Satellite Dish Effects
A focal cloud can render a satellite dish rather inefficient in relation to its size in specific frequencies. If the imperfections are formed in a way that the phase shifts of reflected waves, at a large portion of the dish, are near 180 degrees between them and the feed horn, this results in self-cancellation of the specific frequency.
For countering the effect, there are several techniques, either in construction of the reflectors or lenses, or in the way signal beams are concentrated. In Satellite antennas, scalar rings are used to push the spread signal into the feed horn.
Read more about this topic: Focal Cloud
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