station may be affected by tower construction or modification notwithstanding the criteria set forth above. In such cases, an AM station may submit a showing that its operation has been affected by tower construction or modification. Such a showing shall consist of either a moment method analysis as described in paragraph (c) of this section, or of field strength measurements. The showing shall be provided to:
(1) The tower proponent if the showing relates to a tower that has not yet been constructed or modified and otherwise to the current tower owner; and
(2) To the Commission, within two years after the date of completion of the tower construction or modification. If necessary, the Commission shall direct the tower proponent or tower owner, if the tower proponent or tower owner holds a Commission authorization, to install and maintain any detuning apparatus necessary to restore proper operation of the AM antenna. An applicant for a Commission authorization may not propose, and a party holding a Commission authorization may not locate, an antenna on any tower or support structure that has been shown to affect an AM station's operation pursuant to this subparagraph, or for which a disputed showing of effect on an AM station's operation is pending, unless the applicant, party, or tower owner notifies the AM station and takes appropriate action to correct the disturbance to the AM pattern.
(h) An AM station may submit a showing that its operation has been affected by tower construction or modification that was commenced or completed prior to or on the effective date of the rules adopted in this Part pursuant to MM Docket No. 93-177. Such a showing shall consist of either a moment method analysis as described in paragraph (c) of this section, or of field strength measurements. The showing shall be provided to the current tower owner and the Commission within one year of the effective date of the rules adopted in this Part pursuant to MM Docket No. 93-177. If necessary, the Commission shall direct the tower owner, if the tower owner holds a Commission authorization, to install and maintain any detuning apparatus necessary to restore proper operation of the AM antenna.
(i) An applicant for a Commission authorization may not propose, and a party holding a Commission authorization may not locate, an antenna on any tower or support structure, whether constructed before or after December 5, 2013, that meets the criteria in paragraphs (a) and (b) of this section, unless the analysis and notice process described in this subpart, and any necessary measures to correct disturbances of the AM radiation pattern, have been completed by the tower owner, the party proposing to locate the antenna, or any other party, either prior to construction or at any other time prior to the proposal or antenna location. [78 FR 66295, Nov. 5, 2013]