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SCANNING   METHODS
Monopulse    Scanning

Fire Controlman Volume 02-Fire Control Radar Fundamentals
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Nutation The  nutation  process  is  difficult  to  describe  in words,   but   easy   to   demonstrate.   Hold   a   pencil   with both  hands,  one  hand  at  the  eraser  end  and  the  other hand  at  the  point.  While  holding  the  eraser  end  as  still as  possible,  move  the  point  in  a  circle  to  form  a  cone. The motion of the pencil is called  nutation.  The  pencil point  corresponds  to  the  transmitting  end  of  a  wave- guide  feed. The  important  point  is  that  the  polarization  of  the RF   beam   does   not   change   during   a   nutation   cycle. This  means  that  the  axis  of  the  moving  feed  horn  must not  change  horizontal  or  vertical  orientation  while  the feed  is  moving.  The  movement  might  be  compared  to that  of  a  Ferris  wheel;  the  orientation  of  the  seats  re- mains   vertical,   regardless   of   where   they   are   on   the wheel. Nutating   Waveguide The  waveguide  feed  is  a  metal  pipe,  usually  rec- tangular  in  cross  section.  The  open  end  of  the  wave- guide   feed   faces   the   antenna   disk   reflector,   and   the energy  it  emits  is  bounced  from  the  reflector  surface. A  conical  scan  can  be  generated  by  nutation  of  the waveguide   feed.   In   this   process,   the   axis   of   the waveguide   feed   is   nutated   through   a   narrow   conical pattern. This  movement  is  fast  (from  30  to  60  hertz)  and small   in   amplitude.   To   an   observer,   the   waveguide feed  would  appear  to  be  vibrating  slightly.  The  ampli- tude  of  the  nutation  determines  the  angle  of  the  cone. The  amplitude  is  kept  small  so  that  sufficient  power is  present  in  the  center  of  the  conical  pattern  for  target tracking.   Figure   2-5   shows   the   conical   pattern   pro- duced  by  nutating  the  RF  beam. Angle   Tracking The   radar’s   angle   error-detecting   circuits   provide correction   signals   to   the   antenna   and   director   drive circuits.  The  correction  signals  are  proportional  to  the Figure 2-5.—Nutating lobe. target  displacement  from  the  nutation  axis  of  the  an- tenna.  Target  displacement  is  detected  by  first  locking onto   the   target   in   range,   so   that   only   that   target   is gated  and  used  for  angle  tracking. A  two-phase  reference  generator  in  the  antenna establishes  the  position  of  the  feed  horn  relative  to  the nutation   axis   of   the   antenna.   The   generator   provides the   reference   voltages   for   the   angle   error-detection circuits.  The  elevation  reference  voltage  is  900  out  of phase   with   the   azimuth   reference   voltage. Each   error-detector   circuit   compares   the   phase   of its  reference  voltage  with  the  phase  of  its  video  en- velope.  The  phase  comparison  indicates  the  direction of  the  error.  The  amplitude  modulation  of  the  video envelope   is   compared   to   the   amplitude   of   the   refer- ence  voltage  to  determine  the  amount  of  error. ELECTRONIC    SCANNING Several   techniques   are   used   for   electronic   scan- ning.  The  two  most  common  techniques  used  in  fire- control  radars  are  monopulse  (simultaneous  lobing) scanning   and   phasing   scanning. Monopulse   scanning   does   not   move   the   trans- mitted   beam;   instead,   the   echo   signal   is   scanned   or 2-6






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