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Wireless energy again

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Avalanche

Thu Jun 07 2007, 06:28PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

I thought this was interesting! I remember a discussion here a few months ago, mostly about the efficiency. Looks like they have managed a maximum of 40% efficiency with the current setup.

nrhoades

Thu Jun 07 2007, 07:19PM

Registered Member #610 Joined: Wed Mar 28 2007, 09:44PM
Location: Middletown, RI
Posts: 110

Right now I'm actually working on a "rectenna" device, but I can not release further information :)

Chris Russell

Thu Jun 07 2007, 11:09PM

... not Russel!
Registered Member #1 Joined: Thu Jan 26 2006, 12:18AM
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I would be interested to see how much of the 60% loss is dissipated as heat, and how much is radiated as RF energy. Those coils look like they would make excellent magnetic loop antennas, so I wouldn't be surprised if a large portion of that was, in fact, being radiated. At 10 MHz, loss due to radiation is going to be a difficult hurdle to overcome. The finished product is also going to need a tightly controlled oscillator that generates a nice sine wave, in order to avoid polluting the RF spectrum.

nrhoades

Fri Jun 08 2007, 05:03AM

Registered Member #610 Joined: Wed Mar 28 2007, 09:44PM
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The biggest problem is that the diffraction limit of RF energy is rather large, i.e. a beam from an antenna diverges rather quickly as one gets further from the source. The area of collected energy from a receiver or rectifying antenna is only small portion of the transmitted energy area. The goal is to transmit RF with negligable divergence (like a laser beam). This requires extreme antenna conditions... A group from MIT is working within the diffraction limit, so I'm curious to how they do it.

Steve Conner

Fri Jun 08 2007, 08:59AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

That circuit in the BBC link doesn't make any attempt to direct or radiate the energy. It just creates a big throbbing reactive field that fills most of the room, from which another tuned coil can extract energy. Because it's not radiating anything, it doesn't need to direct. But the flipside of that is that the range is extremely short.

It's basically the same as Tesla's schemes, except they left the Tesla resonators out and just used the primary coils. Without the step-up effect of the resonator, the field is much lower impedance: it's all H with very little E, whereas a Tesla coil gives you all E and hardly any H. So, we can expect that most of the energy losses in the new system will be from eddy current heating of objects in the room, whereas with the classical Tesla system, it would be dielectric losses in your couch.

Regardless, both of these E/H ratios are poor matches to the impedance of free space, so you can be sure that the fields are strongly reactive and not much power will get radiated. This is always the case when an antenna occupies a region that is small compared to a wavelength.

However, even if it only radiated 1% it could still potentially make life misery for the local hams...

Bert

Fri Jun 08 2007, 01:18PM

Registered Member #118 Joined: Fri Feb 10 2006, 05:35AM
Location: Woodridge, Illinois, USA
Posts: 72

Avalanche wrote ...

I thought this was interesting! I remember a discussion here a few months ago, mostly about the efficiency. Looks like they have managed a maximum of 40% efficiency with the current setup.

I just had a look at the actual article in the June issue of "Science Express" - here are some more details. They are using identical self-resonant helical coils, each 60 cm in diameter and 20 cm high, wound with 3mm copper wire. Each coil has 5.25 turns, and a resonant frequency of 9.9 MHz, and both are aligned along a common axis of rotation. Interestingly, they predict a theoretical Q of 2500 and measure an actual Q of 950 +/- 50. RF power is inductively coupled from a 400 watt Colpitts power oscillator into the "sending" coil via a single turn 50 cm diameter loop. Power is extracted via a single-turn loop in close proximity to the resonating receiving coil. For this system, they predict an efficiency that's proportional to the coupling coefficient. They adjusted the distance between the receiving coil and load loop for optimal performance. Wallplug power into the RF driver was 400 watts, lighting the 60 watt lamp 2 meters away, for an overall power transfer efficiency of about 15%. The inventors claim that the receiving coil can be made considerably smaller without decreasing efficiency (as long as resonance is maintained)...

Carbon_Rod

Fri Jun 08 2007, 08:11PM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

Such circuits have been around for a long time (similar to RFID systems, however they rely more on LF inductive coupling for communication.)

As Chris pointed out, inverse field laws still apply... and the lower the Hz generally means its harder to make it directional.

Practical use seems limited (perhaps the VW factory floor is the exception.)

Cheers,

DrZoidberg

Sat Jun 09 2007, 12:00PM

Registered Member #350 Joined: Mon Mar 27 2006, 05:14PM
Location:
Posts: 106

There already is a product on the market for transmitting power over a distance. The wireless extension cord

Rather practical too. You can power your appliances and cook your food at the same time

Avi

Sat Jun 09 2007, 12:38PM

Registered Member #580 Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410

that was an april fools IIRC

Marko

Sat Jun 09 2007, 01:32PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Steve is correct about his point. The entire thing is actually about transmitting the power without radiating it.
Power can be transferred in nearfield via capacitive or inductive coupling, and it is magnetic in this case. And I bet Tesla played with both of these 100 years ago.

A tesla resonator excited at resonance will develop enormous voltage on it's topload.
Another resonator brought in victinity will show a tiny intercapacitance with transmitting resonator.
The 'receiver' is tuned to resonance to present as high as possible impedance to the transmitter, for lowest losses.

High impedance can be converted to low by putting the load on resonator's base or using a magnetically coupled winding.

With each cycle, small amount of energy is transferred trough small coupling capacitance and most of it remains stored in topload capacitance, and some (unfortunately too much for us) is dissipated as ohmic loss in resonators.

Magnetic coupling is pretty analogous. But this time it is paralell so current expeirences resonant rise instead the voltage, and there is small mutual inductance between 'transmitter' and 'receiver'.

Again, small amount of energy is transferred with each cycle while most is reactive.

The receiver is tuned resonant to represent lowest possible impedance to transmitter, and converts it back into high(ish) impedance on it's far ends to power the lightbulb.

They seemingly still needed aditional windings for impedance amtching, though.

Nothe that there is no absolutely any kind of radiation anywhere in both cases, and actually any EM radiation escaping the system is considered a loss.

I don't know how to model radiation characteristics for a helical resonator, my antenna theory knowledge sucks in overall.

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