Rectenna : Basic questions

[msbauer]

Hi.  I'm a mechanical engineer who is just getting into electronics hobby stuff for both home and work projects.
so if you could reply as if I'm a small child, I'd appreciate it...

NOTE: Question is at very bottom of this post after I give background context.

I've been seeing videos on youtube of people using crystal radios to power LEDs.  This made me excited as I thought it meant that I should be able to charge a battery using the same approach.  Or maybe power LEDs in devices in the size range of .5 square ft without batteries.

What became immediately apparent with a little googling:
There are a lot of crazies out there spinning tales about this stuff.
AM radio is perhaps the less desirable spectrum segment for this application in my view since it requires such long antenna/inductor.
Using a set of simple calculations given in an EEVblog youtube video, it seems that one would have to build a device (antenna and circuit) that would be compatible with as many spectrum segments as possible (AM, FM, UHF, VHF, etc.) and tune them all simultaneously to get an amount of power that would be useful. (I've found some papers which are attached that speak to using fractal antenna and special circuit designs)

For example,
Broadcast Antenna Power (Watts)                           50000
Receptor Antenna Distance from antenna (Miles)   2
Receptor Antenna size (m2)                                   0.01
Application Power Requirement (Watt Hours)           2
Power Density @ Receive location (Watts per m2)   0.000384258
Watts received by Receptor Antenna (Watts)           3.84258E-06
Charge Hours (@100% Efficiency)                           520,484.0

It seems to me that this kind of power reception could NEVER power an LED.  Am I completely mis-understanding a fundamental EE concept?

With all of this in mind, I still can't get past the apparent fact that in these videos, hobbyists are lighting LEDs with a crystal radio tuning only 1 frequency.  Is there some simple math that can demonstrate why the LED can be powered, but my numbers above seem so incompatible with that reality?

[SeanB]

If you are living close to the transmitter you will get quite high signal levels from it. As most US AM transmitters start at 1kW and typically are a 10-50kW unit with some being up to 2ookW you will have a very strong signal near to them, definitely enough to provide 10mW or so to a LED to light it. A single wire parallel to a 132kV power line about 2 pylons long and grounded at one end will couple enough power by induction to light up a small lamp. Charging batteries and such from it though is not going to work, the inverse cube law of light ( radio waves are light of a long wavelength) will always prevail. Even wireless charging mats only work with poor efficiency and are just convenient, as you do not have to plug in a plug just place the device directly on top of the mat to power it via a very lossy transformer.

[ElectroIrradiator]

You are not missing anything. Using rectified RF from random radio transmitters are in general not an useful source of electric energy.

To emphasize what SeanB wrote, then there are a few corner cases, where you are *really* close to the transmitter, and where you can get enough energy to light for instance a LED with relative ease. In general though, this is a waste of time. In most locations there is simply not enough total radio energy per unit volume to make this worthwhile for anything but powering pico-power devices. This is another one of those cases, where everybody would already be doing it, if it worked.

As for the YouTube videos, then the people in them are either unaware they have a nearby powerful transmitter, they have a really high and large outside antenna, or they themselves are supplying the RF energy to make an interesting video (Ie. they cheat). Normally, even if you used a very large, outdoor wire antenna, you could only expect maybe a few mW of rectified power. But in most locations the mere cost of building a sufficiently large antenna to light a low current LED, would cover the cost of the electricity to light the LED for centuries...

[G7PSK]

At least in the UK harvesting power from radio or TV transmitters is illegal. There was a case in the 1950,s where a man living near the Crystal palace transmitter set up a massive roof top array and was lighting his house with the power, he was quickly bought to book due to the shadow in reception that he caused.

[Zero999]

It depends on the wavelength, the longer, the further you should be able to grab power from the near field, like a transformer which is why it's possible if you live very near an AM transmitter.

[Crazy Ape]

Energy harvesting/scavenging in various forms has become interesting enough for the big players to get involved.
 
RF from existing transmitters.
http://www.technologyreview.com/news/518111/devices-connect-with-borrowed-tv-signals-and-need-no-power-source/

Energy harvesting from vibrations.
http://www.eetimes.com/document.asp?doc_id=1203136