§  73.151   Field strength measurements to establish performance of directional
antennas.

   The performance of a directional antenna may be verified either by
   field strength measurement or by computer modeling and sampling system
   verification.

   (a) In addition to the information required by the license application
   form, the following showing must be submitted to establish, for each
   mode of directional operation, that the effective measured field
   strength (RMS) at 1 kilometer (km) is not less than 85 percent of the
   effective measured field strength (RMS) specified for the standard
   radiation pattern, or less than that specified in § 73.189(b) for the
   class of station involved, whichever is the higher value, and that the
   measured field strength at 1 km in any direction does not exceed the
   field shown in that direction on the standard radiation pattern for
   that mode of directional operation:

   (1) A tabulation of inverse field strengths in the horizontal plane at
   1 km, as determined from field strength measurements taken and analyzed
   in accordance with § 73.186, and a statement of the effective measured
   field strength (RMS). Measurements shall be made in the following
   directions:

   (i) Those specified in the instrument of authorization.

   (ii) In major lobes. Generally, one radial is sufficient to establish a
   major lobe; however, additional radials may be required.

   (iii) Along additional radials to establish the shape of the pattern.
   In the case of a relatively simple directional antenna pattern, a total
   of six radials is sufficient. If two radials would be more than 90°
   apart, then an additional radial must be specified within that arc.
   When more complicated patterns are involved, that is, patterns having
   several or sharp lobes or nulls, measurements shall be taken along as
   many as 12 radials to definitely establish the pattern(s). Pattern
   symmetry may be assumed for complex patterns which might otherwise
   require measurements on more than 12 radials.

   (2) A tabulation of:

   (i) The phase difference of the current in each element with respect to
   the reference element, and whether the current leads (+) or lags (-)
   the current in the reference element, as indicated by the station's
   antenna monitor.

   (ii) The ratio of the amplitude of the radio frequency current in each
   element to the current in the reference element, as indicated on the
   station's antenna monitor.

   (3) A monitoring point shall be established on each radial for which
   the construction permit specifies a limit. The following information
   shall be supplied for each monitoring point:

   (i) Measured field strength.

   (ii) An accurate and detailed description of each monitoring point. The
   description may include, but shall not be limited to, geographic
   coordinates determined with a Global Positioning System receiver.

   (iii) Clear photographs taken with the field strength meter in its
   measuring position and with the camera so located that its field of
   view takes in as many pertinent landmarks as possible.

   (b) For stations authorized to operate with simple directional antenna
   systems (e.g., two towers) in the 1605-1705 kHz band, the measurements
   to support pattern RMS compliance referred to in paragraphs (a)(1)(ii)
   and (a)(1)(iii) of this section are not required. In such cases,
   measured radials are required only in the direction of short-spaced
   allotments, or in directions specifically identified by the Commission.

   (c) Computer modeling and sample system verification of modeled
   parameters to establish operation of a directional antenna consistent
   with the theoretical pattern. Each element of the directional array
   shall be modeled by use of a method of moments computer program, using
   the physical characteristics of each element to establish a model that
   does not violate any of the internal constraints of the computer
   program. Only arrays consisting of series-fed elements may have their
   performance verified by computer modeling and sample system
   verification.

   (1) A matrix of impedance measurements at the base and/or feed point of
   each element in the array, with all other elements shorted and/or open
   circuited at their respective measurement locations, shall be made. The
   physical model of the individual antenna elements used in the computer
   program may be varied to match the measured impedance matrix, but the
   actual spacings and orientations of the array elements must be used.
   Towers may be modeled using individual vertical wires to represent
   them, or with multiple wires representing their leg and cross-member
   sections. The resulting model description (consisting of the length,
   radius, and number of segments of each wire for arrays using vertical
   wire sections to represent the towers, or the length, end-point
   coordinates, and radius of each wire used to represent leg and
   cross-member sections for arrays using detailed tower structure
   representations) as well as the assumed input feed and base region
   stray reactances shall be used to generate the drive impedances and
   sample system parameter values for the operating directional antenna
   pattern parameters.

   (i) For arrays using vertical wires to represent each tower, the radii
   of cylinders shall be no less than 80 percent and no more than 150
   percent of the radius of a circle with a circumference equal to the sum
   of the widths of the tower sides.

   (ii) For arrays using multiple wires to represent leg and cross-member
   sections, the individual legs of the tower may be modeled at their
   actual diameters with appropriate interconnecting segments representing
   cross-members at regular intervals.

   (iii) No less than one segment for each 10 electrical degrees of the
   tower's physical height shall be used for each element in the array.

   (iv) Base calculations shall be made for a reference point at ground
   level or within one electrical degree elevation of the actual feed
   point.

   (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.

   (vi) For self-supporting towers, stepped-radius wire sections may be
   employed to simulate the physical tower's taper, or the tower may be
   modeled with individual wire sections representing the legs and cross
   members.

   (vii) The lumped series inductance of the feed system between the
   output port of each antenna tuning unit and the associated tower shall
   be no greater than 10 µH unless a measured value from the measurement
   point to the tower base with its insulator short circuited is used.

