Goto Section: 73.160 | 73.183 | Table of Contents
Revised as of December 4, 2012
Goto Year:2011 |
§ 73.182 Engineering standards of allocation.
(a) Sections 73.21 to 73.37, inclusive, govern allocation of facilities
in the AM broadcast band 535-1705 kHz. § 73.21 establishes three
classes of channels in this band, namely, clear, regional and local.
The classes and power of AM broadcast stations which will be assigned
to the various channels are set forth in § 73.21. The classifications
of the AM broadcast stations are as follows:
(1) Class A stations operate on clear channels with powers no less than
10kW nor greater than 50 kW. These stations are designed to render
primary and secondary service over an extended area, with their primary
services areas protected from objectionable interference from other
stations on the same and adjacent channels. Their secondary service
areas are protected from objectionable interference from co-channel
stations. For purposes of protection, Class A stations may be divided
into two groups, those located in any of the contiguous 48 States and
those located in Alaska in accordance with § 73.25.
(i) The mainland U.S. Class A stations are those assigned to the
channels allocated by § 73.25. The power of these stations shall be 50
kW. The Class A stations in this group are afforded protection as
(A) Daytime. To the 0.1 mV/m groundwave contour from stations on the
same channel, and to the 0.5 mV/m groundwave contour from stations on
(B) Nighttime. To the 0.5 mV/m-50% skywave contour from stations on the
(ii) Class A stations in Alaska operate on the channels allocated by
§ 73.25 with a minimum power of 10 kW, a maximum power of 50 kW, and an
antenna efficiency of 282 mV/m/kW at 1 kilometer. Stations operating on
these channels in Alaska which have not been designated as Class A
stations in response to licensee request will continue to be considered
as Class B stations. During daytime hours a Class A station in Alaska
is protected to the 100 µV/m groundwave contour from co-channel
stations. During nighttime hours, a Class A station in Alaska is
protected to the 100 µV/m-50 percent skywave contour from co-channel
stations. The 0.5 mV/m groundwave contour is protected both daytime and
nighttime from stations on adjacent channels.
Note: In the Report and Order in MM Docket No. 83-807, the Commission
designated 15 stations operating on U.S. clear channels as Alaskan
Class A stations. Eleven of these stations already have Alaskan Class A
facilities and are to be protected accordingly. Permanent designation
of the other four stations as Alaskan Class A is conditioned on their
constructing minimum Alaskan Class A facilities no later than December
31, 1989. Until that date or until such facilities are obtained, these
four stations shall be temporarily designated as Alaskan Class A
stations, and calculations involving these stations should be based on
existing facilities but with an assumed power of 10 kW. Thereafter,
these stations are to be protected based on their actual Alaskan Class
A facilities. If any of these stations does not obtain Alaskan Class A
facilities in the period specified, it is to be protected as a Class B
station based on its actual facilities. These four stations may
increase power to 10 kW without regard to the impact on co-channel
Class B stations. However, power increases by these stations above 10
kW (or by existing Alaskan Class A stations beyond their current power
level) are subject to applicable protection requirements for co-channel
Class B stations. Other stations not on the original list but which
meet applicable requirements may obtain Alaskan Class A status by
seeking such designation from the Commission. If a power increase or
other change in facilities by a station not on the original list is
required to obtain minimum Alaskan Class A facilities, any such
application shall meet the interference protection requirements
applicable to an Alaskan Class A proposal on the channel.
(2) Class B stations are stations which operate on clear and regional
channels with powers not less than 0.25 kW nor more than 50 kW. These
stations render primary service only, the area of which depends on
their geographical location, power, and frequency. It is recommended
that Class B stations be located so that the interference received from
other stations will not limit the service area to a groundwave contour
value greater than 2.0 mV/m nighttime and to the 0.5 mV/m groundwave
contour daytime, which are the values for the mutual protection between
this class of stations and other stations of the same class.
Note: See § § 73.21(b)(1) and 73.26(b) concerning power restrictions and
classifications relative to Class B, Class C, and Class D stations in
Alaska, Hawaii, Puerto Rico and the U.S. Virgin Islands. Stations in
the above-named places that are reclassified from Class C to Class B
stations under § 73.26(b) shall not be authorized to increase power to
levels that would increase the nighttime interference-free limit of
co-channel Class C stations in the conterminous United States.
