Antenna Fundamentals

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Antenna Fundamentals
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What is an Antenna?

• The IEEE Standard Definitions of Terms (IEEE Std
  145-1983)
• --A means for radiating or receiving radio
  waves
• The antenna is the transitional structure between
  free space and a guiding device
• A way of converting the guided waves present in a
  waveguide, microstrip or transmission line into
  radiating waves travelling in free space or vice
  versa

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• We have
• Substituting (4) into (3) gives
• The electric field

(4)
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• If ßr is large (i.e., ßrgtgt1, or rgtgt?)

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• The ratio of these electric and magnetic field
  components is
• An expression for the complex power flowing
  density out of a sphere of radius r surrounding
  the ideal dipole

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• Bandwidth
• The bandwidth of an antenna expresses its ability
  to operate
• over a wide frequency range. It is often defined
  as the range
• over which the power gain is maintained to within
  3 dB of its
• maximum value, or the range over which the VSWR
  is no
• greater than 21, whichever is smaller. The
  bandwidth is
• usually given as a percentage of the nominal
  operating
• frequency. The radiation pattern of an antenna
  may change
• dramatically outside its specified operating
  bandwidth.

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