rado wave emitted by inductively coupled power transfer system

[BaSF]

I would like to know if a a inductively coupled power transfer system also emits electromagnetiv waves. The system contains of two inductance coils one generating the magnetic field with AC, the other picking up the magnetic energy and transforms it into an AC, also. The current frequency is above 100 kHz.

 

In my oppinion, the self inductance of the coils do emit electromagnetic waves, but i have read that only an open oscillating circuit by meaning of a straight antenna emits electromagnetic waves.

 

Does anyone knows can help me out here?

[Enthalpy]

Any antenna or coil emits both. It's just a matter of how much.

 

That is, an efficient antenna must be big enough, in terms of wavelength. Shorter than 1/20 wavelength, it's getting frankly difficult to radiate well. Such an antenna creates an important electromagnetic field, which propagates far. The field also accepts power from the transmitter even if no receiver is near, and sends it to the rest of the Universe.

 

A short dipole or loop creates essentially an electric respectively magnetic field, which drops very quickly over the distance, like R^-3. Such a field stores energy but does not radiate power; it accepts power from the transmitter only if a receiver is near enough. Though, the dipole or loop can (and does!) have its own power losses.

 

Imperfect life makes that short dipoles and loops also radiate a small electromagnetic field. A loop would make a magnetic near-field, but as this field diminishes over distance, from the point where it gets the amplitude corresponding to the electric field, you get a honest far-field, which is electromagnetic, and decreases as R^-1. Small but present.

 

This relates directly with the way a loop radiates. Imagine the current flowing in it, in phase everywhere, and along a closed path.

- Near the loop, one side is nearer to the observer, and this side's effect predominates. This is the near field. It's magnetic mainly.

- Farther, the amplitude difference would give only the R^-3 decrease hence vanish. But an other effect is less affected by distance hence predominates: the field emitted from the nearer side arrives earlier than from the farther side. Because the alternating current changes meanwhile (over the propagation time difference), the effects don't cancel out. That's the far field.

 

You see that:

- The far field needs loop dimensions not too small, as compared with the wavelength

- It is less sensitive to the distance, since it's not an imperfect amplitude cancellation. The phase difference doesn't reduce with the distance.

 

The far field doesn't relate simply with the self-inductance. It's more the near field, or in fact, the very near field, since the inductance is just the magnetic flux divided by the current. A toroidal coil for instance would have a flux, a self-inductance, but no significant far field.

 

Sometimes, small antennas are necessary. That's the case with long waves, medium waves, which are kilometric and hectometric. Antennas are small hence inefficient, but they work. They can be coils, preferably on a ferrite core, but air coils are just bigger.

 

Formulas, including the radiation resistance of a small loop, which lets compute the electromagnetic power and fields radiated by the loop, are for instance in "Antennas", from Kraus.

[BaSF]

thank you very much for the information, Enthalpy!

It was really helpful