FCC 73.185
Revised as of December 4, 2012
Goto Year:2011 | 2013
```§  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
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 §  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
§  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 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]