§  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
   follows:

   (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
   adjacent channels.

   (B) Nighttime. To the 0.5 mV/m-50% skywave contour from stations on the
   same channels.

   (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
   station.

   (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
   assignments involved.

   (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
   service.

   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
   interference-free coverage.

   (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
   already included.

   (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
   1605-1705 kHz.

   (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
   existing stations.

   (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
   applicable.

   (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
   antenna case.)

   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
   basis.

   ^2 Groundwave.

   ^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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