AM Station Interference Screening & Detuning

Knowing whether a site requires action to comply with the FCC rules for protecting AM broadcast station patterns is an important factor in planning your wireless build-out. The FCC mandates that any public mobile service licensee that constructs or modifies tower in the immediate vicinity of any AM broadcast station is responsible for measures necessary to correct disturbances to the AM station's antenna pattern.

The licensee must: 

  • Notify the AM broadcast station in advance of planned construction or modification
  • Perform field measurements to document due diligence
  • Install any required detuning apparatus necessary to restore proper performance of the AM station array
  • Provide for continued maintenance and operation of the detuning apparatus

 

Through the use of Waterford's advanced, proprietary screening system offered through the Waterford website, our team of highly reputed engineers and technical personnel routinely deliver expertise on the most complex and high profile AM detuning projects and are able to assist you with efficiently evaluating and insuring your sites' compliance needs.

Waterford provides its customers with their own login credentials, which in turn, allow them to track the progress of their sites online through and effective and timely project management system. This includes the delivery of "Negative Certificates" and proposals (if needed), as well as the ability to freely monitor the status of the work and any documents that were created during the project's execution. The system can also send notification emails to whomever the client specifies when there is a change in job status.

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Waterford offers expertise in all AM compliance solutions including traditional "pre and post-construction measurements" and "Moment Method Modeling (MOM)". The critical new ruling under FCC 47 CRF 1.3000 mandates specific requirements for anyone that owns towers or is performing tower work. Waterford continues to lead the way as the premier expert in this new ruling and welcomes any questions, inquiries or requests for clarification on the compliance criteria or the new "MOM" analysis procedures.

MOMENT METHOD MODELING (MOM)

In light of the recent FCC rule change governing compliance of wireless telecommunication sites new AM broadcast stations, a “MOM” analysis is the newer of two acceptable methods of determining the impact of a structure to an AM station radiation pattern or patterns.

Unlike a traditional AM compliance study that involves physical measurement of the station’s monitor points before and after site construction / antenna installation (aka “Pre / Post Construction Measurements”), a “MOM” analysis is performed using an AM directional antenna Moment Method Modeling program based on the MININEC3 core. The desktop analysis is performed generating two data sets. The first data set contains the station pattern resulting using the station licensed parameters as extracted from the FCC CDBS database including augmentations. The result is a calculated field strength value for each of 360 degrees from the station (expressed in mV/m @ 1 km) from the station reference coordinates. This data is the baseline or unmodified station pattern.

The second generated data set results from the introduction of the Subject structure into the baseline pattern environment. Tabulated data are incorporated into the report by attachment. A comparison of the two data set columns ("Pattern" and "Subject Structure") calculates the extent of deviation from the station baseline plot. The difference between the station standard and the modified pattern field strengths are compared and tabulated as the difference columns expressed in mV/m and calculated in dB. All positive deviations from the station pattern are highlighted in red and thus the maximum deviation is determined. The two patterns are then overlaid to graphically depict pattern impact.

BULK SCREENINGS

Customers who wish to save time by avoiding the data entry associated with a large quantity of sites can have a “bulk screening” completed at no charge by clicking here or by contacting your Waterford representative.

Q & A - AM SCREENING & DETUNING REQUIREMENTS
Why is screening required?

Industry has long since realized that the frequency band transmitted (PCS, LTE etc) does not matter, it’s the structure itself and its impact on nearby AM stations’ broadcast patterns that are the issue. Rule 47 CFR § 1.30000 applies to all licensees (carriers) who constructs, collocates or modifies equipment on a structure within the vicinity of AM broadcast stations.

  • The FCC has no jurisdiction over the tower owner but does over the carrier (via the FCC license). As the carrier has a license in jeopardy, they are vitally concerned about compliance with 47 CFR § 1.30000. The FCC has the authority to cancel a licensee's authorization to operate as well as fine them. Without the licensee (their customer), there is no business for the tower owner, so they should be interested.
  • Carriers are thoroughly familiar with the rule, compliance requirements, remedial actions (detuning) and the short/long term costs. They are unwilling to assume the potential cost/liability of compliance and often require the tower itself to be compliant before even considering locating their site on the tower.
What are the distances from an AM station used for determining when an AM study is required?

For a non-broadcasting tower, the distances are variable; equal to 10λ (the wavelength) of the frequency of a directional AM station and 1λ of the frequency of a non-directional station.

