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Converting radio waves into electricity is nothing new. Actually this is what happens anytime you listen to a radio or watch a television. Their antennas capture the radio waves, transform them into a tiny electrical current that gets amplified and converted into sound/images.
The problem lies in that “tiny”. To charge a battery you need more than a tiny electrical current (notice that the radio waves create an alternate current -since a wave goes “up and down”- so you need to transform it into a direct current to charge a battery, something you do using a diode).
There is plenty of electromagnetic power in the radio waves, an AM station can emit 50,000W o power but that huge power drops down very quickly, at one km from the antenna it is just 394 mV per meter. That means that if you were to harvest the power of radio waves emitted by that antenna at a km distance from the antenna and you had a big antenna able to capture all those 394mV (which is highly unlike) you will still be way below the 1.5V minimum to charge a battery (and it would take several hours to charge your smartphone battery with a 1.5 V: the average smartphone gets recharged at 5V with a current of 1.5A, that is 7.5W… thousands of time more than what you would get by converting 394mV per meter).
Researchers have looked at ways to make this energy conversion more efficient: one approach is to reduce the distance from the transmitting antenna to the receiving one. This is what happens when you place an iPhone X on the wireless recharging pad. The two are basically in contact hence there is very little energy that gets wasted. Another approach is to create a resonance between the transmitting and the receiving antenna. With this method it is possible to illuminate a lamp bulb a few meters away.
Energous, a US start up based in California announced at the end of 2018 to have got the certification from FCC for its power-at-a-distance wireless charging, WattUp Mid Field transmitter. The transmitter converts electricity into a wireless radio beam and receivers, on the devices to be recharged, convert the radio waves into electricity. The trick they use to have an efficient transfer is using an array antenna with software that can focus the beam exactly on the receiver. It is not using resonance techniques but pulses of radio waves. The distance is still limited, up to 15 feet, but sufficient for home and office application.
