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The Federal Communications Commission (FCC) has issued new rules in a Report and Order with the intent of providing AM radio stations better protections from potential signal pattern distortion created by nearby communications towers and other structures. The uniform rules would apply to all services and the rules also recommend the use of moment method modeling to assess the effects of construction of a new tower or significant modifications on towers near AM stations.

The nature of the AM signal and the means by which it is transmitted requires measures to be taken to protect the AM signal from potential signal pattern distortion created by nearby towers. The AM tower itself is the antenna.  The tower structure radiates the signal. This makes AM signal patterns vulnerable to distortion from other nearby towers and structures that can act as unintentional re-radiators of the AM signal. The relationship between the height of communications towers and the wavelength of the AM signal can distort an AM station’s signal pattern.

The radiation pattern of an AM station is authorized and licensed by the FCC after careful consideration of the location and patterns of other AM stations at or near the same frequency. The impact of a distorted pattern can negatively impact other stations far away. Distortion can reduce the market coverage area of a station, thus affecting its revenue earning capabilities.

The rules outlined in Report and Order FCC 13-115 would first require a party proposing to construct a new tower or significantly modify an existing tower, within the FCC mandated critical distance, to provide notice to the AM station at least 30 days prior to the planned commencement of construction. These rules would apply to all towers, no matter the service type housed on it. Moment method modeling would also be required to determining whether the nearby tower would have a negative impact to the AM station’s pattern.

Presumptive AM Impact Parameters - Federal Communications Commission 13-115

Critical distance and tower height parameters

What is the FCC mandated critical distance?

What is commonly called an AM study would be required for a nondirectional AM station if a communications tower that is going to be constructed or significantly modified is 60 electrical degrees or taller and is located within one wavelength of the AM tower.  When looking at a directional AM station, an AM study would be required if the subject tower is 36 electrical degrees or taller and within the lesser of 10 wavelengths or three kilometers of the center of the AM station’s multi-tower array.

As outlined here, critical distances from an AM station are described in terms of the AM wavelength. The AM wavelength, expressed in meters, is computed as follows:

(300 meters)/(AM frequency in megahertz)=AM wavelength in meters.

For example, at the AM frequency of 1000 kHz, or 1 MHz, the wavelength is (300/1 MHz) = 300 meters.

Electrical degrees at the AM frequency is the height of a proposed tower as a function of the frequency of a nearby AM station. Tower height in electrical degrees is computed as follows:

[(Tower height in meters)/AM wavelength in meters)] x 360 degrees = Tower height in electrical degrees.

Using the original example of an AM station at a frequency of 1000 kHz, its wavelength is 300 meters. Let’s suppose that there is a nearby tower where significant modifications are proposed.  That tower is 75 meters tall. The calculations would be as follows:

[75/300] x 360 = 90 electrical degrees tall at the AM frequency

Again, based on the new rules, the criteria where a responsible party would be required to show that a new tower or significantly modified tower would not negatively impact an AM station are based on the critical distance outlined by the rule.  The critical distance for nondirectional AM stations is one wavelength at the frequency of the AM station. The critical distance for directional AMs is any distance less than ten wavelengths of the frequency of the AM station up to a maximum distance of three kilometers.

What defines a significant modification to a tower?

“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,” according to the Report and Order FCC 13-115.

What must the party do that is proposing construction of a new tower or significant modification of an existing tower that is within the FCC critical distance of a nondirectional AM station? (Note: The FCC identifies the party proposing the tower construction or significant modification as the “proponent”.

  1. Notify the AM station at least 30 days in advance of the commencement of construction.
  2. Examine the potential impact of the construction or modification. This likely means ordering a moment method modeling of the site.
  3. If the construction or modification would distort the radiation pattern of the nondirectional AM station by more than 2 dB, the proponent is responsible for the installation and maintenance of a detuning system to restore proper operation of the nondirectional AM station.

What must the party do that is proposing the tower construction or significant modification of an existing tower that is within the FCC critical distance of a directional AM station?

  1. Notify the AM station at least 30 days in advance of the commencement of construction.
  2. Examine the potential impact of the construction or modification. This likely means ordering a moment method modeling of the site.
  3. 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 would be responsible for the installation and maintenance of a detuning system necessary to restore proper operation of the directional AM station.

What must be done by the party that is proposing an AM collocation installation or modifications on a nondirectional AM tower?

  1. Notify the AM station.
  2. Following installation on the nondirectional AM tower, antenna impedance measurements on the AM antenna must be made.
  3. If the resistance of the AM antenna changes by more than 2 percent, an application on FCC Form 302-AM (including a tower sketch of the installation) must be filed with the Commission for the AM station to return to direct power measurement.

What must be done by the party that is proposing an AM collocation installation or modifications on a directional AM tower?

  1. Notify the AM station.
  2. For AM stations licensed using field strength measurements, a partial proof of performance must be conducted by the tower proponent both before and after construction to establish that the AM array will not be and has not been adversely affected.
  3. If the operating parameters of the AM array change following the installation, the results of the partial proof of performance must be made available to the made available to the AM station so it can be filed by the AM station with the Commission on Form 302-AM.
  4. For AM stations licensed via a moment method proof, a base impedance measurement on the tower being modified must be made by the tower proponent.
  5. If the new measured base resistance and reactance values of the affected tower differ by more than +/- 2 ohms and +/- 4 percent from the corresponding modeled resistance and reactance values contained in the last moment method proof, then the station must file Form 302-AM. The Form 302-AM must be accompanied by the new impedance measurements for the modified tower and a new moment method model for each pattern in which the tower is a radiating element. Base impedance measurements for other towers in the array, sampling system measurements, and reference field strength measurements need not be repeated.