   (viii) The shunt capacitance used to model the base region effects
   shall be no greater than 250 pF unless the measured or manufacturer's
   stated capacitance for each device other than the base insulator is
   used. The total capacitance of such devices shall be limited such that
   in no case will their total capacitive reactance be less than five
   times the magnitude of the tower base operating impedance without their
   effects being considered. This "five times" requirement only applies
   when the total capacitance used to model base region effects exceeds
   250 pF and when base current sampling is used.

   (ix) The orientation and distances among the individual antenna towers
   in the array shall be confirmed by a post-construction certification by
   a land surveyor (or, where permitted by local regulation, by an
   engineer) licensed or registered in the state or territory where the
   antenna system is located. Stations submitting a moment method proof
   for a pattern using towers that are part of an authorized AM array are
   exempt from the requirement to submit a surveyor's certification,
   provided that the tower geometry of the array is not being modified and
   that no new towers are being added to the array.

   (x) An AM station that verified the performance of its directional
   antenna system using computer modeling and sampling system verification
   under this rule section, that makes modifications to tower or system
   components above the base insulator, shall follow the procedures set
   forth in section 1.30003(b)(2) of this chapter.

   (2)(i) The computer model, once verified by comparison with the
   measured base impedance matrix data, shall be used to determine the
   appropriate antenna monitor parameters. The moment method modeled
   parameters shall be established by using the verified moment method
   model to produce tower current distributions that, when numerically
   integrated and normalized to the reference tower, are identical to the
   specified field parameters of the theoretical directional antenna
   pattern. The samples used to drive the antenna monitor may be current
   transformers or voltage sampling devices at the outputs of the antenna
   matching networks or sampling loops located on the towers. If sample
   loops are used, they shall be located at the elevation where the
   current in the tower would be at a minimum if the tower were detuned in
   the horizontal plane, as determined by the moment method model
   parameters used to determine the antenna monitor parameters. Sample
   loops may be employed only when the towers are identical in
   cross-sectional structure, including both leg and cross member
   characteristics; if the towers are of unequal height, the sample loops
   shall be mounted identically with respect to tower cross members at the
   appropriate elevations above the base insulator. If the tower height
   used in the model is other than the physical height of the tower, the
   sampling loop shall be located at a height that is the same fraction of
   the total tower height as the minimum in tower current with the tower
   detuned in the model. Sample lines from the sensing element to the
   antenna monitor must be equal in both length (within one electrical
   degree) and characteristic impedance (within two ohms), as established
   by impedance measurements, including at the open-circuit resonant
   frequency closest to carrier frequency to establish length, at
   frequencies corresponding to odd multiples of 1/8 wavelength
   immediately above and below the open circuit resonant frequency closest
   to carrier frequency, while open circuited, to establish characteristic
   impedance, and at carrier frequency or, if necessary, at nearby
   frequencies where the magnitude of the measured impedance is no greater
   than 200 ohms with the sampling devices connected. Samples may be
   obtained from current transformers at the output of the antenna
   coupling and matching equipment for base-fed towers whose actual
   electrical height is 120 degrees or less, or greater than 190
   electrical degrees. Samples may be obtained from base voltage sampling
   devices at the output of the antenna coupling and matching equipment
   for base-fed towers whose actual electrical height is greater than 105
   degrees. Samples obtained from sample loops located as described above
   can be used for any height of tower. For towers using base current or
   base voltage sampling derived at the output of the antenna coupling and
   matching equipment, the sampling devices shall be disconnected and
   calibrated by measuring their outputs with a common reference signal (a
   current through them or a voltage across them, as appropriate) and the
   calibration must agree within the manufacturer's specifications. A
   complete description of the sampling system, including the results of
   the measurements described in this paragraph, shall be submitted with
   the application for license.

   (ii) Proper adjustment of an antenna pattern shall be determined by
   correlation between the measured antenna monitor sample indications and
   the parameters calculated by the method of moments program, and by
   correlation between the measured matrix impedances for each tower and
   those calculated by the method of moments program. 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. The measured matrix impedances must agree with the moment method
   model within ±2 ohms and ±4 percent for resistance and reactance.

   (3) When the application for an initial license for a directional
   antenna system is submitted that is based on computer modeling and
   sample system verification, reference field strength measurement
   locations shall be established in the directions of pattern minima and
   maxima. On each radial corresponding to a pattern minimum or maximum,
   there shall be at least three measurement locations. The field strength
   shall be measured at each reference location at the time of the proof
   of performance. The license application shall include the measured
   field strength values at each reference point, along with a description
   of each measurement location, including GPS coordinates and datum
   reference. New reference field strength measurements are not required
   for subsequent license applications for the same directional antenna
   pattern and physical facilities.

   [ 36 FR 919 , Jan. 20, 1971, as amended at  42 FR 36828 , July 18, 1977;  49 FR 23348 , June 6, 1984;  50 FR 32416 , Aug. 12, 1985;  56 FR 64862 , Dec.
   12, 1991;  63 FR 33876 , June 22, 1998;  66 FR 20756 , Apr. 25, 2001;  73 FR 64561 , Oct. 30, 2008;  82 FR 51165 , Nov. 3, 2017]