The parabolic dish can be a highly useful way of increasing signal strength. In and of itself, the parabolic dish is not an antenna. It is used with an antenna to greatly increase the gain of the antenna as well as the directivity; the antenna and dish work in unison to create this result.
How a Parabolic Dish Works
At a glance, a parabolic dish is a type of reflector, but its unique shape renders it extremely effective at concentrating high amounts of energy. Because of its shape, RF reflecting off of the parabolic surface of the dish will be reflected towards a single point above the middle of the dish. The result is a massive concentration of energy at this focal point. The actual antenna is then placed at the focal point of the dish.
Gain
The amount of gain possible with a dish is enormous. In a receiving type of scenario, the dish will concentrate to its focal point a quantity of RF energy directly related to its size. Generally, RF signal tends to pass through space more or less evenly across a given surface area. So, what the parabolic dish can do is intercept a large area of available RF and concentrate it to its focal point, resulting in an intense signal at the antenna. The bigger the size of the dish, the more surface area of available RF it will intercept and focus to the antenna. Thus, the gain can be increased by merely enlarging the dish.
Limitations
For any parabolic dish, of course, the antenna will need to be placed at the focal point for the characteristic parabolic dish results to be obtained. This can cause mounting difficulties, for the antenna must invariably be suspended somewhere over the top of the dish. Furthermore, bigger dishes will, of course, weigh more and can quickly become unwieldy.
By far the most significant limiting factor, however, is the quality of the dish itself. A parabolic dish relies on its shape to concentrate RF properly. This shape must be maintained without flaws in order to focus power correctly. For small dishes, this is easy enough, though the manufacturing process must still be reasonably precise for good results. As the dish becomes larger, it becomes much more difficult to achieve an exact shape.
In the end, though a dish can, theoretically, be made as large as desired, practical mechanical problems limit the maximum dish size, for the shape needs to be maintained exactly for the proper results. And, as the dish gets larger, small errors towards the edges of the dish can easily result in the RF missing the antenna at the focal point altogether.
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