(3) Class C stations operate on local channels, normally rendering
primary service to a community and the suburban or rural areas
immediately contiguous thereto, with powers not less than 0.25 kW, nor
more than 1 kW, except as provided in § 73.21(c)(1). Such stations are
normally protected to the daytime 0.5 mV/m contour. On local channels
the separation required for the daytime protection shall also determine
the nighttime separation. Where directional antennas are employed
daytime by Class C stations operating with more than 0.25 kW power, the
separations required shall in no case be less than those necessary to
afford protection, assuming nondirectional operation with 0.25 kW. In
no case will 0.25 kW or greater nighttime power be authorized to a
station unable to operate nondirectionally with a power of 0.25 kW
during daytime hours. The actual nighttime limitation will be
calculated. For nighttime protection purposes, Class C stations in the
48 contiguous United States may assume that stations in Alaska, Hawaii,
Puerto Rico, and the U.S. Virgin Islands operating on 1230, 1240, 1340,
1400, 1450, and 1490 kHz are Class C stations.
(4) Class D stations operate on clear and regional channels with
daytime powers of not less than 0.25 kW (or equivalent RMS field of 141
mV/m at one kilometer if less than 0.25 kW) and not more than 50 kW.
Class D stations that have previously received nighttime authority
operate with powers of less than 0.25 kW (or equivalent RMS fields of
less than 141 mV/m at one kilometer) are not required to provide
nighttime coverage in accordance with § 73.24(j) and are not protected
from interference during nighttime hours. Such nighttime authority is
permitted on the basis of full nighttime protection being afforded to
all Class A and Class B stations.
(b) When a station is already limited by interference from other
stations to a contour value greater than that normally protected for
its class, the individual received limits shall be the established
standard for such station with respect to interference from each other
(c) The four classes of AM broadcast stations have in general three
types of service areas, i.e. , primary, secondary and intermittent.
(See § 73.14 for the definitions of primary, secondary, and
intermittent service areas.) Class A stations render service to all
three areas. Class B stations render service to a primary area but the
secondary and intermittent service areas may be materially limited or
destroyed due to interference from other stations, depending on the
station assignments involved. Class C and Class D stations usually have
only primary service areas. Interference from other stations may limit
intermittent service areas and generally prevents any secondary service
to those stations which operate at night. Complete intermittent service
may still be obtained in many cases depending on the station
(d) The groundwave signal strength required to render primary service
is 2 mV/m for communities with populations of 2,500 or more and 0.5
mV/m for communities with populations of less than 2,500. See § 73.184
for curves showing distance to various groundwave field strength
contours for different frequencies and ground conductivities, and also
see § 73.183, “Groundwave signals.”
(e) A Class C station may be authorized to operate with a directional
antenna during daytime hours providing the power is at least 0.25 kW.
In computing the degrees of protection which such antenna will afford,
the radiation produced by the directional antenna system will be
assumed to be no less, in any direction, than that which would result
from non-directional operation using a single element of the
directional array, with 0.25 kW.
(f) All classes of broadcast stations have primary service areas
subject to limitation by fading and noise, and interference from other
stations to the contours set out for each class of station.
(g) Secondary service is provided during nighttime hours in areas where
the skywave field strength, 50% or more of the time, is 0.5 mV/m or
greater (0.1 mV/m in Alaska). Satisfactory secondary service to cities
is not considered possible unless the field strength of the skywave
signal approaches or exceeds the value of the groundwave field strength
that is required for primary service. Secondary service is subject to
some interference and extensive fading whereas the primary service area
of a station is subject to no objectionable interference or fading.
Only Class A stations are assigned on the basis of rendering secondary
Note: Standards have not been established for objectionable fading
because of the relationship to receiver characteristics. Selective
fading causes audio distortion and signal strength reduction below the
noise level, objectionable characteristics inherent in many modern
receivers. The AVC circuits in the better designed receivers generally
maintain the audio output at a sufficiently constant level to permit
satisfactory reception during most fading conditions.
