FCC 73.185 Revised as of October 1, 2007
Goto Year:2006 |
2008
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]
CiteFind - See documents on FCC website that
cite this rule
Want to support this service?
Thanks!
Report errors in
this rule. Since these rules are converted to HTML by machine, it's possible errors have been made. Please
help us improve these rules by clicking the Report FCC Rule Errors link to report an error.