Goto Section: 73.150 | 73.152 | Table of Contents
FCC 73.151
Revised as of October 2, 2015
Goto Year:2014 |
2016
§ 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]
return arrow Back to Top
Goto Section: 73.150 | 73.152
Goto Year: 2014 |
2016
CiteFind - See documents on FCC website that
cite this rule
Want to support this service?
Thanks!
Report errors in
this rule. Since these rules are converted to HTML by machine, it's possible errors have been made. Please
help us improve these rules by clicking the Report FCC Rule Errors link to report an error.
hallikainen.com
Helping make public information public