The FCC released a Rule Making last week, FCC-13-115A1, that harmonizes the Rules associated with protecting AM radio stations. The effort was first initiated in 1993. Communication towers constructed in the near vicinity of an AM station could distort the radiation pattern of a directional antenna array because the tower itself absorbs and re-radiates the AM signal. When the pattern is distorted, there is a potential for interference into other AM stations as far away as hundreds of miles away.
FCC Rules have protected AM stations for some time, but these protections only applied to commercial communication carriers and other broadcasters. The new Rules now apply to all licensees, including critical infrastructure and public safety.
The new FCC Rules require Method of Moments (MoM) modeling rather than the measurement methods that have been used for decades. The MoM analysis is complex, requiring specialized software and engineering resources.The potential for interference is based on the distance from the AM station, its transmit frequency and the height of the tower. Towers that are close a quarter of the AM wavelength have the most possibility of distorting the AM pattern. The most common method for remediation is a process called “de-tuning”, where equipment is installed on the tower to make it invisible to the AM signal. 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. Transmitters may not be installed and operated until the AM station has signed off the construction.UTC will help members and clients screen their proposed construction to determine if new towers are within coordination range of AM stations.
