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Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi Just correct me when I'm wrong in something or talking BS. It happens often in discussion with you big guys!
Now I wish if I had more time to play with these tihngs.
Terry: although there isn't much of information around there, after some lurking around that MIT system and articles you put in (although their information is very very brief) I must admit that these coils *are* series and not paralell resonant (just hanging in air with loose ends) as I first tought they should be for magnetic coupling. You are definitely right about them operating just like tesla coils and expeirencing resonant voltage rise.
Neither now I'm not sure how this benefits claimed magnetic coupling wich would imply maxed current inside the coils, and having a mismatched system as a result.
Now, I don't know what to think about your capacitive coupling theory, it would be defeating for them! Surely, this would be easily veryfyable by simple tests like by turning one of coils by 90 degrees.
One thing I must ask you, how did you come to having all the resonators sharing the same ground plane on 'one end' (like in my cartoon) although they are actually hanging in free air?
Capacitive coupling can surely still be but closes trough ''influence''. All capacitances between resonators and ground play role so I would say it's much more difficult to model.
There is also posible existence of a wall right off that receiving resonator wich is not shown anywhere but could play significant part in coupling capacitance, and etc.
Regarding radiation losses, my understanding is still really poor.
I could think of magnetic field ''reaching out'' from the coil to point where it's at half of wavelength and inducing associated displacement current, getting out of the system as farfield wave.
But since field strength is poor at that point from the coil the impedance match is as well.
Similary displacement current from example TC topload could induce associated magnetic field at a point from topload, but the current is tiny and impedance match is poor again, I guess...
If chris's formula applies then it's clearly right to reduce frequency if radiation losses are proportional to 1/Λ^4. So I would also be evil and say that MIT system operates at way too high frequency.
Now, I don't know what to think about your capacitive coupling theory, it would be defeating for them! Surely, this would be easily veryfyable by simple tests like by turning one of coils by 90 degrees.
If they turn the coil 90 degrees it should not work, but both capacitive and magnetic coupling suggest that. A grounded counterpoise bewteen the coils would not affect it magnetically but would stop the capacitive coupling. Years ago Richard Hull had a video of an unpowered Tesla coil run by another Tesla coil nearby. The disconnected coil had nice little arcs until a grounded conductor was placed between them that blocked the capacitive coupling of the high voltage. If Hull had wired a light bulb to the primary of the unpowered coil, he should have shown just about the same things as the MIT group.
One thing I must ask you, how did you come to having all the resonators sharing the same ground plane on 'one end' (like in my cartoon) although they are actually hanging in free air?
Spice needs lots of grounds or the models fail. The free air coils act like a 1/2 wave coil with the center grounded so I could probably rework the model some there. They couple the power supply and load to the coils of one end which probably messes up tuning and symmetry some. My model is not perfect by any means, but as a first try, it shows the basics.
All capacitances between resonators and ground play role so I would say it's much more difficult to model.
The stray capacitance is easy to deal with since we do it with Tesla coils all the time. With the loaded "Q" of the coils so low, tuning should not be that hard. I wonder how much fiddling they did trying to get the thing in tune.
If chris's formula applies then it's clearly right to reduce frequency if radiation losses are proportional to 1/Λ^4. So I would also be evil and say that MIT system operates at way too high frequency.
I am not sure what the radiation losses are but Chris's calculation seems fine. If they go low in frequency, like the ~300kHz Tesla coils, then they loose the ability to drive current by about 1/30th unless they go to enormous voltages like Tesla coils do. The higher the frequency the better for power transfer until radiation and other problems start to become too much.
Of course, it might not be very practical and it has probably all been done in various ways hundreds, if not thousands, of times before. The MIT group's thing is just a very nice example that "looks good" and photographs well. A very nice example of a very mysterious art
Terry
UPDATE:
I have made up some WiTricity "lenses", for lack of a better name. Just those free floating 1/2 wave Tesla coil transformers...
Just a 1 inch length of #24 wire wound on a 6 inch from. I coated it with EnviroTex and slide it off the forms and trimmed it as sort of a free coil form held together with the epoxy. Had to freeze it all to shrink the form so I could get the darn things off... But they turned out cool...
Then I made some coupling loops with just big wire for the transmitter and receiver:
I think it is #8 bare copper with #22 twisted coming off the ends.
I got out the "toys" and worked on calibrating and compensating them since it has been awhile since they were used for critical stuff:
I am trying to keep things "symmetrical" to avoid odd tuning issues... But it is looking cool!!
Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
Some awesome replies guys, and nice work Terry - I'm going to have to make time to read through this thread properly as I've only been skimming over it.
It'll be interesting to see if homebrew 'WiTricity' devices eventually become commonplace within the HV community. I wish I had more time to mess with this stuff myself!
Registered Member #65
Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155
Terry, I like the Halo antennas you made (split rings). They should form a rather stable resonance with the air gap and wire-twist capacitor..... They come pre-tuned... =)
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Well, I'm pleased to report that I managed to repeat the Wi-tricity experiment. The results were not as impressive as the original MIT experiment, but still positive, and they pretty much confirmed the opinions expressed here.
First I made a pair of what Terry called "Wi-tricity lenses". I used some heavy gauge PVC insulated wire wrapped round two glass dishes. I know PVC and glass aren't terribly good RF dielectrics, but they were the best materials I could find to hand!
Then I tuned the so-called "lenses" by messing with the length of wire and turn spacing, using a signal generator and scope, until they both resonated at 14.3 MHz. This is in the 14MHz ham band, so I'm allowed to splatter around all the RF I want. My signal generator only goes up to 10MHz, so I set it to square wave and used the third harmonic.
Next, I made a coupling loop out of coax, and another receiving loop with a 240V, 40 watt light bulb on it. I connected the coupling loop to the antenna tuner on my 100 watt ham transmitter, and sure enough, I was able to light the bulb. The coax got very hot, however.
Finally, I stuck the coupling loops to the lenses with duct tape (well known for its dielectric properties... :P) and pointed them at each other. With the transmitting "lens" coupled up to the HF rig, the noise level from the receiver increased markedly. That suggests that the lens is indeed functioning as an antenna, and would radiate as Chris suggested. When the receiving lens with its lamp was brought near, the noise level decreased again.
Transmitting at as high a power as possible, with the lenses about 2ft apart, the original bulb barely lit. At 3ft spacing, the bulb would not light at all. I doubt I was getting the full 100 watts, since I couldn't find a particularly good match on the antenna tuner.
I substituted a smaller bulb, and it lit brightly enough at 2ft that I could get a photo. Turning the receiving lens through 90 degrees made the bulb go out. Removing the receiving lens altogether had the same effect: the receiving loop with the bulb had to be brought within 2" of the transmitting lens before it would light again. So the lenses do seem to be directing the RF in some way.
Bringing a hand near either lens would detune it and make the bulb go out.
Conclusion: The "lenses" are probably just small magnetic loop antennas.
Registered Member #477
Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546
Steve Conner wrote ...
When I tuned the so-called "lenses" by messing with the length of wire and turn spacing, using a signal generator and scope, until they both resonated at 14.3 MHz. This is in the 14MHz ham band, so I'm allowed to splatter around all the RF I want.
Wow, neat! I hope you blinked out your call letters on that lightbulb; gotta identify those WiTricity transmissions, after all
Maybe I should fire up the HF rig and give this a shot...
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi steve, terry, everyone,
Steve, you are cool! Could you maybe get the Fres down and see if you can hit 'true' resonance without harmonics with that thing?
I would say that those operate just like bipolar HF tesla coils. They develop HV across their ends and have definite ability to strongly couple capacitively.
This is how I think the most simplified version of mit system should look like. SUrely you guys have to fill in a lot in there, so sorry if I'm just being a nuisance.
There is clearly both electric and magnetic coupling present, although I'm also starting to believe magnetic actually plays minor role in the story. There is no truly heavy currents flowing anywhere in system that could produce required B in few turns of wire.
Terry: I did have a brain fart, you are right that in any case the thing wouldn't work turned 90 degrees. In that case both ends of the coil will be at closely same potential and no current will be induced.
You are also right about the way for getting rid of capacitive coupling. When same side ends of both resonators are grounded and phasing is correct only effect of magnetic coupling should be seen... although for some reason I don't believe it would do the job very well.
I imagine it would be the same as if without ''lenses'' and with coupling loops only like steve tried out. The resonators don't actually give any ''magnetic field boost''.
I would gladly play with this stuff now but I'm out of time and ill with serious polyprojectitis.
I might try something this summer but will have to improvise with lower freq's and homemade drivers.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Hi Firkragg,
Maybe I didn't make it clear, but I only used the signal generator and square wave for tuning. To do the actual experiment I switched over to my HF transmitter, which does drive the system at its true resonance.
I'm going to try a Faraday screen between the transmitter and receiver some other time. That would stop the E-field and only allow the H-field through.
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Wireless energy again
