Sec.  80.769   Shadow loss.

   Where  the transmission path is obstructed the received signal must be
   adjusted to include shadow loss. Attenuation due to shadowing must be taken
   from  Sec. 80.769 Graph 1, as follows:

   (a) Inspect the map(s) to determine if a hill(s) obstructs an imaginary line
   of sight (dashed line on illustrative profiles of  Sec. 80.769 Graph 1 from the
   average terrain elevation at the coast station antenna to the water level at
   the ship location. If average terrain elevation exceeds the actual ground
   elevation at the antenna site, the latter elevation must be used as the
   average terrain elevation.

   (b) If a hill appears to obstruct the radio path, plot the antenna site
   elevation, the obstruction elevation and the height of the ship station on
   rectangular coordinate paper using elevation above mean sea level as the
   vertical scale and distance in statute miles as the horizontal scale. Then
   draw a straight line between the antenna and the ship.

   (c) If a hill obstructs the imaginary line of sight, determine its height
   (H) above the imaginary line and its distance (D) from either the coast or
   ship station, whichever is nearer, as illustrated by examples “A” and “B” on
   Graph 1.

   (d) Read the shadow loss from this Graph 1 and subtract that loss from the
   computed received signal.

   (e) Where more than one hill obstructs the transmission path, determine the
   height and position of a single equivalent hill, as illustrated by example
   “C” on this graph. Read the shadow loss from this graph for the equivalent
   hill.
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