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From this answer: How much energy can be harvested from nearby radio towers or power lines? What circuits would you use? How much would you have to absorb for it to be noticeable by the producers or by other consumers?

I see a lot of stories, rumors and anecdotes online, but I'd like facts and references better.

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    \$\begingroup\$ As far as "can be harvested", I would like to know both meanings, "how much is legal" and "how much is practically possible". I remember hearing the phone company would get mad if you used too much from the phone line, but "rumors and anecdotes" aren't very useful except for spreading more rumors and anecdotes. \$\endgroup\$ – Jack Schmidt Oct 1 '10 at 0:26
  • \$\begingroup\$ "what is actually possible" is interesting thing. and does it means if we don't harvest , it get wasted in heating air ? \$\endgroup\$ – iamgopal Oct 1 '10 at 5:21
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    \$\begingroup\$ It doesn't get wasted in heating air, since air doesn't absorb the energy. Except maybe a little, but we transmit on certain frequencies specifically because the air doesn't absorb them. en.wikipedia.org/wiki/… \$\endgroup\$ – endolith Oct 1 '10 at 14:13
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    \$\begingroup\$ i'd rather gather energy from that huge object that emits electromagnetic waves in the range of IR, visible light, and even UV. unfortunately, it's only available at day. \$\endgroup\$ – Stefan Paul Noack May 15 '12 at 13:38
  • \$\begingroup\$ Related: Electromagnetic charging \$\endgroup\$ – Phil Frost Jan 17 '14 at 19:19
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Back when I was in grade school I had a crystal radio set. A crystal radio contains no amplifier. The output signal is completely powered by what is picked up from the antenna. I had around a 50 foot length of wire running out my bedroom window to a shed in the back yard as a antenna. With that I could pick up a 50 kW AM station over 20 miles away quite clearly. It was reasonably loud with headphones, a few kΩ impedance. I hooked up a impedance matching transformer to drive a 8 Ω speaker with it. The radio program was easily audible with my ear up to the speaker. Sometimes I left it on at night to annoy my brother. You couldn't make out what was said accross the room, but you could hear what sounded like distant talking, enough to be annoying if you didn't know what it was.

I can't say how much power that actually was, but enough to harvest and use by a low power intermittent device is possible.

As for loading the transmitter, that only happens in the near field. The radio station I mentioned broadcasts at 1.03 MHz, so the wavelength is about 290 m. For anything much beyond that, the power is already propagating irrevocably from the transmitter such that it can't see any loading. Put another way, the transmitter has already been loaded with that power, whether you use it or let it propagate into space forever. Since I was over 20 miles away, I was well past the near field. My receiving the signal only reduced the field very slightly in the vicinity of my antenna. As far as I know, there is nothing illegal about using a 50 foot wire antenna completely on your own property to receive AM radio stations.

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  • \$\begingroup\$ I had one too. We called them "long wire" antennas. I had an early germanium diode but I also had my Dad's galena crystal and cat-whisker (still do). You have to assume the people who own the transmitter are expending the power so you can use it to listen. If you use it for something else, the courts (in the U.S. decided long ago that you don't own the radio power passing through your house or property, so it is an ethical question. \$\endgroup\$ – C. Towne Springer Jan 18 '14 at 3:06
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Without trying to attempt to do any mind numbing number crunching I am just going to suggest that harvesting energy from radio towers would not be very cost effective. It would be much cheaper and more realistic to look at other sources in the environment for energy harvesting. EM signals transmits minuscule amounts of energy to an antenna that is amplified by another power source....

EDIT:

I know this post is old, but I stumbled on a device today that made me revisit the idea.. There are devices on the market that does exactly what you are proposing. An example of such is the Powercast.

Looking at the datasheet for the P1110 we can see that the max input is 20dBm (100mW). At 20 dBm the data sheet states that the Efficiency would be around 60% which would give us a 60mW output.

A typical transmission power of a FM radio station would give an output of 100kWs. Say we were 1Km from the source and assume the antenna gain to be 1, than using the formula below the power flux would be ~7960 W/Km². This would mean that the antenna surface area would have to be around 12.6 m² which is reasonable. While you may not be able to run your television off such power, you would be able to run smart devices such as the MSP430F2001 which is a microcontroller that only takes several hundred micro amps to run in active mode.

$$ w = \frac{p_r G_r}{4 \pi d^2} $$

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Wireless power distribution is a load of bollocks! Pardon my directness, however it is not realistic.

Nikola Tesla was determined to do this along with General electric as his main sponsor. He proposed a network of towers emitting power for people to use, the major downfall was that there was no way to bill people. So GE pulled the plug. The other issues are to do with interference from such a high voltage electric field, it caused all sorts of problems where it was trialed, so it's not a realistic option.

