Angle of Incidence and Critical Angle

particular  layer,  it  may  still  be  refracted  by  a higher  layer  if  its  frequency  is  lower  than  the higher layer’s critical frequency. Angle of Incidence and Critical Angle When  a  radio  wave  encounters  a  layer  of  the ionosphere, that wave is returned to earth  at  the same  angle  (roughly)  as  its  angle  of  incidence. Figure  1-6  shows  three  radio  waves  of  the  same frequency  entering  a  layer  at  different  incidence angles.  The  angle  at  which  wave  A  strikes  the layer  is  too  nearly  vertical  for  the  wave  to  be refracted  to  earth,  However,  wave  B  is  refracted back to earth. The angle between wave B and the earth is called the critical angle. Any wave, at a given  frequency,  that  leaves  the  antenna  at  an incidence angle greater than the critical angle will be  lost  into  space.  This  is  why   wave  A was   not refracted.   Wave   C   leaves   the   antenna   at   the smallest angle that will allow it to be refracted and still  return  to  earth.  The  critical  angle  for  radio waves   depends   on   the   layer   density   and   the wavelength of the signal. Figure 1-6.—Incidence angles of radio waves. As the frequency of a radio  wave  is  increased, the critical angle must be reduced for refraction to occur.  Notice  in  figure  1-7  that  the  2-MHz  wave strikes the ionosphere at the critical angle for that frequency  and  is  refracted.  Although  the  5-MHz line (broken line) strikes  the  ionosphere  at  a less critical  angle,  it  still  penetrates  the  layer  and  is lost  As  the  angle  is  lowered,  a  critical  angle  is finally  reached  for  the   5-MHz   wave   and   it   is refracted back to earth. Figure 1-7.—Effect of frequency on the critical angle. 1-6


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