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I was studying this radio receiver circuit enter image description here Then I read that radio waves broadcasted by radio stations will keep traveling in the air forever unless absorbed by something and that radio waves can penetrate most things. I wonder that if, some radio waves broadcasted some years ago are not absorbed by anything till now, can they still be traveling in the air now and like, be picked up by this radio circuit and we hear old broadcasted waves now?

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In air radiowaves will eventually dissipate. The waves that escaped into space however are still speeding away from us, for ever. Much like the sunlight from the day the the dinosaurs were wiped out by an asteroid.

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  • $\begingroup$ Why do they dissipate in the air? $\endgroup$ – user3407319 Feb 15 at 19:22
  • $\begingroup$ And if they do so, how do they reach the receiver circuit anyways? $\endgroup$ – user3407319 Feb 15 at 19:23
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    $\begingroup$ Just like you dissipate energy when walking in loose sand, radiowaves lose energy to the molecules in air. They also lose their coherence. So after seconds a radiowave is completely dissipated. However that is enough to be able to reach a receiver. $\endgroup$ – my2cts Feb 15 at 20:06
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the radio waves that this particular radio was designed to receive are prevented from escaping into space by the ionosphere, and hence bounce around between the earth and the underside of the ionosphere. Each bounce dissipates some of the energy of the radio wave, to the extent that after the 3rd or 4th bounce, the radio wave's strength is less than that of the background noise at that frequency and the signal disappears into the noise. This is true even for extremely powerful transmitted signals.

This means you cannot today pick up old radio broadcasts from the past.

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