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

[[Page 56]]

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 deg.00'00'N 
and 100 deg. 00'00'W and the station to be protected is located at an 
azimuth of 45 deg. true at geographic coordinates of 44 deg.26'05'N and 
93 deg.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 deg. and 
16.6 deg.. 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 deg. and 16.6 deg. 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 
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 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 Tmp 
represent the number of hours before and after sunrise and sunset at the 
path midpoint.

[[Page 57]]

                        (i) Presunrise Constants

                                                                        
------------------------------------------------------------------------
             Tmp                  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

                                                                        
------------------------------------------------------------------------
             Tmp                  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 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 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 deg. should be used in case the mid-point of 
the path lies below 35 deg. North and latitude 50 deg. should be used in 
case the mid-point of the path lies above 50 deg. 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]