§ 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 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.

   (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.

   (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) Reference field strength measurement locations shall be established in
   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.

   [ 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]

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