§  73.185   Computation of interfering signal.

   (a) Measured values of radiation are not to be used in calculating
   overlap, interference, and coverage.

   (1) In the case of an antenna which is intended to be non-directional
   in the horizontal plane, an ideal non-directional radiation pattern
   shall be used in determining interference, overlap, and coverage, even
   if the antenna is not actually non-directional.

   (2) In the case of an antenna which is directional in the horizontal
   plane, the radiation which shall be used in determining interference,
   overlap, and coverage is that calculated pursuant to §  73.150 or
   §  73.152, depending on whether the station has a standard or modified
   standard pattern.

   (3) In the case of calculation of interference or overlap to (not from)
   a foreign station, the notified radiation shall be used, even if the
   notified radiation differs from that in paragraphs (a) (1) or (2) of
   this section.

   (b) For skywave signals from stations operating on all channels,
   interference shall be determined from the appropriate formulas and
   Figure 6a contained in §  73.190.

   (c) The formulas in §  73.190(d) depicted in Figure 6a of §  73.190,
   entitled "Angles of Departure versus Transmission Range" are to be used
   in determining the angles in the vertical pattern of the antenna of an
   interfering station to be considered as pertinent to transmission by
   one reflection. To provide for variation in the pertinent vertical
   angle due to variations of ionosphere height and ionosphere scattering,
   the curves 2 and 3 indicate the upper and lower angles within which the
   radiated field is to be considered. The maximum value of field strength
   occurring between these angles shall be used to determine the
   multiplying factor to apply to the 10 percent skywave field intensity
   value determined from Formula 2 in §  73.190. The multiplying factor is
   found by dividing the maximum radiation between the pertinent angles by
   100 mV/m.

   (d) Example of the use of skywave curves and formulas: Assume a
   proposed new Class B station from which interference may be expected is
   located at a distance of 724 kilometers from a licensed Class B
   station. The proposed station specifies geographic coordinates of
   40DEG00'00'' N and 100DEG00'00'' W and the station to be protected is
   located at an azimuth of 45DEG true at geographic coordinates of
   44DEG26'05'' N and 93DEG32'54'' W. The critical angles of radiation as
   determined from Figure 6a of §  73.190 for use with Class B stations
   are 9.6DEG and 16.6DEG. If the vertical pattern of the antenna of the
   proposed station in the direction of the existing station is such that,
   between the angles of 9.6DEG and 16.6DEG above the horizon the maximum
   radiation is 260 mV/m at one kilometer, the value of the 50% field, as
   derived from Formula 1 of §  73.190, is 0.06217 mV/m at the location of
   the existing station. To obtain the value of the 10% field, the 50%
   value must be adjusted by a factor derived from Formula 2 of §  73.190.
   The value in this case is 8.42 dB. Thus, the 10% field is 0.1616 mV/m.
   Using this in conjunction with the co-channel protection ratio of 26
   dB, the resultant nighttime limit from the proposed station to the
   licensed station is 3.232 mV/m.

   (e) In the case of an antenna which is non-directional in the
   horizontal plane, the vertical distribution of the relative fields
   should be computed pursuant to §  73.160. In the case of an antenna
   which is directional in the horizontal plane, the vertical pattern in
   the great circle direction toward the point of reception in question
   must first be calculated. In cases where the radiation in the vertical
   plane, at the pertinent azimuth, contains a large lobe at a higher
   angle than the pertinent angle for one reflection, the method of
   calculating interference will not be restricted to that just described;
   each such case will be considered on the basis of the best knowledge
   available.

   (f) In performing calculations to determine permissible radiation from
   stations operating presunrise or postsunset in accordance with
   §  73.99, calculated diurnal factors will be multiplied by the values
   of skywave field strength for such stations obtained from Formula 1 or
   2 of §  73.190.

