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Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi guys,
I have several questions over here I'd like to ask.
Firstly and mainly, can I or can't I call the form of power transmission shown in the picture ''non radiative''? I'm more and more troubled with that question and I need an answer quickly.
These guys apparently had no trouble calling it nonradiative.
BTW, most of you RF guys over here would call the energy 'radiated' only if it was lost on radiation resistance, naively thinking, vent out into vacuum.
I ended being led into a trap with just agreeing with this without really thinking deep enough.
Many of you probably now consider this article
as legendary. I knew for it for long time, but until recently I was just way too dumb to understand anything from it.
Now after sitting and rereading it I was amazed how much I understood and how it made perfect sense. That's why I would love to stick to it in this discussion.
I' going to copy most important sentences from the article:
Paul Nicholson wrote ...
It's important to appreciate straight away that you cannot separate the field from the charges. Indeed you cannot draw any real distinction between a charged particle and the EM field that is associated with it, and that it doesn't make much sense to say that energy is either stored in the particle or stored in its field. When you apply energy, it goes into the particle/field system and that's all you can say about it.
In the end, it makes perfect sense to think of EM radiation simply as interaction between charges delayed by limited speed of light.
To me, this at first looked completely against Steve Conner's statement that electricity is never in wires - apparently it indeed is in the wires, as electric or magnetic field don't really store energy on themselves.
Still, once I radiate energy from antenna I lose it, and I may not have clue whether this energy will affect any charges anywhere, or that any other charges exist at all?
(the thought experiment of empty universe with an antenna: does antenna radiate energy or not?)
This is so utterly confusing, I hope someone can clarify it.
How can I always imagine that it's always *EMwaves+charged particles* that store energy?
Paul Nicholson wrote ...
At the quantum level, the EM field is often described in terms of a cloud of so-called 'virtual' photons which radiate out from a charged object for some distance before returning. Each virtual photon borrows a little energy and heads out into the world. If it can't convert it to real energy (by finding a load or absorber) it returns its borrowed energy back to the source.
We can see that the quantum description of the process by which a charge influences its neighbours mirrors that of the classical radiation described above. We have a reactive region around the source, containing mostly 'imaginary' or borrowed energy, and this is similar in principle to the cloud of virtual photons surrounding a charged particle. As with the reactive field, the virtual photon can only borrow its energy, and can therefore exist, for a time of around a cycle of its oscillation frequency. You could perhaps think of a virtual photon as a photon with its E and H out of phase, so that it is 'virtual' in the same sense that reactive power is 'imaginary' power, although we would not want to take these mental pictures too literally.
So, I can consider real photons those who *do* transfer energy and have B/H in phase, while virtual photons have B/H 90 degrees out of phase and don't transfer energy but back to source!
Now if I have real photons in the system, which is needed to send some energy over any distance at all, how can it ever be 'non-radiative'?
Then from the other side again, I'm not radiating anything into space, since output and input impedances of my sender and receiver are very low compared to 377ohms and truly radiated power is negligible.
I want to know if following statements are right:
The real photons absolutely must exist, but they never get out of the system.
Rather, they *tunnel* from transmitter into receiver, which is very close to transmitter compared to their wavelength and thus they have no trouble doing it!
Only then I understood what witricity guys are talking about, and it's just matter of opinion what you consider radiation, just the existence of real photons, or their existence outside the transmitter and receiver - that is, radiation into space?
If it's just the second thing, then they are completely right to call their system ''nonradiative''.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
I think there's a lot of ways to look at this. My way - if you have a second receiver far away it will pick up a much smaller signal/power, but it will pick it up so the system must be radiating SOMETHING that allows remote reception, so I'd have to call it Radiative.
(if there's no one there does smell it when a bear s**ts in the woods etc.)
Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
Duh, I'll have a shot at this tomorrow, I am too tired right now. Basically I agree with Sulaiman (and Dick Feynman for that matter), that there are a lot of "equivalent" ways of looking at things, and none of them is necessarily more correct than the other. So in a way, it IS just a matter of opinion. Nobody has ever seen or touched a field, just like a virtual particle it is just a convenient theoretical construct that makes it slightly easier to get to terms with this "action at a distance" that electricity involves.
... not Russel! Registered Member #1
Joined: Thu Jan 26 2006, 12:18AM
Location: Tempe, Arizona
Posts: 1052
Marko wrote ...
