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March 23, 2018 | Author: hmalrizzo469 | Category: Antenna (Radio), Waves, Radio, Electromagnetic Radiation, Physical Sciences


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IEMS  5701  Wireless  Communica7ons  Part  II:  Radio  Wave  Propaga7on  Basics                                    Antenna  Basics   Electromagne7c  Waves   •  Wireless  signals  propagate  in  the  air  in  form  of   electromagne7c  waves   Recalling  Some  High  School  Physics   •  Sta7c  electric  field   •  Sta7c  magne7c  field   . Recalling  Some  High  School  Physics   •  Ampere’s  Law   •  Faraday’s  law  of  induc7on   .  then  both  fields   must  be  considered  together  as  a  coupled   electromagne7c  field  using  Maxwell’s  equa7ons   . t) •  Magne7c  field   B(x. t) •  If  either  E  or  B  are  7me  dependent. y. y.Electromagne7c    Field   •  Electric  field   E(x. z. z. Star7ng  at  the  Beginning   Maxwell’s  Equa7ons  published  in  1865   •  A  set  of  par7al  differen7al  equa7ons  that  form  the   founda7on  of  classical  electrodynamics   . EM  Wave  Propaga7on  in  Free  Space   •  Maxwell’s  equa7ons  in  a  simple  media  with  no   current  and  free  charge:   ∇iE = 0 ∇iB = 0 ∇×E= − ∂B ∂t ∇ × B = µ0 ε 0 ∂E ∂t •  These  equa7ons  predicts  EM  waves   . Radio  Spectrum   . Radio  Spectrum   . What  Happens  When  an  EM  Wave  Encounters  a   New  Media   •  •  •  •  Reflec7on   Diffrac7on   ScaVering   Absorp7on   .   .Reflec7on   •  Wave  hits  an  object  with   dimension  much  larger  than         the  radio  wavelength  will  be   par7ally  reflected  off  the   surface  of  the  object   •  Amount  of  reflec7on   depends  on  the  incident   angle.  materials.  etc. Diffrac7on   •  Apparent  bending  of  waves   around  small  obstacles  and   the  spreading  out  of  waves   past  small  openings       •  The  effect  of  diffrac7on  is   most  pronounced  when  the   wavelength  is  roughly   similar  to  the  dimensions  of   the  diffrac7ng  objects   . ScaVering   •  Waves  are  forced  to   deviate  from  a  straight   trajectory  by  one  or   more  localized  non-­‐ uniformi7es  in  the   propaga7on  medium   •  Rayleigh  ScaVering:   –  EM  wave  scaVered  by   par7cles  that  are  much   smaller  in  diameter  than   the  wavelength   . g.   heat   .  e..Absorp7on   •  The  absorp7on  of  EM  wave  is  o[en  called   aVenua7on     •  The  energy  of  EM  wave  is  taken  up  by  some  maVer   and  transformed  to  other  forms  of  energy. Update   •  We  have  learned  how  radio  waves  propagate  in  the   open  air   •  We  now  discuss   –  Antennas:  to  convert  EM  waves  in  a  wired  circuit  to  EM   waves  radiated  in  the  open  air  and  vice  versa   . A  Simplified  Block  Diagram  of  Wireless   Communica7on  System   •  Example:  speech  or  music  transmission  via   microphone   . Antenna   . Radia7on  PaVerns   Omni-­‐direc7onal   Direc7onal   Direc7onal   Direc7onal   . Antennas  and  Antenna  Arrays   . Antenna  Parameters   •  Efficiency:  the  ra7o  of  power  actually  radiated  by  an  antenna   to  the  electrical  power  it  receives     •  Bandwidth:  the  range  of  frequency  within  which  the   performance  of  the  antenna  conforms  to  a  specified  standard   •  Direc7vity:  the  ra7o  between  the  maximum  radia7on   intensity  radiated  by  the  antenna  in  the  maximum  direc7on   and  the  intensity  radiated  by  a  hypothe7cal  isotropic  antenna   radia7ng  the  same  total  power   . Antenna  Parameters  (Cont’d)   •  Aperture  of  an  antenna:  the  area  that  captures  energy  from  a   passing  radio  wave   •  Gain:  the  ra7o  between  the  intensity  radiated  by  the  antenna   in  a  given  direc7on  at  an  arbitrary  distance  divided  by  the   intensity  radiated  at  the  same  distance  by  an  hypothe7cal   isotropic  antenna              For  example:     G = 4πη A λ2 . e..  i.Reciprocity  Between  Transmit  and  Receive   Antennas   •  A  transmit  antenna  setup  can  be  used  to  serve  as  a   receive  antenna   •  The  antenna  gains  are  the  same.    Gr=Gt   .
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