§  73.190   Engineering charts and related formulas.

   (a) This section consists of the following Figures: 2, r3, 5, 6a, 7, 8,
   9, 10, 11, 12, and 13. Additionally, formulas that are directly related
   to graphs are included.

   (b) Formula 1 is used for calculation of 50% skywave field strength
   values.

   Formula 1. Skywave field strength, 50% of the time (at SS+6):

   The skywave field strength, F c (50), for a characteristic field
   strength of 100 mV/m at 1 km is given by:
   eCFR graphic ec01mr91.068.gif

   The slant distance, D , is given by:
   eCFR graphic ec13no91.020.gif

   The geomagnetic latitude of the midpoint of the path, ΦM , is given by:

   ΦM =arcsin[sin a M sin 78.5° + cos a M cos 78.5° cos(69 + b M
   )]degrees                        (Eq. 3)

   The short great-circle path distance, d, is given by:
   eCFR graphic ec13no91.021.gif

   Where:

   d ° = arccos[sin a T sin a R + cos a T cos a R cos( b R − b T
   )]degrees                              (Eq.5)

   Where:

   a T is the geographic latitude of the transmitting terminal (degrees)

   a R is the geographic latitude of the receiving terminal (degrees)

   b T is the geographic longitude of the transmitting terminal (degrees)

   b R is the geographic longitude of the receiving terminal (degrees)

   a M is the geographic latitude of the midpoint of the great-circle path
   (degrees) and is given by:

   b M is the geographic longitude of the midpoint of the great-circle
   path (degrees) and is given by:
   eCFR graphic ec01mr91.069.gif eCFR graphic ec01mr91.070.gif

   Note (1): If |FM | is greater than 60 degrees, equation (1) is
   evaluated for | FM | = 60 degrees.

   Note (2): North and east are considered positive; south and west
   negative.

   Note (3): In equation (7), k = −1 for west to east paths ( i.e. , b R >
   b T ), otherwise k = 1.

   (c) Formula 2 is used for calculation of 10% skywave field strength
   values.

   Formula 2. Skywave field strength, 10% of the time (at SS+6):

   The skywave field strength, F c (10), is given by:

   Fc (10) = Fc (50) + Δ      dB(µV/m)

   Where:

   Δ = 6 when | FM | < 40

   Δ = 0.2 | FM | − 2 when 40 ≤ | FM | ≤ 60

   Δ = 10 when | FM | > 60

   (d) Figure 6a depicts angles of departure versus transmission range.
   These angles may also be computed using the following formulas:
   eCFR graphic ec01mr91.071.gif

   Where:

   d=distance in kilometers

   n=1 for 50% field strength values

   n=2 or 3 for 10% field strength values

   and where

   K1 =0.00752

   K2 =0.00938

   K3 =0.00565

   Note: Computations using these formulas should not be carried beyond
   0.1 degree.

   (e) In the event of disagreement between computed values using the
   formulas shown above and values obtained directly from the figures, the
   computed values will control.
   eCFR graphic ec01mr91.072.gif

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   eCFR graphic ec01mr91.073.gif

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   eCFR graphic ec01mr91.074.gif

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   eCFR graphic ec01mr91.075.gif

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   eCFR graphic ec01mr91.076.gif

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   eCFR graphic ec01mr91.077.gif

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   eCFR graphic ec01mr91.078.gif

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   eCFR graphic ec01mr91.079.gif

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   eCFR graphic ec01mr91.080.gif

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   eCFR graphic ec01mr91.081.gif

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   eCFR graphic ec01mr91.082.gif

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   eCFR graphic ec01mr91.083.gif

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   eCFR graphic ec01mr91.084.gif

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   [ 28 FR 13574 , Dec. 14, 1963, as amended at  30 FR 12720 , Oct. 6, 1965;
    33 FR 15420 , Oct 17, 1968;  48 FR 42959 , Sept. 20, 1983;  49 FR 43963 ,
   Nov. 1, 1984;  50 FR 18844 , May 2, 1985;  51 FR 4753 , Feb. 7, 1986;  52 FR 36879 , Oct. 1, 1987;  56 FR 64869 , Dec. 12, 1991]

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Subpart B—FM Broadcast Stations

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