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 [mu]V/m groundwave contour from co-channel 
stations. During nighttime hours, a Class A station in Alaska is 
protected to the 100 [mu]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

[[Page 48]]

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 beprotected 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 themutual 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. Fornighttime 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

[[Page 49]]

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 [mu]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

[[Page 50]]

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.


[[Page 51]]


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%

[[Page 52]]

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):

-------------------------------------------------------------------------------
---------------------------------
                                            Signal strength contour of area    
  Permissible interfering signal
                                              protected from objectionable     
             ([mu]V/m)
  Class of station    Class of channel         interference \1\([mu]V/m)       
---------------------------------
                            used        ---------------------------------------
                                              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 esta
blished 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 usin
g the 50% exclusion method are
  required to either reduce their contributions to that RSS by 10%, or to a lev
el at which their contributions
  no longer enter into the 50% RSS value, whichever is the lesser amount of red
uction. Those interferers that
  contribute to a station's RSS using the 25% exclusion method but do not contr
ibute to that station's RSS using
  the 50% exclusion method may make changes not to exceed their present contrib
ution. Interferers not included
  in a station's RSS using the 25% exclusion method are permitted to increase r
adiation 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 tr
eat all Class B stations assigned
  to 1230, 1240, 1340, 1400, 1450 and 1490 kHz in Alaska, Hawaii, Puerto Rico a
nd 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.

-------------------------------------------------------------------------------
---------------------------------
                                                    Desired Groundwave to:
  Frequency separation of desired to  -----------------------------------------
---------  Desired 50% Skywave to
       undesired signals (kHz)           Undesired groundwave    Undesired 10% 
Skywave    Undesired 10% Skywave
                                                 (dB)                     (dB) 
                    (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.

[[Page 53]]

    (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]