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Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
Firkragg wrote ...
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.
Very good explanation Firkragg, it is trivial thinking of it this way, but in terms of EM radiation I had doubts I would ever get what is going on
Firkragg wrote ...
Note 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 am not happy with how the discussion turned out to emphasize the differece between "radiation" on one hand and "capacitive coupling" / "mutual inductance" on the other. These things cannot be distinguished, as is explained in here Usually Steve links to this article, but he seems to have gotten tired of it What we are really talking about here is the difference between near and far field, and this is also what limits the range of this whytricity gizmo so much. In the near field power falls off with r^-4 rather than r^-2, so this is presumably what limits the practical range to a few feet.
Still good enough for charging cell phones in your pocket, I'll buy one when the product hits the market
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I don't see how what I said disagrees with the content of Paul Nicholson's article. Capacitive/inductive coupling is how you transfer power in the near field, radiation is how you transfer it in the far field.
So capacitive/inductive coupling is different from radiation, in the same way that the near field is different from the far field. But I agree that this is indeed a near field device and the 1/r^4 restriction applies.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi joe!
I am not happy with how the discussion turned out to emphasize the differece between "radiation" on one hand and "capacitive coupling" / "mutual inductance" on the other.
I just wanted to take that part out to keep things short stupid and avoid confusing people. I believe these things can actually be very well modeled as simple RCL networks.
I know for that article for a long time but until recently I didn't understand a thing out of it. How does the guy calculate radiation resistances of a TC?
What we are really talking about here is the difference between near and far field, and this is also what limits the range of this whytricity gizmo so much. In the near field power falls off with r^-4 rather than r^-2, so this is presumably what limits the practical range to a few feet.
I'm sorry, joe, but I would say this is just misleading, especially saying that ''range is limited to few feet''. As long as resonators are efficient enough you can send any amount of power across any range. (both with magnetic and with capacitive coupling).
IOW, the transmittable power will depend on mutual inductance/capacitance of resoantors (wich is a function of their size and distance), their Q's, and input power.
PS. the r^4 figure is for magnetic coupling, IIRC?
I have a spice model of the MIT system now and I get 17.5% efficiency for their pictured system assuming some things. I am not sure it will be terribly useful in the real world, but it does works on paper. Time is short now but I will write it up in awhile...
Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
Sorry Steve and Marco, I didn't mean to sound like I'd disagree with you, my post was just meant as a quick reminder of the Nicholson article.
I am quite sure about my claim with the limited range though. The r^-4 figure applies to the near field in general, so I suppose it does not matter if the field is E or H. You say that "power will depend on mutual inductance/capacitance of resoantors (wich is a function of their size and distance), their Q's, and input power", which is exactly the point: Mutual capacitance and inductance will fall off quickly with distance, the Q is always limited by "couch losses" as Steve put it, and by increasing the input power you do not increase the efficiency but only the possibly harmful interaction with nearby electronic equipment.
I am not saying that I don't see practical applications for this, all I am saying is that this method applies best when you only need a little power over a small range. Perfect for charging cell-phones, not so good for powering electric vehicles or rendering mains obsolete.
I think I have the MIT system pretty much figured out now. They are optical physicists and not electrical engineers so their terminology and perspectives is very different. They are trying to make electronics work on the macro scale the some way photons work on the sub-microscopic scale which they are used to. They know all about Maxwell's equations and all that... They just look at a horse from the side instead of the front so their descriptions sound strange, but we are all still looking at the same horse. Optics is their subject and not electronics, so they are a bit lost on the electronics side of things.
I wrote up some notes here:
I think it is all displacement currents and magnetic coupling has nothing to do with it. Since their coils are in free air without a ground or ground plane, the coils have a front and back electrostatic region like a dipole as they predicted. In their supplement and other papers they keep trying to blame the effect on magnetic coupling and even try a few tests:
But I think it is all capacitive coupling.
I think I can whip up a little duplicate system here easily enough driven by a 50 ohm signal generator (1 watt) and using back to back LEDs as a receiver. I don't have any way to generate tunable high powers in the 1 to 10MHz range so have to go with very low power. Should be cool!
... not Russel! Registered Member #1
Joined: Thu Jan 26 2006, 12:18AM
Location: Tempe, Arizona
Posts: 1052
Steve Conner wrote ...
I don't know how to model the radiation characteristics either. But I think, given that the antenna structure doesn't have any dimension larger than 60cm, and the wavelength of 10MHz is 30m, it's a fairly safe bet that it will suck completely as a radiator. That is only 1/50 of a wavelength so the radiation efficiency would be a few %. The measured Q factor of 950 also suggests that it is a lousy radiator.
You'd be surprised. I've used my 5' loop to work several states on the 80m band, and that is roughly 1/50th of a wavelength across. So-called "magnetic" loops, that generate only a magnetic component in the near field, can be surprisingly efficient, given their extremely small sizes.
The radiation characteristics of a loop are pretty easy to figure out:
Where RRAD is the radiation resistance of the loop, A is the effective area (cross sectional area multiplied by number of turns) and lambda is, of course, the wavelength. This formula is only good for "magnetic" loops, i.e., loops where the total length of wire does not exceed 1/3 of a wavelength or so.
If we take the figures given here by Bert, we have a 5.25 turn loop with a 0.6m diameter, giving an effective area of 1.48m^2. Operating at a frequency of 9.9MHz, the wavelength is 30.3m. That gives us a radiation resistance of .0821 ohms, distributed along the coil. If the sending coil is constructed of 9.90m of 3mm copper wire, the losses in the wire at 9.9MHz should be about .865 ohms. So the overall efficiency of the loop as a radiator is about 9.49%. So, in the example, they start with 400W, 60W ends up in the light bulb, and about 32W gets radiated as RF. The rest is lost as heat. While not a terribly efficient radiator, it still does a pretty good job. Forget problems with local hams; 32W on 30m will get you across the world for at least a few hours most days. The real implication of this, though, is that even if you were using superconductors, even if you were doing this far away from any potential "couch" losses, there is a radiation loss that cannot be avoided.
Registered Member #16
Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554
As a testament to just how much of a problem 32w at 10Mhz could be, I'll share a story from my early days of ham radio.
My good friend Jim, N3OJI was working on a home built morse code transmitter that put out roughly 70w on the 7 and 14Mhz bands. We were calibrating the oscillator to the tuning dial, with him transmitting using a 100w light bulb as a dummy load (not advisable, but it will work in a pinch). This is a throw back to the old days when non-inductive, high power resistors were damn hard to find.
He sat on one side of his garage, transmitting into the light bulb, and I sat on the other, listening on another radio. He sent his callsign and some random words a few times, and a station in Chesterton, Indiana (approximately 400 miles) heard him and responded. He ended up working the station (which I believe was W3UP) for several minutes, listening on his high end receiver and dipole, and transmitting on a 100w lightbulb.
Given that this was at the very peak of the solar cycle, and the HF bands were extremely active, but it illustrates the point.
The lightbulb changes impedance pretty drastically as it heats up so its difficult to calculate the efficiency, but even with so much reverse RF in the coax that the shield begins to radiate, the best case scenario is a few hundred milliwatts. Even a coil optimized to power devices across a room at 10Mhz would be putting out a signal several orders of magnitude greater, and on a frequency just as conducive to world wide communications.
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Wireless energy again
