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One of Maxwell's equation gives the reason why a perfect or even a good conductor can reflect EM waves (with a phase change). This essentially states that the magnetic flux is zero on the plane of a conducting surface, therefore the energy is reflected with a phase change to maintain the boundary condition. This means that even the surface of the earth reflect EM waves. As such you can and i think they have built large reflectors using craters or natural valleys between mountains. The earth acts as the reflector.
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