I have lots of radio waves (wasting) producing at a distance 20km ,I want to utilize them for electicity production. The antenna is used,does this 20km distance is efficient to generate electricity?
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\$\begingroup\$ This will get you started: en.wikipedia.org/wiki/Faraday%27s_law_of_induction \$\endgroup\$– Houston FortneySep 2, 2015 at 0:45
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\$\begingroup\$ Do you understand why electricity is generated by a changing magnetic field? \$\endgroup\$– Andy akaSep 2, 2015 at 9:49
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You're not going to be able to harness a usable amount of power thanks to the Inverse Square Law. As described in that article, the radio-waves being generated from 20km away will radiate as a sphere, where the intensity at any point "d" distance away will be P/(4*pi*d^2), where P is the power of the source.
If you were using, for example, a 1m by 1m antenna that could perfectly harness the power from the radio waves from a 100kW radio transmitter 20km away, that antenna would only be able to generate about .02mW of power. That's an incredibly small amount of power -- your system would have to run for 8.5 years just to generate the same amount of energy stored in a AAA battery!