(h) Intermittent service is rendered by the groundwave and begins at
the outer boundary of the primary service area and extends to a
distance where the signal strength decreases to a value that is too low
to provide any service. This may be as low as a few µV/m in certain
areas and as high as several millivolts per meter in other areas of
high noise level, interference from other stations, or objectionable
fading at night. The intermittent service area may vary widely from day
to night and generally varies over shorter intervals of time. Only
Class A stations are protected from interference from other stations to
the intermittent service area.
(i) Broadcast stations are licensed to operate unlimited time, limited
time, daytime, share time, and specified hours. (See § § 73.1710,
73.1725, 73.1720, 73.1715, and 73.1730.) Applications for new stations
shall specify unlimited time operation only.
(j) Section 73.24 sets out the general requirements for modifying the
facilities of a licensed station and for establishing a new station.
Sections 73.24(b) and 73.37 include interference related provisions
that be considered in connection with an application to modify the
facilities of an existing station or to establish a new station.
Section 73.30 describes the procedural steps required to receive an
authorization to operate in the 1605-1705 kHz band.
(k) Objectionable nighttime interference from a broadcast station
occurs when, at a specified field strength contour with respect to the
desired station, the field strength of an undesired station (co-channel
or first adjacent channel, after application of proper protection
ratio) exceeds for 10% or more of the time the values set forth in
these standards. The value derived from the root-sum-square of all
interference contributions represents the extent of a station's
(1) With respect to the root-sum-square (RSS) values of interfering
field strengths referred to in this section, calculation of nighttime
interference-free service is accomplished by considering the signals on
the three channels of concern (co- and first adjacencies) in order of
decreasing magnitude, adding the squares of the values and extracting
the square root of the sum, excluding those signals which are less than
50% of the RSS values of the higher signals already included.
(2) With respect to the root-sum-square values of interfering field
strengths referred to in this section, calculation of nighttime
interference for non-coverage purposes is accomplished by considering
the signals on the three channels of concern (co- and first
adjacencies) in order of decreasing magnitude, adding the squares of
the values and extracting the square root of the sum, excluding those
signals which are less than 25% of the RSS values of the higher signals
(3) With respect to the root-sum-square values of interfering field
strengths referred to in this section, calculation is accomplished by
considering the signals on the three channels of concern (co- and first
adjacencies) in order of decreasing magnitude, adding the squares of
the values and extracting the square root of the sum. The 0% exclusion
method applies only to the determination of an improvement factor value
for evaluating a station's eligibility for migration to the band
(4) The RSS value will not be considered to be increased when a new
interfering signal is added which is less than the appropriate
exclusion percentage as applied to the RSS value of the interference
from existing stations, and which at the same time is not greater than
the smallest signal included in the RSS value of interference from
(5) It is recognized that application of the above “50% exclusion”
method (or any exclusion method using a per cent value greater than
zero) of calculating the RSS interference may result in some cases in
anomalies wherein the addition of a new interfering signal or the
increase in value of an existing interfering signal will cause the
exclusion of a previously included signal and may cause a decrease in
the calculated RSS value of interference. In order to provide the
Commission with more realistic information regarding gains and losses
in service (as a basis for determination of the relative merits of a
proposed operation) the following alternate method for calculating the
proposed RSS values of interference will be employed wherever
(6) In the cases where it is proposed to add a new interfering signal
which is not less than 50% (or 25%, depending on which study is being
performed) of the RSS value of interference from existing stations or
which is greater that the smallest signal already included to obtain
this RSS value, the RSS limitation after addition of the new signal
shall be calculated without excluding any signal previously included.
Similarly, in cases where it is proposed to increase the value of one
of the existing interfering signals which has been included in the RSS
value, the RSS limitation after the increase shall be calculated
without excluding the interference from any source previously included.
(7) If the new or increased signal proposed in such cases is ultimately
authorized, the RSS values of interference to other stations affected
will thereafter be calculated by the “50% exclusion” (or 25% exclusion,
depending on which study is being performed) method without regard to
this alternate method of calculation.
(8) Examples of RSS interference calculations:
(i) Existing interferences:
Station No. 1—1.00 mV/m.
Station No. 2—0.60 mV/m.
Station No. 3—0.59 mV/m.
Station No. 4—0.58 mV/m.
The RSS value from Nos. 1, 2 and 3 is 1.31 mV/m; therefore interference
from No. 4 is excluded for it is less than 50% of 1.31 mV/m.
