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A silvered mirror will reflect Radio waves. A ocean wave to a Tea cup saucer would smash through and not even notice the saucer. The larger wave length of a RF wave would require a bigger surface to produce a result. Given it is a bigger wave the surface does not have to be a solid it can be as Fred pointed out made of chicken wire mesh. For Microwave as its smaller wave you need a more solid surface.
Also as rightly pointed out by Kal you would need a Feed Antenna at the focal point to convert the EM wave to an Electric current that you can detect. For the microwave spectrum they use a waveguide to pikcup the signal and convert it to electrical currents.
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