PUBLIC NOTICE
Federal Communications Commission
445 12th St., S.W.
Washington, D.C. 20554
News Media Information 202 / 418-0500
Internet: http://www.fcc.gov
TTY: 1-888-835-5322
DA 09-2340
October 29, 2009
MEDIA BUREAU CLARIFIES PROCEDURES FOR
AM DIRECTIONAL ANTENNA PERFORMANCE VERIFICATION
USING MOMENT METHOD MODELING
On September 26, 2008, the Commission released a Second Report and Order in MM Docket 93-
177 amending its Rules (Rules) to permit the use of computer modeling techniques, known as moment
method modeling, to verify that AM directional antennas perform as authorized.1 The new Rules, which
set forth the requirements for a moment method proof of performance, are optional for eligible AM
stations. 2
The Media Bureau has received numerous questions concerning the new procedures. To assist
future applicants, the Media Bureau has prepared this Public Notice to answer the most common
questions. The following information is intended to provide general guidance reflecting the staff’s initial
interpretation of the new Rules. It is not intended to establish binding precedent. The staff will make
specific rulings in response to actual applications on a case-by-case basis.
Types of antenna systems eligible for moment method proofs
The Rules permit use of moment method modeling only for directional antenna arrays consisting
of series-fed radiators. Directional antennas with top-loaded elements are eligible for moment method
modeling , provided the towers are fed in series. Folded unipoles and sectionalized antennas are
ineligible. In addition, directional antennas using the new proof techniques must have standard ground
systems, as defined in Section 73.189(b)(4) of the Commission’s Rules. The record supporting the new
Rules does not include any discussion of the suitability of moment method techniques for directional
antennas with non-standard ground systems, such as ground systems consisting of short, elevated radials.
We will therefore require that directional antennas with non-standard ground systems continue to use the
proof of performance Rules based on field strength measurements.
1 An Inquiry Into the Commission’s Policies and Rules Regarding AM Radio Service Directional Antenna
Performance Verification, Second Report and Order, 23 FCC Rcd 14267 (2008) (“Second Report and Order”).
2 See 47 C.F.R. § 73.151 et seq.
2
Tower location tolerance
The new Rules require a post-construction surveyor’s certification to verify that the towers in a
directional array have the proper spacing and orientation. Because some variability in tower location is
normal, we will allow a tolerance of 1.5 electrical degrees for the location of each tower in a directional
array.3 This tolerance follows the recommendation of an informal committee of members of the
Association of Federal Communications Consulting Engineers (AFCCE). The AFCCE’s committee
concluded that the recommended tolerance would allow reasonable flexibility without significantly
increasing the potential for interference. The “as built” location of each tower shall be determined with
respect to a reference tower. The same reference tower shall be used to describe the location of each of
the other towers in the array. The tolerance is represented as a circle with a radius of 1.5 electrical degrees
at the AM station’s frequency around each tower location, as specified on the construction permit. In
cases where the “as built” location differs from the authorized location by more than 1.5 electrical
degrees, applicants may submit, concurrently with the license application, a construction permit
application, FCC Form 301-AM, to change spacing and orientation, and theoretical operating parameters
as necessary, provided that no interference would result.
Exemption of formerly licensed facilities from survey requirement
We will exempt licensed stations applying to be re-licensed under the new Rules from the
requirement to submit a surveyor’s certification, provided there is no change in the authorized theoretical
pattern or patterns.
Accounting for base region effects when base sampling is used
The Rules require applicants to measure the impedance of each tower in the array with other
towers short- or open-circuited. The impedance values predicted by the moment method software must
agree, within specified limits, with the measured impedance values. In order to account for lead
inductance, base capacitance, and shunt elements such as isolation coils, the Rules permit the
incorporation of actual and assumed circuit elements between the tower base and the antenna tuning unit,
where measurements are typically performed. When base circuit elements are used in comparing
measured and calculated impedances, the model may be considered to have two parts: the moment
method model and the circuit model. Stations using base current (or voltage) sampling must take into
account both the moment method model and the circuit model when calculating operating parameters.