What is required in the notification sent to an AM station at least 30 days in advance of the commencement of construction?

  1. Proponent’s name and address.
  2. Coordinates of the tower to be constructed or modified.
  3. Physical description of the planned structure.
  4. Results of the analysis showing the predicted effect on the AM pattern, if performed.

As a catch all, the Commission acknowledges that there could be some instances where an AM station may be affected by tower construction or modification even though the station falls outside the critical distance or other criteria that would otherwise require action by a tower proponent.

What can an AM station do if it falls outside the critical distance requirement but feels a nearby tower is impacting its operation?

  1. Submit a showing that its operation has been affected by tower construction or modification. This can include either a moment method analysis, or field strength measurements.
  2. “The showing shall be provided to 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 to the Commission, within two years after the date of completion of the tower construction or modification,” according to the FCC Report and Order.
  3. The Commission could direct the tower proponent or tower owner, if the tower proponent or tower owner holds a Commission authorization, to install and maintain a detuning system necessary to restore proper operation of the AM antenna.
Detuning system on a cell tower near an AM array

Detuning system on a cell tower near an AM array

What is AM detuning?

The process of AM detuning mainly involves the installation and adjustment of hardware to cancel AM radiation from the subject tower. Typically, the AM detuning system hardware consists of a network of wires about a tower, often from near the top to near the bottom of the tower.  The system also includes a tuning box with components to resonate the system to the AM frequency. While deceptively simple, the devil is in the details, and almost any disturbance to the mechanical or electrical configuration of the system can compromise the AM detuning effects.

A detuning system effectively reduces the electrical height of a tower on which it is installed.  This reduces or eliminates the potential for the tower to re-radiate the signal of a nearby AM station and thus distort its signal pattern.

There is no required inspection period for AM detuning systems in the FCC rules; however, these new rules restate the need for maintaining detuning systems.  It is generally thought that a two year inspection and recalibration interval is appropriate, and LBA concurs with that. Of course, a special inspection would be appropriate if irregularities are noted in between times, major tower modification or antenna construction is undertaken, heavy ice or wind occurs, or the site is vandalized.

When the AM detuning system is installed, it is too often a “set it and forget it” procedure. Left undisturbed, that initial setting is possibly good for many years. Unfortunately, that happy state is rarely achieved on a communications tower.  By restating the need for detuning maintenance in this latest Report and Order, the Commission is emphasizing that this needs to be a regular and continuing responsibility of the owner.

Wireless professionals are well versed in cellular systems; however AM technology concepts are often not included in their education or experience. Thus, it’s easy to overlook detuning maintenance. AM detuning systems are entirely passive, have no failure bells and whistles, and their failure in no way affects the wireless carrier operation. The systems are just some more wires and cables among many on a tower.

LBA Group, Inc. is celebrating 50 years of providing RF asset solutions and risk management for industrial and telecommunications infrastructure. The company’s professional technical consultancy, Lawrence Behr Associates, Inc., specializes in AM protection and its manufacturing division has been producing high quality detuning systems for decades.  The company is well versed in the new rules outlined in this article and is prepared to continue offering services to meet the requirements of FCC 13-115 once implemented by the Commission.

Lawrence Behr Associates, Inc. would be happy to answer any questions related to AM protection and the new rules introduced by FCC 13-115.  Contact Mike Britner, 252-752-0279 or mike.britner@lbagroup.com.

Keep up with the LBA companies on Facebook at: www.facebook.com/LBAGroup.


 

David HornDavid Horn is an award-winning business and marketing development specialist with LBA Group, Inc. He helps some of the largest companies in the country implement regulatory compliance programs. LBA also utilizes his decades of experience in communications and new media to supplement the global marketing initiatives of the company. He specializes in turning complex topics into informative and entertaining stories.

 

 

3 Comments on "New FCC Communications Tower AM Protection Rules Explained"

  1. Steven Wendt September 6, 2013 at 11:01 am · Reply

    From what i can read this will not affect 99% of hams. Now the cell phone companies well that is a different story. For me i am not concerned.

    Steve
    KB9RDS
    US Navy (Ret)

    Like or Dislike: Thumb up 0 Thumb down 0

  2. Lawrence Behr September 6, 2013 at 11:01 am · Reply

    Hi Steve,You are correct that this will affect few individuals since very few hams have towers of sufficient height. However, all hams with towers should conduct the required evaluation. For instance, a 75′ tower within about a mile of an AM DA at, say 1500 kHz, would need to be detuned. A greater possibility relates to repeaters. Traditionally, many repeaters have been hosted on communications towers. Many of these licensees have disregarded earlier FCC AM protection policies. Now compliance will be mandatory by all licensees, even hams. Repeater operators need to bring this to the attention of the host tower owner.LawrenceK4JRZ

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