Commission adopts new comprehensive standards for determining both the adverse effects of construction on nearby AM signals and who should be liable for correcting those effects.
If you’re an AM licensee looking to protect your signal from distortion caused by newly-built (or modified) structures nearby, the FCC has made your life a bit simpler.
The Commission has decided that ALL FCC-regulated services – broadcast and non-broadcast alike – will have to protect AM stations from signal distortion arising from construction or modification of nearby towers. (For purposes of the new rules, the term “tower” includes a building or any other structure on which a new or modified antenna or antenna-supporting structure is being installed.)
Why do AM’s get this special treatment?
Unlike most other radio services – in which the signal is transmitted from an antenna which is mounted on a tower or other structure – AM towers are themselves the antenna. The entire AM tower structure radiates the signal. Other nearby metallic structures can unintentionally re-radiate the signals and distort the AM station’s pattern. The problem is aggravated when the AM is directional, because directional AM stations have more than one radiating tower/antenna, with signals phased so as reinforce and/or cancel one another to create the desired directional coverage pattern. Distortion caused to the signal coming off any one of the towers will affect the overall pattern; if the interfering feature(s) – buildings, water towers, etc. – distort(s) the signals coming off more than one of the towers, the effect is exacerbated.
Fixing such distortion usually involves a process called de-tuning, where insulators are installed on one or more structures to alter the electrical height and make the structure a poor re-radiator. De-tuning is often not a cheap or easy process.
The FCC has traditionally required newcomers – i.e., anybody constructing new or modified facilities – to take corrective action for the benefit of pre-existing stations. However, the specific requirements imposed by that “newcomer” policy have varied from one service to another; and as far as protecting AM broadcasting is concerned, there are no rules at all for some services, including those covered by Parts 24 and 90.
The FCC’s new approach eliminates all that by establishing a uniform set of rules applicable to all services.