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Temperature Inversion
Optimum Working Frequency

Electronics Technician Volume 07-Antennas and Wave Propagation
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atmospheric  levels  in  the  ionosphere  account  for  a large part of these energy losses. There are two other types    of    losses    that    also    significantly    affect propagation.   These   losses   are   known   as   ground reflection  losses  and  freespace  loss.  The  combined effect   of   absorption   ground   reflection   loss,   and freespace loss account for most of the losses  of  radio transmissions propagated in the ionosphere. GROUND REFLECTION LOSS When propagation is accomplished  via  multihop refraction, rf energy is lost each time the radio wave is  reflected  from  the  earth’s  surface.  The  amount  of energy lost depends on the frequency of the wave, the angle  of  incidence,  ground  irregularities,   and   the electrical conductivity of the point of reflection. FREESPACE LOSS Normally, the major loss of energy is because of the spreading out of the wavefront as it travels from the transmitter. As distance increases, the area of the wavefront  spreads  out,  much  like  the  beam  of   a flashlight.    This    means    the    amount    of    energy contained  within  any  unit  of  area  on  the  wavefront decreases   as   distance   increases.   By   the   time   the energy    arrives    at    the    receiving    antenna,    the wavefront is so spread out that the receiving antenna extends  into  only  a  small  portion  of  the  wavefront. This is illustrated in figure 1-15. FREQUENCY SELECTION You  must  have  a  thorough  knowledge  of  radio- wave  propagation  to  exercise  good  judgment  when selecting  transmitting  and  receiving  antennas  and operating  frequencies.  Selecting  a  usable  operating frequency    within    your    given    allocations     and availability  is  of  prime  importance  to  maintaining reliable communications. For  successful  communication  between  any  two specified locations at any given time of the day, there is a maximum frequency, a lowest frequency and an optimum frequency that can be used. Figure 1-15.—Freespace loss principle. MAXIMUM USABLE FREQUENCY The  higher  the  frequency  of  a  radio  wave,  the lower   the   rate   of   refraction   by   the   ionosphere. Therefore, for a given angle of incidence and time of day, there is a maximum frequency that can be used for communications between two given locations. This frequency  is  known  as  the  MAXIMUM  USABLE FREQUENCY (muf). Waves    at    frequencies    above    the    muf    are normally refracted so slowly that they return to earth beyond  the  desired  location  or  pass  on  through  the ionosphere and are lost. Variations in the ionosphere that  can  raise  or  lower  a  predetermined  muf  may occur at anytime. his is especially true for the highly variable F2 layer. LOWEST USABLE FREQUENCY Just  as  there  is  a  muf  that  can  be  used  for communications  between  two  points,  there  is  also  a minimum   operating   frequency   that   can   be   used known as the LOWEST USABLE FREQUENCY (luf). As the frequency of a radio wave is lowered, the rate of refraction increases. So a wave whose frequency is below the established luf is refracted back to earth at a shorter distance than desired, as shown in figure 1- 16. 1-13






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