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I know this probably varies quite a bit from place to place on earth. But just some rough estimates: if I were to pull power via multiple antennae tuned to a variety of different frequencies, how much "ambient" power could I draw from the atmosphere? Perhaps I'm not asking the right question, but I'm just trying to understand how much unused energy is floating around me right now and if I could use it to say, recharge batteries slowly without an active power source.

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It may give you hint that one of my professors liked to collect plans for "the next revolutionary invention" (mostly perpetual motion and inertia-less drives), and had a folder for these... –  dmckee Sep 22 '11 at 3:29
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@dmckee: what does your comment have to do with the question? –  Ron Maimon Sep 22 '11 at 3:39
    
My prof had a folder full of plans that he had been assured by earnest inventors would solve the energy crisis by taking advantage of the atmospheric electric potential gradient...and there is a big one under good conditions, but there is almost no current available, and so very little power. Or have I misunderstood the question? –  dmckee Sep 22 '11 at 3:44
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There is a voltage gradient in the atmosphere, due to lightning, but it is very hard to extract. You can certainly charge a small battery in a thunderstorm, this is Benjamin Franklin's famous experiment. –  Ron Maimon Sep 22 '11 at 3:46
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@dmckee: this guy wants to charge a battery, not run a city. –  Ron Maimon Sep 22 '11 at 3:47
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The main problem with RF energy harvesting is the low power density (<1µW/cm^2, unless near a transmitter). Other approaches are generally more useful, though even picowatts can be enough for some applications. Some links:

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During WWII in Greece there were radio receivers with only crystals, absolutely no external energy source. That is how the underground and people in general kept in touch with what the allies were doing and what was happening in the world. BBC. I remember playing/listening to one ( ear muffs necessary, power too small) around '46,'47. It is a way of utilizing RF. –  anna v Sep 22 '11 at 13:26
    
@anna v:+1, this is fantastic, thank you for sharing! –  Slaviks Sep 22 '11 at 13:33
    
en.wikipedia.org/wiki/Crystal_radio –  anna v Sep 22 '11 at 15:47
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