What is ‘‘MOM’’ evaluation and when is it used?
  • The Moment of Method (MOM) analysis is a desktop computer study of the impact caused to a broadcast pattern and consists of two steps. First, from the FCC data, a computer model of the station’s antenna(s) and operating parameter(s) is created in free space. Second, the new or modified structure is inserted into a free space model. A comparison of the two model’s results demonstrates the impact to the FCC licensed pattern caused by the new or modified structure.
  • MOM analysis is appropriate to determine if a new or modified tower (where a height change exceeding five electrical degrees is involved) will result in the need for detuning. It is not appropriate to determine if an existing structure with detuning equipment is properly detuned; this requires field adjustment and measurements. Waterford Consultants will recommend MOM analysis or field actions as appropriate to each case.
What is the primary objective of ‘’FCC Research/Maps & Licenses’’?

In order to perform measurements of the AM station’s pattern, we have to verify the FCC license for the station. It contains the pertinent data on the station and specific measurement locations (ie. FCC licensed Monitor Points). The maps are prepared in advance detailing the area where the measurements are performed. These are later annotated to reflect the exact position where the measurements were actually made. These maps are then used to insure that the pre- & post-construction measurements were completed at exactly the same location; this insures that an outside error is notintroduced into the comparison measurements. A displacement of as little as 5’ can make a difference in the measurement values.

When pre-construction measurments are required, what are we actually measuring?

All measurements are made to document the shape of the station’s pattern. Think of each set of measurements as a “snapshot” of what the pattern looks like today. Our evaluation typically consists of 2 such snapshots; one just prior to the construction/modification and another as soon as possible after the construction/modification is completed. We compare the 2 sets of measurements and any difference is attributed to our client’s construction/modification. This procedure also protects the client. For a variety of reasons, the station’s pattern could already be out of “licensed limits” before construction/modification starts. The measurements are designed to determine if (regardless of current compliance condition) the shape station’s pattern is changed by the construction/modification, while at the same time insuring that the client is not responsible for the station being out of compliance to start with. This is not possible with post-construction only measurement values compared to FCC licensed limits.

What is the unit of measure criteria involved that will ultimately lead to a determination of compliance?

The station’s signal strength is measured in milivolts per meter (mV/m) at specific locations relative to the station. The concept is for the comparison measurement values to vary by no more than ±5%. This number is dictated by the calibration accuracy of the measurement device. Any more than ±5% becomes quantifiable and documented as pattern disturbance.

How long are pre-measurments good for?
  • Waterford recommends completing post-construction measurements within 90-days of pre-construction baseline measurements because AM field strengths demonstrate significant variation with changes in ambient temperature and ground conductivity at the time of measurement. This will minimize the possibility of environmental factors distorting the measurement comparison and falsely indicate a need for detuning, as we have observed significant variation in temperature and precipitation patterns from one season to the next in all areas of the United States.
  • An additional factor that can impact the measurement results is other civil or carrier related construction activity taking place near the station which can/will impact the RF environment at the station frequency. Again, these actions can distort the measurement comparison and falsely indicate a need for detuning.
Are post-construction measurements measured for day and night differences?

AM stations operate in two distinct propagation conditions. This involves the physics of layers in the upper atmosphere and how they react to ionization and thermals produced by the sun’s energy. Thus stations frequently have 2 configurations. This equates to one set of stations that they could potentially interfere with during daylight hours and another set of stations (much further away) during the nighttime hours. Consequently they can have completely different patterns for day time and night time operations. Both patterns require measurements and the FCC licensed monitor points are necessarily at different locations for each of the two patterns and have different limits. All measurements are made on the station’s frequency, and at locations designed to document the shape of their pattern(s) and detect any changes to it (them).

If data analysis result in conclusive evidence, what recommendation Waterford will make?

We typically recommend detuning the structure. This is the long standing AM industry recognized solution to pattern disturbance. Detuning effectively removes the possibility that re-radiated energy from the tower is causing pattern disturbance.

What are the risks the tower owner/carrier/contractor face as a result of non-compliance?

The result of non-compliance can be a complaint filed with the FCC by the impacted AM station. The possible consequences are to the licensed carrier are:

  • A FCC levied fine for failure to comply with the 47 CFR § 1.30000 rule
  • Loss of license for the carrier
  • FCC imposed conditions on future and continued operations (when willful/repeated violations of the rule are found)