Agreement between calculated and measured tower base impedances
According to Section 73.151(c)(2)(ii), modeled tower impedances must agree with measured
values within +/- 2 ohms and +/- 4 percent for resistance and reactance. The following example shows
the calculation of the permissible tolerance for a measured base impedance of 20 – j32 ohms:
The tolerance for the resistance term is (2 + 0.04 x 20), or 2.80 ohms. Calculated base resistance
must therefore be within +/- 2.80 ohms of 20 ohms.
The tolerance for the reactance term is (2 + 0.04 x 32), or 3.28 ohms. Calculated base reactance
must therefore be within +/- 3.28 ohms of –j32.
3 A tolerance of 1.5 electrical degrees equals 2.3 meters at the lowest AM frequency and 0.7 meter at the highest
AM frequency.
3
Requirement to measure impedance of sampling line with sampling device connected
The matrix of measurements described in Section 73.151(c)(2)(i) includes a requirement for
impedance measurements of the sampling line at or near carrier frequency “with the sampling device
connected” (emphasis added). Some applicants have failed to submit this measurement. This particular
impedance measurement is used as a baseline for the periodic re-certification measurements set forth in
Section 73.155.
As part of the re-certification measurements, Section 73.155(a)(3) requires licensees using
sampling loops to repeat the impedance measurements of the sampling line with the loop connected, as
noted in the preceding paragraph. This section stipulates that the frequencies measured must be the same
as those used in the most recent proof, and also that the measured resistance and reactance must agree
within +/- 2 ohms and +/- 4 percent of the proof values. These same provisions will apply for applicants
using base sampling devices.
Determination of licensed parameters vs. operating tolerances
Regarding the proper adjustment of the antenna array, Section 73.151(c)(2)(ii) states: “The
antenna monitor sample indications must be initially adjusted to agree with the moment method model
within +/- 5 percent for the field ratio and +/- 3 degrees in phase.” Some applicants have misinterpreted
this statement to mean that any set of operating parameters within the specified tolerance of the moment
method-determined parameters may appear in Section III, FCC 302-AM. Applicants should specify the
same antenna monitor parameters that were determined by the moment method, as modified to account
for any base region effects. The tolerance of +/-5 percent and +/-3 degrees cited above is simply the
standard operating tolerance for an AM directional antenna, as set forth in Section 73.62. Applicants may
not apply this tolerance twice, once in selecting parameters to appear on the license and again when
comparing a set of operating parameters to the licensed values.
Modeled antenna height
The moment method Rules allow variation of the physical dimensions used in the model within
specified percentages of the actual tower dimensions. According to Section 73.151(c)(1)(v), “For
uniform cross-section towers represented by vertical wires, each wire used for a given tower shall be
between 75 to 125 percent of the physical length represented.” The same adjustment may be used for the
overall height of towers represented by more complex models.
Antenna monitor calibration
A moment method proof should include a certificate or statement of calibration verifying that the
antenna monitor is properly calibrated according to the manufacturer’s specifications, as Section 73.69(e)
requires.
Filing fees and procedures
A directional AM station may opt to file a moment method proof to cover new facilities
authorized by a construction permit, or as support for a new license without monitoring points. In either
case, commercial licensees must pay both the new license fee and the directional antenna fee.4 The fees
4 See 47 C.F.R. §§ 1.1104(2)(d) and (e).
4
are assessed because a moment method proof requires the same scope of work, regardless of whether it is
performed to cover newly constructed facilities or to issue a new license for an existing operation.5
Stations filing a moment method proof for either purpose should indicate, on Section III of FCC 302-AM,
that the purpose of the application is “Station License.” If possible, licensees should indicate on the form
that a moment method proof is being submitted. All moment method proofs will receive a file number
with the prefix “BMML.”
Licensees wishing to convert to operation authorized by a moment method proof shall request and
receive Special Temporary Authority to operate with antenna monitor parameters determined by the
moment method before readjusting the array to the new operating parameters.
For further information, contact Ann Gallagher or Susan Crawford, Audio Division, Media
Bureau, (202) 418-2700.
-FCC-
5 The informal guidance published in “Application Fee Filing Guide for Media Bureau,” available at the
Commission’s website (http://hraunfoss.fcc.gov/edocs_public/attachmatch/DOC-290290A3.pdf), will be updated to
reflect the fee requirements for moment method proofs.