[TowerTalk] Takeoff Angles and Non-Reciprocal Propagation

[David Gilbert]

Mike, thanks again for the reply.

Yes, I fully agree with your description of reciprocity below.  No argument.

In the context of a radio signal traveling both directions between two 
points, however, that isn't quite the situation we have.  The wave 
leaving A gets diffracted and dispersed into a bunch of waves as it 
passes over a terrain feature (surface), but (for the sake of argument) 
only one of them leaves at the proper angle to propagate through the 
ionosphere to arrive at B.  When B sends a return wave back down that 
same path, and it arrives back in the vicinity of A (presumably at the 
same angle), it arrives as a single wave ... not the collection of waves 
that comprised the original wave.  So for the single wave coming from B 
all bets are off, and it sees what it sees ... which for an asymmetrical 
feature might look entirely different (relative to a wavelength) from 
the B side than what the original signal saw from the A side.  That 
would result in a different diffraction pattern for the waves continuing 
on in the direction of A.

I think I'll still work up a diagram ...

73,
Dave   AB7E


Michael Keane K1MK wrote:
>
>> My basic hypothesis is still the same, though, at least until 
>> someone can help me wrap by mind around an explanation of my error, 
>> that an HF signal (a wave of energy) could react differently to an 
>> asymmetric terrain feature when traveling from opposite directions 
>> along the exact same path as long as the asymmetry was significant 
>> relative to a wavelength.
>>     
>
> Dave, there is no error in your thinking. The only confusion is about 
> exactly what "react differently" means.
>
> Suppose I send a plane wave at some complicated diffractive surface. 
> Let's say this plane wave was initially moving from left-to-right in 
> one direction with a uniform amplitude. Upon encountering the 
> surface, the wave gets diffracted and ends up going off to the right 
> in a bunch of different directions with differing amplitudes and phases.
>
> What reciprocity says is if I send in a bunch of waves from the right 
> from different directions with differing amplitudes and phases such 
> that they replicate the output seen above, I'll get a copy of the 
> original wave moving to the left.
>
> 73,
> Mike K1MK
>
> Michael Keane K1MK
> k1mk at alum.mit.edu
>
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