Figure 1-3.—Ducting effect on the radar wave. Water   droplets   and   dust   particles   diffuse   radar energy   through   absorption,   reflection,   and   scattering. This  leaves  less  energy  to  strike  the  target,  so  the return  echo  is  smaller.  The  overall  effect  is  a  reduc- tion  in  usable  range.  Usable  range  varies  widely  with such  weather  conditions.  The  higher  the  frequency  of the  radar  system,  the  more  it  is  affected  by  weather conditions,  such  as  rain  or  clouds. Since   all   radar   systems   perform   the   same   basic functions   of   detection,   they   all   have   the   same   basic equipment   requirements. BASIC   RADAR   SYSTEMS Radar   systems,   like   other   complex   electronics systems,   are   composed   of   several   major   subsystems and  many  individual  circuits.  Although  modern  radar systems  are  quite  complicated,  you  can  easily  under- stand  their  operation  by  using  a  basic  block  diagram of  a  pulse-radar  system. FUNDAMENTAL   RADAR   SYSTEM Since  most  radars  used  today  are  some  variation of  the  pulse-radar  system,  this  section  discusses  those used  in  a  pulse  radar.  All  other  types  of  radars  use some   variation   of   these   units.   Refer   to   the   block diagram  in  figure  1-4. 1-6 Figure 1-4.—Block diagram of a fundamental radar system. Modulator You  can  see  on  the  block  diagram  that  the  heart  of the  radar  system  is  the  modulator.  It  generates  all  the necessary  timing  pulses  (triggers)  for  use  in  the  radar and   its   associated   systems.   Its   function   is   to   ensure that  all  subsystems  of  the  radar  system  operate  in  a definite   time   relationship   with   one   another   and   that the  intervals  between  pulses,  as  well  as  the  pulses themselves,  are  of  the  proper  length. Transmitter The  transmitter  generates  powerful  pulses  of  elec- tromagnetic   energy   at   precise   intervals.   The   required power  is  obtained  by  using  a  high-power  microwave oscillator  (such  as  a  magnetron)  or  a  microwave  am- plifier  (such  as  a  klystron)  that  is  supplied  by  a  low- power   RF   source.


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