In the case of light, only a small portion of the light impinges on the
knife edge (from either direction). We're not talking about yagi
antennas in free space with lots of capture area and nothing in between,
but yes of course that is a reciprocal scenario.
In the case of a real antenna at Station A with asymmetrical terrain
features in front of it, the net radiation pattern is the sum of all of
the waves that travel directly over, reflect off, and diffract over the
edges of the terrain, but (for the sake of argument) only a very small
segment of that energy takes off at the proper angle to reach a distant
Station B. Assuming reciprocity in the ionosphere, a signal returning
from Station B will arrive at that same specific angle to the terrain in
the foreground of Station A, except in the case of asymmetric terrain it
doesn't see the same terrain profile as the outgoing signal did, and
therefore the path taken by the return signal (the sum of all the
direct, reflected,and refracted rays) could easily be different. That
signal may now arrive at the antenna weaker in strength (different sum
of rays) or it may (as K8RI has pointed out) arrive at a different angle.
Dave AB7E
Dan Zimmerman N3OX wrote:
>> Now pick any
>> spot on the wall that has been illuminated and shine a light back toward
>> the edge .
>>
>
> Imagine a pair of yagis in free space. What one hears from the other
> one depends on their relative orientation, correct?
>
> The pattern of an antenna in free space is an interference pattern...
> the radiation from different parts of the antenna interferes to form a
> pattern on transmit. Much of the power transmitted from one antenna
> "goes out in random directions" and will not be intercepted by the
> receiving antenna. Yet I doubt you'll tell me that a pair of
> identical yagis in free space won't obey reciprocity...
>
> Adding the knife edge makes your antenna's response to incoming
> signals from beyond the knife edge rather complicated, but it's still
> just big antenna with a pattern formed by interference. Everywhere
> there's a bright spot has high response, everywhere there's a dark
> spot is a null. The earth is just part of your antenna.
>
>
> I'll revisit this with coffee in the morning or maybe someone else can
> chime in about antenna reciprocity itself. It's too late for me to do
> anything but mumble "time reversal symmetry" and go to bed.
>
>
> 73,
> Dan
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