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.
These rules require anyone looking to construct or modify structures of a certain size within a certain distance of an AM station to (a) notify the AM station before construction is undertaken and (b) provide the AM licensee the opportunity to complain to the FCC about the impact of new towers. The new rules thus provide uniform codification of the historical first-come, first-served principle.
How big does the new/modified construction have to be, and how close to an AM station, to trigger the new requirements?
That depends on a number of factors, most of them determined by reference to the AM station in question.
If the AM is non-directional, any new or modified tower must be analyzed if it is 60 electrical degrees or taller and located within one wavelength of the single AM tower. If the AM is directional, the size/distance criteria are expanded to 36 electrical degrees or taller and within the lesser of 10 wavelengths or three kilometers of the center of the AM array as shown in CDBS.
Wavelength? Height in electrical degrees? No problem. The FCC provides handy formulas.
To determine an AM station’s wavelength in meters, insert the station’s frequency into this equation:
(300 meters)/(AM frequency in megahertz) = AM wavelength in meters.
(Example: if the AM station is operating on 1000 kHz – which is the equivalent of one megahertz – the calculation would be: 300 meters/1 MHz = 300 meters. Answer: the station’s wavelength is 300 meters.)
To determine a structure’s height in electrical degrees, use the following:
[(Structure height in meters)/(AM wavelength in meters)] x 360 degrees = Structure height in electrical degrees
(Example, again using a station on 1000 kHz – which, as we saw in the calculation above, has a wavelength of 300 meters – and a structure that’s 75 meters tall: (75 meters/300 meters) x 360 degrees = 90 degrees tall at 1000 kHz.)
The proponent of any construction or modification of any towers meeting the criteria described above must notify the affected AM licensee at least 30 days before commencement of construction and “examine the potential impact” on the AM pattern using the recently adopted “moment method” analysis. The proponent will be responsible for the “installation and maintenance of any detuning apparatus necessary to restore proper operation” IF that examination shows that: (a) in the case of non-directional AM’s, the tower construction/modification would distort the AM pattern by more than 2 dB; or (b) in the case of directional AM’s, the tower construction/modification would “result in radiation in excess of the AM station’s licensed standard pattern or augmented standard pattern values”.
Where an antenna or antenna-supporting structure is being installed or modified on top of a building, the FCC will evaluate the added structure alone – not the building beneath it. It has not escaped the FCC’s notice that AM pattern distortion can be caused by structures over which the FCC has no jurisdiction, like power lines, water towers, and buildings. But the Commission does control its own licensees. So the Commission has decided that, rather than claim jurisdiction over the likes of water towers, it will simply forbid any party holding an FCC license from operating from a structure which has not been subjected – by the structure’s owner or a tenant – to a compliance check for impact on AM stations. Determining compliance won’t be so simple for operators who lease a little space on a big tower full of who-knows-what kind of stuff. It looks like we may need a new clause in tower leases where the tower owner represents and warrants that the tower does not have any adverse impact on AM stations.
The new rules will be phased in, applying as soon as they are effective to new construction near pre-existing AM stations. Proponents of new or modified structures may ask for expedited action, in which case they may start construction immediately upon receiving an OK from the AM station. Emergency construction must be followed by notice to the AM station within 5 days and a promise to cooperate in resolving any problems.
If a new structure is built, a pre-existing AM station will have two years to discover problems and file a complaint if problems cannot be privately resolved. If an AM station is newly built or modified near a pre-existing tower, the AM station must take the other tower into account in adjusting its pattern. Where both the AM station and other tower were built before the new rules became effective, the AM station will have one year to file a complaint. Complaints that are currently pending will be resolved under the prior rules if possible; otherwise, the FCC may require the complaint to be dismissed and re-filed under the new rules.
Stations trying to resolve potential pattern distortion situations should remember that they may take advantage of the FCC’s moment method of predicting whether a new structure will adversely affect their pattern. That method increases reliance on computer modeling and reduces the need for signal strength measurements in the field.
The new rules include new “information collections” and will, therefore, not take effect until the Office of Management and Budget has checked them out pursuant to the Paperwork Reduction Act. Check back here for updates on that front.
(While the Commission’s recent action relates only to the protection of AM stations, we note that it ringingly affirms the first-come, first-served “newcomer” principle mentioned in our recent post about complaints of interference from spurious FM broadcast radiation to new LTE wireless services.)