bending the earth!
If the sender and the receiver are too far what happens? We find a problem: the earth is a sphere and the propagation of the electromagnetic wave is straight. Fortunately there are some natural features that gives us an hand: the effect of the surface waves and the refraction by the ionosphere. So we can transmit in HF also at more than 300 kms.
The first thing that we want to transmit is the voice, after understood how the voice is working (at what frequencies) we arrived at the conclusion that we need to "shift" our information (the voice) up in the spectrum of frequencies, to do this we need to build a moltiplicator and to understood how to recover the original voice information.
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