Sec.  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  Sec. 73.150 or  Sec. 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  Sec. 73.190.

   (c) The formulas in  Sec. 73.190(d) depicted in Figure 6a of  Sec. 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  Sec. 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 40 °00'00" N and 100 °00'00" W and
   the  station  to  be protected is located at an azimuth of 45 ° true at
   geographic coordinates of 44 °26'05" N and 93 °32'54" W. The critical angles
   of radiation as determined from Figure 6a of  Sec. 73.190 for use with Class B
   stations are 9.6 ° and 16.6 °. 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 9.6 ° and 16.6 ° 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  Sec. 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  Sec. 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  Sec. 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  Sec. 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  Sec. 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 D[f]=a+bF+cF^2 +dF^3 where:

   D[f]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  Sec. 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 T[mp]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,  Sec. 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 theFederal Registerat 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  Sec. 73.190 is used in place of Figure 1a and
   1b and the formulas of  Sec. 73.190. In using Figure 2 of  Sec. 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  Sec. 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  Sec. 73.190, latitude 35 ° should be used in
   case the mid-point of the path lies below 35 ° North and latitude 50 ° should
   be used in case the mid-point of the path lies above 50 ° 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]