Hi guys,
These guys apparently had no trouble calling it nonradiative.
They also had no problem taking it to the press, complete with buzzwords (WiTricity, really? really??), and all sorts of unfounded hype. They'd probably have claimed it cured cancer if it would get people to sponsor their research.
Marko wrote ...
Then from the other side again, I'm not radiating anything into space, since output and input impedances of my sender and receiver are very low compared to 377ohms and truly radiated power is negligible.
Patently untrue. Unless the two coils are perfectly coupled, there will be a significant loss of energy to radiation resistance. You can see the previous thread on the matter for the specific math that I posted, but the simple fact is that energy fed into the "sending" coil is going to end up as heat in the coil, end up being sent to the receiving coil, or end up "lost" as radiated energy. In a system like that, the radiated energy is on the order of a few percent. Move the coils a significant distance apart, and this figure will grow.
Marko wrote ...
Only then I understood what witricity guys are talking about, and it's just matter of opinion what you consider radiation, just the existence of real photons, or their existence outside the transmitter and receiver - that is, radiation into space?
If it's just the second thing, then they are completely right to call their system ''nonradiative''.
Nonradiative? Sure. Except for the part where a significant percentage of the energy put into the device is lost as radiation. That's not how it transfers its power from one coil to another, sure, but calling something that radiates well enough to be picked up on the other side of the world "nonradiative" seems a bit dishonest to me. That's why the whole mess is doomed to failure. Large coils actually make fairly good antennas, and the radiation resistance of the coil will set an efficiency cap that just can't be overcome. If it weren't for that glaring problem, I think "nonradiative" would be a fair description. Some people might call it high Q inductive coupling, others might bring virtual photons into the mix, but it's all ways of looking at the same thing.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Sure, I've got nonradiative radiation right here. I keep it in a virtual cupboard with the non-stick glue and the water-resistant water
What we have here (IMO) is radiation of the ordinary, radiative sort, but trapped inside a waveguide. The arrangement of ferrite blocks and coils used doesn't look anything like a coax cable or microwave plumbing, but it is a waveguide nonetheless.
The fields generated by the apparatus have a non-reactive part, they must do in order to transfer energy. But the non-reactive part probably has an impedance greatly different to the impedance of free space, so it can't propagate well in free space. It can only exist inside the "waveguide" whose impedance is a better match to it. So, I'd rather refer to it as guided radiation than "non-radiative".
That's what I think, anyway. I find all this stuff very confusing too!
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi guys,
Sulaiman wrote ...
I think there's a lot of ways to look at this. My way - if you have a second receiver far away it will pick up a much smaller signal/power, but it will pick it up so the system must be radiating SOMETHING that allows remote reception, so I'd have to call it Radiative.
(if there's no one there does smell it when a bear s**ts in the woods etc.)
Dr. Shark wrote ...
Duh, I'll have a shot at this tomorrow, I am too tired right now. Basically I agree with Sulaiman (and Dick Feynman for that matter), that there are a lot of "equivalent" ways of looking at things, and none of them is necessarily more correct than the other. So in a way, it IS just a matter of opinion. Nobody has ever seen or touched a field, just like a virtual particle it is just a convenient theoretical construct that makes it slightly easier to get to terms with this "action at a distance" that electricity involves.
Yeah, the radiativeness/nonradiativeness thing may be nothing more but a nomenclature problem, still if someone asks me about this I'd love to have this solved.
Patently untrue. Unless the two coils are perfectly coupled, there will be a significant loss of energy to radiation resistance. You can see the previous thread on the matter for the specific math that I posted, but the simple fact is that energy fed into the "sending" coil is going to end up as heat in the coil, end up being sent to the receiving coil, or end up "lost" as radiated energy. In a system like that, the radiated energy is on the order of a few percent. Move the coils a significant distance apart, and this figure will grow.
Chris, your reaction is like I did some worst sin, well I didn't really mean that no power gets radiated into space, just that it is negligible compared to amount of power getting through evanescent wave coupling/tunneling/induction or whatever.
Naively thinking, If I had to make a distinction between ''radiative'' and ''nonradiative'' power transmission,
in a radiative system the power radiated into far-field would be the main thing transmitting the energy, and the transmitter would lose energy as radiation whether there is some kind of receiver or it isn't (unless there are reflections).
Such system would need to be highly directional to work well.
