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Originally Posted by phlogiston
In my case, an antenna 10 meters away focuses almost 50 cm closer to the ... apex of the reflector and an antenna 1000 meters away focuses very close to the calculated focal point.
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Unless I'm missing something I think what you are seeing isn't what one would expect from a perfect parabola. I think you are seeing hot zones brought about by imperfections in your construction.
In theory all source wave/signal should arrive at the focal point in phase having traveled the same distance, irregardless of distance from source.
Now on the other hand there may very well be some minimum distance between the source and dish that you begin to see gain diminish.