[TowerTalk] Diffraction Reciprocity

[Chris Wendling]

Well, why not wade into this interesting topic? Would have sooner, but  spent most of the weekend working the SS CW contest with W8RJL's group  under the K4IX call.
  Had a clean sweep and had a blast!
  
  Let me put my 2 cents in by proposing a simple thought experiment.
  At the local transmitting antenna, a radio wave is emitted which spreads out.
  
  Were the transmitting antenna an isotropic radiator in free space, the  wave front would  remain spherical in shape as it spreads.   In our case, the wave still spreads out,  not exactly spherical,  but nonetheless it has a curved wave front, who's radius of curvature  generally decreases as it moves away from the antenna. This wavefront  can reflect and diffract from the surrounding physical environment,  setting up a pattern of waves with destructive and constructive  "interference", depending on the wave phases from the reflections and  diffractions. These interference patterns are ingeniously employed by  hams with yagi's to modify the radiated power pattern to their  advantage, proving gain in the direction of interest along with other  advantages.
  
 Now, consider a far away transmitter, sending a  similar frequency signal to our local antenna.   In this  case, as the radio waves approach our local antenna, the wave front  radius of curvature is VERY much less than the radius of curvature that  our local antenna emitted towards the local diffracting objects.   Because of this, the times of arrival  of the wavefront to those  diffracting objects will be different, and the resultant wave phasings  will be different.
 As we all know, if the phasing of the wave  fronts arriving at the antenna are different, the constructive and  destructive wave interference patterns must also be different.  Therefore, I have a problem believing that "reciprocity" can exist in a  diffracting environment when the geometries are not symmetrical.
  I'm sure this will further fan the discussion rather than resolve it,  as these issues are seldom settled in this kind of communications  environment; perhaps reciprocity doesn't exist on Internet reflectors  either! But remember: Just some food for thought, and have fun with  it!  
  Respectfully submitted,
  CW, AI4MI