High voltage transmission lines, and radio towers, do emit energy, it is spherical and goes out in three dimensions, so for a larger radio tower, of for example, 100 kW transmitter, for each meter you are away from it, the amount of power that you can possibly receive is calculated by the inverse square relationship. At 100 m away, the amount of energy is 100 kW/(4·pi·r2), where r = 100 m. This equates to 79.58 W/m2. So if your antenna dish is one square meter in area, and you stand at 100 m from the 100 kW transmitter, at best, you have enough power to run a incandescent light bulb. not much really. move away another 100 m, and you are down to 40 W. This is a large antenna, and for not a great return, and you will most definitely be questioned. I doubt, however, it would really have an effect on people receiving a signal, you will only create a tiny blind spot in the transmitters path.

On the other hand high voltage transmission lines conduct most of their energy, to there is far less radiated out. Similar maths can be performed to work out the radiated energy and potential recieved energy. In doing this, you are not "stealing" energy, just collecting waste, and recycling it, so it should not pose a problem to anyone, it may raise some eyebrows with your neighbors, depending on where you live.

All of these forms are far field radiation, near field radiation does work for transmission of energy quite effectively, but in this situation, you are transmitting and receiving, so it is not free!!!

There are of course, RFID devices, which do not consume much energy, so do work quite well using far field radiation, but not much energy is available, just enough to power some small microprocessors.

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    \$\begingroup\$ I'm not sure this is accurate. Radio tower patterns are spherical? I thought they had high gain in the horizontal direction. Why radiate sports radio upward into space? Why build a dish antenna? Wouldn't a resonant antenna pick up more energy? Would a 1 m dish really create a blind spot? Wouldn't the waves just diffract around it? I'm not sure what you mean by "power lines conduct energy". Energy is carried in the fields surrounding power lines, not within the lines themselves. \$\endgroup\$ – endolith Oct 23 '10 at 14:46
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    \$\begingroup\$ "In doing this, you are not "stealing" energy, just collecting waste, and recycling it" - Is incorrect. By placing energy collecting circuits along a transmission line you change the impedance (this is by definition - otherwise you wouldn't get any energy out) which removes more energy from the lines than the environment normally would. If you excavated all the earth underneath them, and only changed the impedance as much as the earth was doing, then you could say that you aren't stealing more than the environment naturally does. \$\endgroup\$ – Adam Davis Oct 23 '10 at 16:47
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    \$\begingroup\$ Power companies measure line loss, and if it's different than expected, they inspect the lines and surrounding land. Changes in impedance often signal problems that should be examined prior to a failure. \$\endgroup\$ – Adam Davis Oct 23 '10 at 16:51
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    \$\begingroup\$ What costs less, several square meters of metal mesh, or a tiny solar panel that captures the same amount of energy? Guessing the solar panel, and nobody will come sue you. \$\endgroup\$ – Matt B. Jul 8 '11 at 20:03
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    \$\begingroup\$ I don't like the Tesla / General Electric anecdote. I see it so often and I'm sure GE would have been concerned about not being able to bill people for it, but why is there never any mention of how wasteful wireless transmission would be? That seems like something Tesla would have worried about before GE even had the chance to get on board with the project. \$\endgroup\$ – bhillam Mar 19 '13 at 5:17
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In EEVBlog #55 Dave Jones dissects the (bogus) claims of a gadget which can harvest energy from Wi-Fi signals. Most of the mathematics should be applicable to radio towers.

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  • \$\begingroup\$ Wi-Fi routers transmit using milliwatts. Radio towers transmit using kilowatts. \$\endgroup\$ – endolith Oct 1 '10 at 16:03
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    \$\begingroup\$ Indeed, so the maths should scale up... \$\endgroup\$ – Toby Jaffey Oct 1 '10 at 16:14
  • \$\begingroup\$ They're also different frequencies. 12 cm wavelength for Wi-Fi vs 560 m for AM. \$\endgroup\$ – endolith Oct 1 '10 at 17:45
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    \$\begingroup\$ Distances also scale up...usually you are feet away from wifi and miles away from radio towers \$\endgroup\$ – davr Oct 1 '10 at 19:55
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    \$\begingroup\$ For an (arbitrarily decided) FM station on 101 (ish) MHz, you're looking at (very roughly) 72 dB of path loss for 1 km. For a 300 Watt station, that means a received power of something like 30 uW. This could be plus or minus 50 times, but it still wouldn't matter. On the optimistic side of 50 times, it's still only 1.5 mW. \$\endgroup\$ – Jesse Oct 3 '10 at 8:20
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If you are literally within the shadow of a broadcast AM radio broadcast antenna tower (but likely NOT an FM or TV), or if you were LITERALLY directly under the path of a very high voltage AC transmission line, then you might be able to practically "harvest" a few milliWatts of power. But zoning laws in most places prohibit people from living in either of those places.

There is nothing of a practical nature you can construct that will materially affect the performance of the broadcast pattern or the losses of the power transmission line. But I recall cases where electric utilities took a dim view of people laying out big coils or antennas under their transmission right-of-way.

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