density  is  the  greatest.  As  a  radio  wave  passes  into the ionosphere, it loses some of its energy to the free electrons and ions present there. Since the amount of absorption  of  the  radio-wave  energy  varies  with  the density  of  the  ionospheric  layers,  there  is  no  fixed relationship between distance and signal strength for ionospheric propagation. Absorption fading occurs for a  longer  period  than  other  types  of  fading,  since absorption    takes    place    slowly.    Under    certain conditions,  the  absorption  of  energy  is  so  great that communication  over  any  distance  beyond  the  line  of sight becomes difficult. Although  fading  because  of  absorption  is   the most serious type of fading, fading on the ionospheric circuits is mainly a result of multipath propagation. Multipath Fading MULTIPATH  is  simply  a  term  used  to  describe the multiple paths a radio wave may follow between transmitter   and   receiver.   Such   propagation   paths include   the   ground   wave,   ionospheric   refraction, reradiation by the ionospheric layers, reflection from the earth’s surface or from more than one ionospheric layer, and so on. Figure 1-11 shows a few of the paths that a signal can travel between two sites in a typical circuit.  One  path,  XYZ,  is  the  basic  ground  wave. Another path, XFZ, refracts the wave at the F layer and passes it on to the receiver at point Z. At point Z, the  received  signal  is  a  combination  of  the  ground wave  and  the  sky  wave.  These  two  signals,  having traveled different paths, arrive at point Z at different times. Thus, the arriving waves may or may not be in phase with each other. A similar situation may result at  point  A.  Another  path,  XFZFA,  results  from  a greater  angle  of  incidence  and  two  refractions  from the  F  layer.  A  wave  traveling  that  path  and  one traveling  the  XEA  path  may  or  may  not  arrive  at point  A  in  phase.  Radio  waves  that  are  received  in phase  reinforce  each  other  and  produce  a    stronger signal  at  the  receiving  site,  while  those  that  are received   out   of   phase   produce   a   weak   or   fading signal.  Small  alterations  in  the  transmission  path may change the phase relationship of the two signals, causing periodic fading. Figure 1-11.—Multipath transmission. Multipath fading may be minimized by practices called    SPACE    DIVERSITY    and    FREQUENCY DIVERSITY In space diversity, two or more receiving antennas  are  spaced  some  distance  apart.  Fading does  not  occur    simultaneously  at  both  antennas. Therefore, enough output  is  almost  always  available from one of the antennas to provide a useful signal. In frequency diversity, two transmitters and two receivers  are  used,  each  pair  tuned  to  a  different frequency,     with     the     same     information     being transmitted   simultaneously   over   both   frequencies. One of the two receivers will almost always produce a useful signal. Selective Fading Fading   resulting   from   multipath   propagation varies with frequency since each frequency arrives at the receiving  point  via  a  different  radio  path.  When  a wide band of frequencies is transmitted simultaneously, each  frequency  will  vary  in  the  amount  of  fading. This variation is called SELECTIVE FADING. When selective    fading    occurs,    all    frequencies    of    the transmitted signal do not retain their original phases and  relative  amplitudes.  This  fading  causes  severe distortion  of  the  signal  and  limits  the  total  signal transmitted. Frequency  shifts  and  distance  changes  because of daily variations of the different ionospheric layers are summarized in table 1-1. 1-9