Basic principle of a graded absorber

  • If incoming wave "sees" a medium with very different electrical properties, the medium will act as a reflector due to the impedance discontinuity. The higher the dielectric constant, the more is reflected. Hence a very lossy medium will act as a reflector
  • Need to match the impedance at the front face of absorber closely to air to minimize reflection

     
     

  • Absorption bandwidth can be increased by cascading multiple layers, similar to bandpass filter design. 20 dB absorption bandwidths of N:1 (n= number of layers) achievable.
  • Wide absorption bandwidth can also be achieved by a gradual impedance taper.
  • Reflections occur at impedance discontinuities with the level of reflection proportional to the amount of impedance discontinuity.
  • By tapering the impedance from close to free space at the front of the absorber to more lossy at the base, reflections are minimized.
  1. Material layer taper (Eccosorb AN)-Material consists of discrete layers graded from low loss at front to high loss at back
  2. Material property taper (Eccosorb HR) - Flat sheet of material graded from lower loss at the front face to higher loss at the rear face.
  3. Physical taper (Eccosorb VHP) - Geometric taper of material to create impedance taper. Typically pyramidal in shape.