A 'non-radiative' system would most importantly differ in it that it's poorly matched to space impedance, and energy is sent mainly over induction; power going into the transmitter will greatly depend on the state of the receiver.
If I try to push the efficiency limits up as you say, the radiation resistance will indeed become the ultimate limiting factor. Still I'm not really going to get to that if I'm just going to charge a toothbrush or a cell phone.
Nonradiative? Sure. Except for the part where a significant percentage of the energy put into the device is lost as radiation. That's not how it transfers its power from one coil to another, sure, but calling something that radiates well enough to be picked up on the other side of the world "nonradiative" seems a bit dishonest to me. That's why the whole mess is doomed to failure. Large coils actually make fairly good antennas, and the radiation resistance of the coil will set an efficiency cap that just can't be overcome. If it weren't for that glaring problem, I think "nonradiative" would be a fair description. Some people might call it high Q inductive coupling, others might bring virtual photons into the mix, but it's all ways of looking at the same thing.
So, do you agree with me on that part?
It's not whether you will be able to pick up some of radiation that got out of the system somewhere on the globe, but just the main way the energy is transferred.
If my math is right, at 800kHz, the radiation resistance of the transmitting coil I posted in a picture is about 1 picoohm. Just completely negligible for any practical purpose, the loop would just melt far before this becomes of any significance.
But that's not really what was the main question..
Sure, I've got nonradiative radiation right here. I keep it in a virtual cupboard with the non-stick glue and the water-resistant water
What we have here (IMO) is radiation of the ordinary, radiative sort, but trapped inside a waveguide. The arrangement of ferrite blocks and coils used doesn't look anything like a coax cable or microwave plumbing, but it is a waveguide nonetheless.
The fields generated by the apparatus have a non-reactive part, they must do in order to transfer energy. But the non-reactive part probably has an impedance greatly different to the impedance of free space, so it can't propagate well in free space. It can only exist inside the "waveguide" whose impedance is a better match to it. So, I'd rather refer to it as guided radiation than "non-radiative".
I think I can sum the question to this:
When is something really considered to be *radiating*?
Is it just the existence of real photons? Or their propagation into free space and loss of energy that way?
witricity must use real photons (that is, electromagnetic waves) to wirelessly send any power, but still, apart from small amount of couch radiation they aren't getting into space really well.
Those that transfer energy in nearfield don't exist in space between the transmitter and receiver, but are rather tunneling between.
In space between and around the transmitter and receiver I only have a big reactive field, virtual particles. In my case it's the alternating magnetic field, which is poor at producing electric field in the space.
Electric field (needed to constitute real photons) is only produced on the tank circuits themselves, and this is why I find it so hard to imagine where are the photons in the system after all
(are they then in the wires, or what?)
As steve said, I could indeed build a waveguide just by putting a number of those tank circuits (like the blue coil and cap in the pic, but without lightbulb) next to each other, and it would work well for transmitting a low frequency EM wave.
But looking at it, where is really the damn electromagnetic wave, I only have reactive magnetic fields in the ''waveguide''!
And actually nearly all of my electric field is concentrated inside the capacitors, and I need electric+magnetic field in phase to have an EM wave at all.
I can even put the capacitors some distance away from the coils and everything would still work - and where can I find the damn real photons in there? I just can't imagine that, I think it would help a lot if I could. What are the photons after all?
I can put their electric field in a capacitor far away from magnetic field and they still exist.
... not Russel! Registered Member #1
Joined: Thu Jan 26 2006, 12:18AM
Location: Tempe, Arizona
Posts: 1052
It seems like a question of semantics to me. I would say that power transferred by the near field is not radiative, but I am unsure if anyone has specifically defined what "radiative" and "nonradiative" transfer of power really means. At any rate, calling a system that does indeed radiate significant amounts of RF "nonradiative" just smacks of dishonesty.
My main gripe with this article is that it's a collection of buzzwords intended to "sell" some really unimpressive research. At 800kHz, transferring any significant power over a distance is going to be hopeless. Higher frequencies work better, but then you're pissing away energy into the RF spectrum, rather than into heating the wire. The fact that a 60% loss between two large coils six feet apart is considered good demonstrates the hopelessness of the situation.
If you want to get hung up on what it means to transfer power in a nonradiative manner, that's a fair question, and it's worth looking into, in my opinion. However, all the hand waving in the world won't make this system into what it pretends to be: a clean, efficient means of transferring power over useful distances.
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''nonradiative radiation''
