Goto Section: 73.150 | 73.152 | Table of Contents
Revised as of September 1, 2021
Goto Year:2020 |
§ 73.151 Field strength measurements to establish performance of directional
The performance of a directional antenna may be verified either by
field strength measurement or by computer modeling and sampling system
(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
(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 90Ā°
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 (ā)
the current in the reference element, as indicated by the station's
(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
(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
(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
(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
(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 ĀµH 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 āfive timesā 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 1ā8 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 Ā±5 percent for the field ratio and Ā±3 degrees in
phase. The measured matrix impedances must agree with the moment method
model within Ā±2 ohms and Ā±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]
Goto Section: 73.150 | 73.152
Goto Year: 2020 |
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