   (1) The diurnal factor is determined using the time of day at the
   mid-point of path between the site of the interfering station and the
   point at which interference is being calculated. Diurnal factors are
   computed using the formula Df =a+bF+cF2 +dF3 where:

   Df represents the diurnal factor,

   F is the frequency in MHz,

   a,b,c, and d are constants obtained from the tables in paragraph (k)(2)

   A diurnal factor greater than one will not be used in calculations and
   interpolation is to be used between calculated values where necessary.
   For reference purposes, curves for presunrise and postsunset diurnal
   factors are contained in Figures 13 and 14 of §  73.190.

   (2) Constants used in calculating diurnal factors for the presunrise
   and postsunset periods are contained in paragraphs (f)(2) (i) and (ii)
   of this section respectively. The columns labeled Tmp represent the
   number of hours before and after sunrise and sunset at the path
   midpoint.

(i) Presunrise Constants

   T[mp]   a      b      c      d
   -2    1.3084  .0083 -.0155  .0144
   -1.75 1.3165 -.4919  .6011 -.1884
   -1.5  1.0079  .0296  .1488 -.0452
   -1.25  .7773  .3751 -.1911  .0736
   -1     .6230  .1547  .2654 -.1006
   -.75   .3718  .1178  .3632 -.1172
   -.5    .2151  .0737  .4167 -.1413
   -.25   .2027 -.2560  .7269 -.2577
   SR     .1504 -.2325  .5374 -.1729
   +.25   .1057 -.2092  .4148 -.1239
   +5.    .0642 -.1295  .2583 -.0699
   +.75   .0446 -.1002  .1754 -.0405
   +1     .0148  .0135  .0462  .0010

(ii) Postsunset Constants

   T[mp]   a      b      c      d
   1.75   .9495 -.0187  .0720 -.0290
   1.5    .7196  .3583 -.2280  .0611
   1.25   .6756  .1518  .0279 -.0163
   1.0    .5486  .1401  .0952 -.0288
   .75    .3003  .4050 -.0961  .0256
   .5     .1186  .4281 -.0799  .0197
   .25    .0382  .3706 -.0673  .0171
   SS     .0002  .3024 -.0540  .0086
   -.25   .0278  .0458  .1473 -.0486
   -.5    .0203  .0132  .1166 -.0340
   -.75   .0152 -.0002  .0786 -.0185
   -1.0  -.0043  .0452 -.0040  .0103
   -1.25  .0010  .0135  .0103  .0047
   -1.5   .0018  .0052  .0069  .0042
   -1.75 -.0012  .0122 -.0076  .0076
   -2.0  -.0024  .0141 -.0141  .0091

   Editorial Note: At  56 FR 64867 , Dec. 12, 1991, §  73.185 was amended by
   redesignating paragraphs (d), (e), (h), and (k) as (c), (d), (e), and
   (f), resulting in two consecutive paragraph (f)'s. These paragraphs
   will be correctly designated by a Federal Communication Commission
   document published in the Federal Register at a later date.

   (f) For stations operating on regional and local channels, interfering
   skywave field intensities shall be determined in accordance with the
   procedure specified in (d) of this section and illustrated in (e) of
   this section, except that Figure 2 of §  73.190 is used in place of
   Figure 1a and 1b and the formulas of §  73.190. In using Figure 2 of
   §  73.190, one additional parameter must be considered, i.e. , the
   variation of received field with the latitude of the path.

   (g) Figure 2 of §  73.190, "10 percent Skywave Signal Range Chart,"
   shows the signal as a function of the latitude of the transmission
   path, which is defined as the geographic latitude of the midpoint
   between the transmitter and receiver. When using Figure 2 of §  73.190,
   latitude 35DEG should be used in case the mid-point of the path lies
   below 35DEG North and latitude 50DEG should be used in case the
   mid-point of the path lies above 50DEG North.

   [ 30 FR 13783 , Oct. 29, 1965, as amended at  33 FR 15420 , Oct. 17, 1968;
    46 FR 11995 , Feb. 12, 1981;  48 FR 42958 , Sept. 20, 1983;  50 FR 18843 ,
   May 2, 1985;  56 FR 64867 , Dec. 12, 1991]

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