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According to Wikipedia a Tesla coil

transfers energy (via loose coupling) from one oscillating resonant circuit (the primary) to the other (the secondary) over a number of RF cycles.

which I don't quite get. RF is likely opposed to near field and the near field spawns about 1 wavelength from the source and that's 30 meters even at 10MHz, so looks like it's a plain old near field transformer, just with an air core and with coils separated rather far from each other.

So does Tesla coild use near or far field for transmitting energy?

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Tesla coils generally use near-field energy transfer. But ...

The page cited is sloppy in its wording and it would better to say "near field" where it says "RF". But, (again) ...

The answer is necessarily not black and white.
"Near field" has a precise meaning BUT obtaining ONLY near field coupling in a given case is not certain.

There is a gradual transition from NF to FF and a boundary region where both can occur. Near Field essentially occurs where certain geometry dependant terms form a major or significant part in describing interactions between transmit and receive structures - these relate to cyclical transfer of energy between the antenna I and V and adjacent electrostatic and magnetic fields. As distance increases these terms become less significant until they can be ignored.

You'll always get "some of both" with "some" varying as you move away from the antenna.



(1) At distances of well under a wavelength there is substantial interaction between the electric and magnetic fields produced by the aerial and current and voltage in the antenna. Energy is transferred to and fro between fields and aerial throughout with losses caused by non idealities but no energy loss due to energy "leaving" the aerial structure. This close in zone is termed the "reactive zone" where power may be absorbed by a tuned load which has voltage and current induced in it and which then dissipates energy (ie has a resistive component). Coupling involving power transfer is magnetic.

(3) [note number] "RF communications or energy transfer occur at distances beyond several wavelengths form the"antenna" structure. Here the ratio if electric and magnetic coupling have "settled down" and any energy present is not coupled to the structure I & V so is "lost", whether "received" or not. One way of viewing this one is that the two aerials are geometrically distance and secondary terms which account for the filed coupling and which have a strong distance dependent component have become insignificant - the field has become essentially homogeneous over lengths of the order of the receiving antenna.

(2) At distances past about half a wavelength the "second order" terms on which pure NFC coupling depends start to get small and the field starts to become homogeneous. This is termed the "Fresnel zone" (the guy has his name all over) and there is a degree of non ideality in field coupling to the antenna.

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This wikipedia page on near and far field does a better than usual job of commenting.

Their summary section says, in part:

  • The near-field is remarkable for reproducing classical electromagnetic induction and electric charge effects on the EM field, which effects "die-out" with increasing distance from the antenna (with magnetic field strength proportional to the inverse-cube of the distance and electric field strength proportional to inverse-square of distance), far more rapidly than do the classical radiated EM far-field (E and B fields proportional simply to inverse-distance). Typically near-field effects are not important farther away than a few wavelengths of the antenna. Far near-field effects also involve energy transfer effects which couple directly to receivers near the antenna, affecting the power output of the transmitter if they do couple, but not otherwise. In a sense, the near-field offers energy which is available to a receiver only if the energy is tapped, and this is sensed by the transmitter by means of answering electromagnetic near-fields emanating from the receiver. Again, this is the same principle that applies in induction coupled devices, such as a transformer which draws more power at the primary circuit, if power is drawn from the secondary circuit. This is different with the far-field, which constantly draws the same energy from the transmitter, whether it is immediately received, or not.
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  • \$\begingroup\$ IIRC, the far field starts when the ratio E/H has settled down to approximately the impedance of free space, i.e. about 377 Ohms. Nearer to the source you get standing waves due to the impedance mismatch between the circuit and space. \$\endgroup\$ – Eryk Sun Nov 15 '11 at 1:54
  • \$\begingroup\$ @erkysun - At distances far more than a wavelength your image is a good one. From 0 on out to a few wavelengths the picture is complex and the more you read the less you know (at least so if you are me - and I happen to have been reading extensively on this recently for unrelated reasons). . There are various models for visualising what is happening. What I wrote was an attempt to simplify something that is in no way simple :-). \$\endgroup\$ – Russell McMahon Nov 15 '11 at 2:03
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My question on physics.SE is about the same stuff. My possibly poor understanding:

The resonant LC tank stores energy, with efficiency increasing as Q increases (Tesla talked about submerging the coils in liquid air to reduce their resistance). So you put energy into the coil, and it oscillates back and forth between L and C, with the EM fields returning energy back to the circuit instead of radiating it (which is the reactive near-field). If you bring another LC circuit nearby at the same frequency, you now have two coupled LC oscillators, and the energy can slosh back and forth between the entire circuits, like this Falstad simulation.

If one LC has a resistive load, then energy will be dissipated in it. If the other LC is connected to a source, then the energy in that LC can be replenished. So the efficiency of transfer really has to do with the LCs storing energy efficiently without radiating it or wasting it in resistance.

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    \$\begingroup\$ Even more than that - near field transfers energy between the "antenna" structure and energy stored "in space" in elecrostatic and magnetic fields. This energy moves in and out of the antenna structure and may be picked up by another structure as/if required. This can only happen losslessly within volume where energy is stored this way. This is the "reactive field" area out to about half a wavelength. \$\endgroup\$ – Russell McMahon Nov 14 '11 at 16:45
  • \$\begingroup\$ @RussellMcMahon: Electrical energy always travels through space, even in a DC circuit. :) \$\endgroup\$ – endolith Nov 16 '11 at 22:03
  • \$\begingroup\$ "Moves through" via something else, yes. "Is stored in" in the sense intended, not always, or even ofen. \$\endgroup\$ – Russell McMahon Nov 17 '11 at 0:53
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I'm not an expert in RF, so take this with a grain of salt, but my guess is that it could use either the near or far field -- but if you want it to be efficient power transfer (the Wikipedia page gives a figure of up to 85% for a "well designed Tesla coil"), it would need to be near field.

Far field effects rely on EM radiation rather than quasistatic electric/magnetic coupling, so to get a significant portion of the energy from primary to secondary you'd somehow need to control the radiation pattern so that it's aimed at the secondary. This seems rather difficult, although I assume you could probably improve things with parabolic reflectors.

(Loose analogy: Think of a very large pool, and in one end is a motor-powered wave generator; in the other end is a resonant system where wave energy is turned into electrical energy via an electric machine. The further away you are, the more of the wave energy goes someplace else besides the receiver. But it still works to transfer energy.)

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it is all near field. near field works within the length of the first wave length. at 500 khz a quarter wave is 500 feet. would be cool if it was far field ,you could run 20 secondaries from 1 primary with out any coupling.

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Tesla coils Shuttle power back and forth between transceivers. RF has nothing to do with it, low frequently works the same. The capacitance (sphere) is used to store coulombs and discharge them. Resonant receivers can receive the discharge.

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