Sec.  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.  Sec. 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  Sec. 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  Sec. 73.25.

   (i) The mainland U.S. Class A stations are those assigned to the channels
   allocated by  Sec. 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  Sec. 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  �V/m groundwave contour from co-channel stations. During nighttime
   hours, a Class A station in Alaska is protected to the 100  �V/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  Sec.  Sec. 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  Sec. 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  Sec. 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
    Sec. 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  Sec. 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  Sec. 73.184 for curves
   showing distance to various groundwave field strength contours for different
   frequencies and ground conductivities, and also see  Sec. 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  �V/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  Sec.  Sec. 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  Sec. 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  Sec. 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  Sec. 73.186, or in
   the absence of such measurements, by reference to the propagation curves of
    Sec. 73.184. The existence or absence of objectionable interference due to
   skywave  propagation  shall be determined by reference to Formula 2 in
    Sec. 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  Sec. 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
    Sec. 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  Sec. 73.190(d).

   (p) The distance to any specified groundwave field strength contour for any
   frequency may be determined from the appropriate curves in  Sec. 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( �V/m) Permissible interfering
   signal ( �V/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

   ^1When 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.

   ^2Groundwave.

   ^3Skywave field strength for 10 percent or more of the time.

   ^4During 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]