Some  distribution  systems  will  be  more  complex. That is, four antennas can be patched to four receivers, or  one  antenna  can  be  patched  to  more  than  one receiver  via  the  multicouplers. RECEIVING   MULTICOUPLER AN/SRA-12 The   AN/SRA-12   filter   assembly   multicoupler provides seven radio frequency channels in the 14-kHz to  32-MHz  frequency  range.  Any  of  these  channels may  be  used  independently  of  the  other  channels,  or they  may  operate  simultaneously.  Connections  to  the receiver  are  made  by  coaxial  patch  cords,  which  are short  lengths  of  cable  with  a  plug  attached  to  each end. ANTENNA   COUPLER   GROUPS AN/SRA-38,  AN/SRA-39,  AN/SRA-40, AN/SRA-49,   AN/SRA-49A,   and   AN/SRA-50 These  groups  are  designed  to  connect  up  to  20 mf and hf receivers to a single antenna, with a highly selective  degree  of  frequency  isolation.  Each  of  the six  coupler  groups  consists  of  14  to  20  individual antenna  couplers  and  a  single-power  supply  module, all are slide-mounted in a special electronic equipment rack.  An  antenna  input  distribution  line  termination (dummy  load)  is  also  supplied.  In  addition,  there  are provisions  for  patching  the  outputs  from  the  various antenna  couplers  to  external  receivers. RADAR   ANTENNAS Radar  antennas  are  usually  directional  antennas that radiate energy in one lobe or beam. The two most important  characteristics  of  directional  antennas  are directivity  and  power  gain.  Most  radar  systems  use parabolic   antennas. These  antennas  use  parabolic reflectors in different variations to focus the radiated energy  into  a  desired  beam  pattern. While  most  radar  antennas  are  parabolic,  other types such as the corner reflector, the broadside array, and  horn  radiators  may  also  be  used. PARABOLIC  REFLECTORS To  understand  why  parabolic  reflectors  are  used for most radar antennas, you need to understand how radio  waves  behave.  A  point  source,  such  as  an omnidirectional  antenna  produces  a  spherical  radiation pattern,  or  spherical  wavefront.  As  the  sphere  expands, the energy contained in a given surface area decreases rapidly.   At   a   relatively   short   distance   from   the antenna, the energy level is so small that its reflection from  a  target  would  be  useless  in  a  radar  system. A  solution  to  this  problem  is  to  form  the  energy into  a  PLANE  wavefront,  In  a  plane  wavefront,  all of   the   energy   travels   in   the   same   direction,   thus providing  more  energy  to  reflect  off  of  a  target.  To concentrate  the  energy  even  further,  a  parabolic reflector is used to shape the plane wavefront’s energy into  a  beam  of  energy.  This  concentration  of  energy provides a maximum amount of energy to be reflected off  of  a  target,  making  detection  of  the  target  much more   probable. How  does  the  parabolic  reflector  focus  the  radio waves?  Radio  waves  behave  much  as  light  waves  do. Microwaves  travel  in  straight  lines  as  do  light  rays. They  may  be  focused  or  reflected,  just  as  light  rays may  be.  In  figure  2-39,  a  point-radiation  source  is placed  at  the  focal  point  F.  The  field  leaves  this antema  with  a  spherical  wavefront.  As  each  part  of the  wavefront  moving  toward  the  reflector  reaches the reflecting surface, it is shifted 180 degrees in phase and sent outward at angles that cause all parts of the field to travel in parallel paths. Because of the shape of a parabolic surface, all paths from F to the reflector and  back  to  line  XY  are  the  same  length.  Therefore, all parts of the field arrive at line XY at the same time after  reflection. Figure  2-39.—Parabolic reflector radiation. Energy that is not directed toward the paraboloid (dotted lines in fig. 2-39) has a wide-beam characteris- 2-23