TRANSMITTERS Search   and   fire-control   radars   require   high- powered   oscillators   and   amplifiers   to   produce   the transmitted  RF  signal.  The  high-power  levels  required for  these  radars  enable  the  detection  and  tracking  of targets   at   extended   ranges.   Solid-state   transmitters have  replaced  most  of  the  radar  transmitters  that  once used  vacuum  tubes  to  provide  high-power  RF  energy. The   high-power   tubes   used   fall   into   two   cate- gories-crossed   field   and   linear   beam.   Each   type   has different  characteristics,  making  one  tube  more  suited for  one  application  than  another. CROSSED-FIELD    TUBES Crossed-field   tubes   get   their   name   from   the   fact that   the   dc   electric   field   and   the   magnetic   field   are crossed  at  right  angles.  One  of  the  first  crossed-field tubes  used  in  early  radars  was  the  magnetron,  and  it was   the   only   one   available   for   quite   a   few   years. Crossed-field  tubes  are  also  known  as  M-type   devices, since   they   deal   with   propagation   of   waves   in   a magnetic   field. There   are   two   types   of   crossed-field   tubes:   res- onant   and   nonresonant. .   Resonant  Tubes:   Resonant  tubes  are  oscilla- tors   and   generate   the   RF   signal.   The   most   common resonant  crossed-field  tube  used  in  radars  is  the magnetron. .   Nonresonant   Tubes:   Nonresonant    crossed- field  tubes  are  amplifiers  and  generally  will  not  oscil- late,  but,  instead,  will  amplify  an  applied  RF  signal. The  amplifiers  are  subclasses  as  to  whether  they  use the  forward  or  backward  wave  and  whether  they  are reentrant.   (Reentrant  means   whether   the   electrons emitted  by  the  cathode  that  return  to  the  cathode  can reenter  the  charge  that  travels  to  the  anode  or  are  then lost  [wasted].)  Only  one  type  of  nonresonant  crossed- field  tube  has  found  wide  use  in  radar  application:  the crossed-field  amplifier  (CFA).  The  CFA,  discussed  in a  later  subsection,  is  nonresonant,  backward  wave, and   reentrant. 2-9 Magnetrons Basically,   the   magnetron   is   a   diode   and   has   no grid.  A  magnetic  field  in  the  space  between  the  plate (anode)  and  the  cathode  serves  as  a  grid.  The  plate  of a  magnetron  does  not  have  the  same  physical  appear- ance  as  the  plate  of  an  ordinary  electron  tube.  Since conventional   inductance-capacitance   (LC)   networks become   impractical   at   microwave   frequencies,   the plate  is  fabricated  into  a  cylindrical  copper  block  con- taining  resonant  cavities  that  serve  as  tuned  circuits. The   magnetron   base   differs   greatly   from   the conventional  base,  as  it  has  short,  large-diameter  leads that   are   carefully   sealed   into   the   tube   and   shielded. The  cathode  and  filament  structure  is  at  the  center  of the  tube  and  is  supported  by  the  filament  leads,  which are  large  and  rigid  enough  to  keep  the  structure  fixed in  position.  The  output  lead  is  usually  a  probe  or  a loop   extending   into   one   of   the   tuned   cavities   and coupled  into  a  waveguide  or  a  coaxial  line. The  plate  structure  is  a  solid  block  of  copper.  The cylindrical   holes   around   its   circumference   are   res- onant   cavities.   A   narrow   slot   runs   from   each   cavity into   the   central   portion   of   the   tube   and   divides   the inner  structure  into  as  many  segments  as  there  are cavities.   Alternate   segments   are   strapped   together   to put  the  cavities  parallel  to  the  output.  These  cavities control   the   output   frequency.   The   straps   are   circular metal  bands  that  are  placed  across  the  top  of  the  block at  the  entrance  slots  to  the  cavities. Since  the  cathode  must  operate  at  high  power,  it must  be  fairly  large  and  be  able  to  withstand  high  op- erating  temperatures.  It  must  also  have  good  emission characteristics,   particularly   under   back   bombardment, because  much  of  the  output  power  is  derived  from  the large  number  of  electrons  emitted  when  high-velocity electrons  return  to  strike  the  cathode.  The  cathode  is indirectly   heated,   and   is   constructed   of   a   high-emis- sion  material.  The  open  space  between  the  plate  and the  cathode  is  the  interaction  space,  where  the  electric and   magnetic   fields   interact   to   exert   force   upon   the electrons. The   magnetic   field   is   usually   provided   by   a strong,   permanent   magnet   mounted   around   the   mag-