[TowerTalk] Takeoff Angles and Non-Reciprocal Propagation

[Richard (Rick) Karlquist]

Directional couplers are reciprocal.  Are you implying
that they might not be?

An example of a non reciprocal circuit is a circulator.
This requires a non-reciprocal medium (ie ferrite).
All electromagnetic structures with reciprocal media such
as air as reciprocal.  If there were a way to make a
non reciprocal microwave structure using only air, it would
have been discovered and exploited long ago.  It would
be worth a lot of money.

I understand what you are saying below.  What you are
saying is that non-reciprocal propagation is possible
if diffraction is involved.   What I am trying to say
is that non-reciprocal propagation in general is impossible,
including diffraction effects.  It is not necessary to
specifically prove diffraction is reciprocal.

The reason why I mentioned the other concepts was because
you brought up HFTA.  It would address those other
concepts (except noise) but not reciprocity.
Again, if there were non-reciprocity, HFTA
would ask you to specify the direction of propagation.

Perhaps an optical analogy would be easier to understand.
Show me an example of where I can shine a laser beam at
a corner cube mirror and have the beam reach the mirror
but not be able to reflect back to the laser source.  You
can include diffraction effects in the path.


Rick N6RK

David Gilbert wrote:
> Rick,
> 
> I don't see how your 2-port S-parameter example is analogous at all.  If 
> anything, a directional coupler might be a closer analogy to this 
> discussion. 
> 
> Also, I know that reciprocity does not require the takeoff angle of the 
> transmitted signal to be the same as the arrival angle of the receiving 
> station at the far end, and I did not say or imply that it was.  I'm 
> also not implying differences in noise levels on the two ends of a path, 
> I'm not implying differences in path loss for the two directions, and 
> I'm for sure not referring to station A working Asia from the east side 
> of a hill while station B works Africa from the west side of the same hill.
> 
> I'll try to simplify the hypothesis.  Assume Station A in Colorado has a 
> yagi pointed east and it is located a few thousand feet west of an 
> asymmetrical hill (steep on the west side, blunt on the east side) high 
> enough to cause diffraction products that bend his signal lower than it 
> would be without the hill.  Let's say the partially diffracted signal 
> has a takeoff angle of 4 degrees.  Station A's signal travels from the 
> hill upward at that 4 degree angle where it interacts with the 
> ionosphere and eventually returns to earth (at some arbitrary angle 
> depending upon propagation) at Station B in Spain.  Station B is 
> identical to Station A in terms of power, antenna gain and efficiency, 
> ground conductivity, and whatever other factors may normally affect 
> signal strength.  Station B receives the signal from Station A and sends 
> a reply.  That reply travels back toward Station A along the same path, 
> suffering the same path losses, and presumably arrives at the 
> asymmetrical hill at the same 4 degree angle that Station A's signal 
> left it.  However .... the signal from Station B is not "processed" by 
> the asymmetrical hill the same way that the signal from Station A was.  
> The asymmetrical hill (blunt on the east side) does not bend Station B's 
> signal downward toward Station A's antenna the same amount as the signal 
> going from A to B was bent. 
> 
> It seems to me that a good portion of Station B's signal arriving at the 
> hill could overshoot Station A because it wasn't diffracted enough.  The 
> path taken by the signal from B to A would be identical to the path 
> taken by the signal from A to B EXCEPT for the portion between Station A 
> and the hill.  Think directional coupler.
> 
> So where am I wrong?
Di
> Dave   AB7E
> 
>