Waveguide Impedance Matching

Figure 3-43.—Slot coupling in a waveguide. Minimum  reflections  occur  when  energy  is  injected or  removed  if  the  size  of  the  slot  is  properly  propor- tioned  to  the  frequency  of  the  energy. After learning how energy is coupled into and out of a waveguide with slots, you might think that leaving the  end  open  is  the  most  simple  way  of  injecting  or removing energy in a waveguide. This is not the case, however,  because  when  energy  leaves  a  waveguide, fields  form  around  the  end  of  the  waveguide.  These fields  cause  an  impedance  mismatch  which,  in  turn, causes  the  development  of  standing  waves  and  a  drastic loss   in   efficiency. Various   methods   of   impedance matching and terminating waveguides will be covered in  the  next  section. WAVEGUIDE   IMPEDANCE MATCHING Waveguide  transmission  systems  are  not  always perfectly  impedance  matched  to  their  load  devices. The standing waves that result from a mismatch cause a power loss, a reduction in power-handling capability, and  an  increase  in  frequency  sensitivity.  Imped- ance-changing   devices   are   therefore   placed   in   the waveguide to match the waveguide to the load. These devices  are  placed  near  the  source  of  the  standing waves. Figure 3-44 illustrates three devices, called irises, that  are  used  to  introduce  inductance  or  capacitance into a waveguide. An iris is nothing more than a metal plate that contains an opening through which the waves may  pass.  The  iris  is  located  in  the  transverse  plane of  either  the  magnetic  or  electric  field. An  inductive  iris  and  its  equivalent  circuit  are illustrated  in  figure  3-44,  view  A.  The  iris  places  a shunt  inductive  reactance  across  the  waveguide  that is  directly  proportional  to  the  size  of  the  opening. Notice that the inductive iris is in the magnetic plane. The  shunt  capacitive  reactance,  illustrated  in  view B,  basically  acts  the  same  way.  Again,  the  reactance is directly proportional to the size of the opening, but the  iris  is  placed  in  the  electric  plane.  The  iris, illustrated  in  view  C,  has  portions  in  both  the  magnetic Figure  3-44.—Waveguide  irises. 3-20


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