(ii) Station A receives interferences from:
Station No. 1—1.00 mV/m.
Station No. 2—0.60 mV/m.
Station No. 3—0.59 mV/m.
It is proposed to add a new limitation, 0.68 mV/m. This is more than
50% of 1.31 mV/m, the RSS value from Nos. 1, 2 and 3. The RSS value of
Station No. 1 and of the proposed station would be 1.21 m/Vm which is
more than twice as large as the limitation from Station No. 2 or No. 3.
However, under the above provision the new signal and the three
existing interferences are nevertheless calculated for purposes of
comparative studies, resulting in an RSS value of 1.47 mV/m. However,
if the proposed station is ultimately authorized, only No. 1 and the
new signal are included in all subsequent calculations for the reason
that Nos. 2 and 3 are less than 50% of 1.21 mV/m, the RSS value of the
new signal and No. 1.
(iii) Station A receives interferences from:
Station No. 1—1.00 mV/m.
Station No. 2—0.60 mV/m.
Station No. 3—0.59 mV/m.
No. 1 proposes to increase the limitation it imposes on Station A to
1.21 mV/m. Although the limitations from stations Nos. 2 and 3 are less
than 50% of the 1.21 mV/m limitation, under the above provision they
are nevertheless included for comparative studies, and the RSS
limitation is calculated to be 1.47 mV/m. However, if the increase
proposed by Station No. 1 is authorized, the RSS value then calculated
is 1.21 mV/m because Stations Nos. 2 and 3 are excluded in view of the
fact that the limitations they impose are less than 50% of 1.21 mV/m.
Note: The principles demonstrated in the previous examples for the
calculation of the 50% exclusion method also apply to calculations
using the 25% exclusion method after appropriate adjustment.
(l) Objectionable nighttime interference from a station shall be
considered to exist to a station when, at the field strength contour
specified in paragraph (q) of this section with respect to the class to
which the station belongs, the field strength of an interfering station
operating on the same channel or on a first adjacent channel after
signal adjustment using the proper protection ratio, exceeds for 10% or
more of the time the value of the permissible interfering signal set
forth opposite such class in paragraph (q) of this section.
(m) For the purpose of estimating the coverage and the interfering
effects of stations in the absence of field strength measurements, use
shall be made of Figure 8 of § 73.190, which describes the estimated
effective field (for 1 kW power input) of simple vertical
omnidirectional antennas of various heights with ground systems having
at least 120 quarter-wavelength radials. Certain approximations, based
on the curve or other appropriate theory, may be made when other than
such antennas and ground systems are employed, but in any event the
effective field to be employed shall not be less than the following:
Class of station Effective field (at 1 km)
All Class A (except Alaskan) 362 mV/m.
Class A (Alaskan), B and D 282 mV/m.
Class C 241 mV/m.
Note (1): When a directional antenna is employed, the radiated signal
of a broadcasting station will vary in strength in different
directions, possibly being greater than the above values in certain
directions and less in other directions depending upon the design and
adjustment of the directional antenna system. To determine the
interference in any direction, the measured or calculated radiated
field (unattenuated field strength at 1 kilometer from the array) must
be used in conjunction with the appropriate propagation curves. (See
§ 73.185 for further discussion and solution of a typical directional
Note (2): For Class B stations in Alaska, Hawaii, Puerto Rico and the
U.S. Virgin Islands, 241 mV/m shall be used.
(n) The existence or absence of objectionable groundwave interference
from stations on the same or adjacent channels shall be determined by
actual measurements made in accordance with the method described in
§ 73.186, or in the absence of such measurements, by reference to the
propagation curves of § 73.184. The existence or absence of
objectionable interference due to skywave propagation shall be
determined by reference to Formula 2 in § 73.190.
(o) Computation of skywave field strength values: —(1) Fifty percent
skywave field strength values (clear channel). In computing the fifty
percent skywave field strength values of a Class A clear channel
station, use shall be made of Formula 1 of § 73.190, entitled “Skywave
Field Strength” for 50 percent of the time.
(2) Ten percent skywave field strength values. In computing the 10%
skywave field strength for stations on a single signal or an RSS basis,
Formula 2 in § 73.190 shall be used.
(3) Determination of angles of departure. In calculating skywave field
strength for stations on all channels, the pertinent vertical angle
shall be determined by use of the formula in § 73.190(d).
(p) The distance to any specified groundwave field strength contour for
any frequency may be determined from the appropriate curves in § 73.184
entitled “Ground Wave Field Strength vs. Distance.”
(q) Normally protected service contours and permissible interference
signals for broadcast stations are as follows (for Class A stations,
see also paragraph (a) of this section):
Class of station Class of channel used Signal strength contour of area
protected from objectionable interference ^1(µV/m) Permissible
interfering signal (µV/m)
Day ^2 Night Day ^2 Night ^3
A Clear SC 100 SC 500 50% SW SC 5 SC 25
AC 500 AC 500 GW AC 250 AC 250
A (Alaskan) ......do SC 100 SC 100 50% SW SC 5 SC 5
AC 500 AC 500 GW AC 250 AC 250
B Clear 500 2000 ^2 25 25
Regional AC 250 250
C Local 500 No presc.^4 SC25 Not presc.
D Clear 500 Not presc SC 25 Not presc.
Regional AC 250
^1 When a station is already limited by interference from other
stations to a contour of higher value than that normally protected for
its class, this higher value contour shall be the established
protection standard for such station. Changes proposed by Class A and B
stations shall be required to comply with the following restrictions.
Those interferers that contribute to another station's RSS using the
50% exclusion method are required to either reduce their contributions
to that RSS by 10%, or to a level at which their contributions no
longer enter into the 50% RSS value, whichever is the lesser amount of
reduction. Those interferers that contribute to a station's RSS using
the 25% exclusion method but do not contribute to that station's RSS
using the 50% exclusion method may make changes not to exceed their
present contribution. Interferers not included in a station's RSS using
the 25% exclusion method are permitted to increase radiation as long as
the 25% exclusion threshold is not equalled or exceeded. In no case
will a reduction be required that would result in a contributing value
that is below the pertinent value specified in the table. This note
does not apply to Class C stations; or to the protection of Class A
stations which are normally protected on a single signal, non-RSS
^3 Skywave field strength for 10 percent or more of the time.
^4 During nighttime hours, Class C stations in the contiguous 48 States
may treat all Class B stations assigned to 1230, 1240, 1340, 1400, 1450
and 1490 kHz in Alaska, Hawaii, Puerto Rico and the U.S. Virgin Islands
as if they were Class C stations.
Note: SC=Same channel; AC=Adjacent channel; SW=Skywave; GW=Groundwave
(r) The following table of logarithmic expressions is to be used as
required for determining the minimum permissible ratio of the field
strength of a desired to an undesired signal. This table shall be used
in conjunction with the protected contours specified in paragraph (q)
of this section.
Frequency separation of desired to undesired signals (kHz) Desired
Groundwave to: Desired 50% Skywave to Undesired 10% Skywave (dB)
Undesired groundwave (dB) Undesired 10% Skywave (dB)
0 26 26 26
10 6 6 not presc.
(s) Two stations, one with a frequency twice of the other, should not
be assigned in the same groundwave service area unless special
precautions are taken to avoid interference from the second harmonic of
the station operating on the lower frequency. Additionally, in
selecting a frequency, consideration should be given to the fact that
occasionally the frequency assignment of two stations in the same area
may bear such a relation to the intermediate frequency of some
broadcast receivers as to cause “image” interference, However, since
this can usually be rectified by readjustment of the intermediate
frequency of such receivers, the Commission, in general, will not take
this kind of interference into consideration when authorizing stations.
(t) The groundwave service of two stations operating with synchronized
carriers and broadcasting identical programs will be subject to some
distortion in areas where the signals from the two stations are of
comparable strength. For the purpose of estimating coverage of such
stations, areas in which the signal ratio is between 1:2 and 2:1 will
not be considered as receiving satisfactory service.
Note: Two stations are considered to be operated synchronously when the
carriers are maintained within 0.2 Hz of each other and they transmit
identical program s.
[ 56 FR 64862 , Dec. 12, 1991; 57 FR 43290 , Sept. 18, 1992, as amended at
58 FR 27950 , May 12, 1993]
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