antenna-theory.com

[splendor]

Hi, Bigsteve,

I saw the NFC antenna the section. It is really interesting. Do you know how do they deal with the efficiency? Or they just simply don't care? where does the energy go?

Thanks

splendor

[bigSteve]

It really just is a big inductor. I don't think they measure actual radiation efficiency, because the NFC antennas are not designed to capture or send radiated fields. They simply want to couple at short distances. The main design technique is to generate a bunch of inductance, and then test it to see if it couples properly to the opposite (receiver) antenna.

The energy is initially reactive - stored in the near field. Just like feeding a regular inductor. When it comes into contact with another inductor that is connected to a receiver, the mutual coupling transforms the impedance of the inductor into a resistive impedance, and power is delivered. It is similar to radiation resistance of an antenna, except in this case it is a function of whether another mutually coupled inductor is present.

[splendor]

Hi, Bigsteve,

Thanks for your answer. I sounds like a hot topic recently. -- the wireless power transfer technology. I've seen the video of Witricity, (origin from MIT group, http://www.witricity.com/pages/technology.html)

They try to transfer energy through big coils, coupling with each other in near field.

As we are talking about efficiency. (1) I don't think they care about radiation efficiency either (2) overall efficiency, a coil may have very low resistance and very high inductance. I am very curious how thease applications can make the energy transfer to it efficiently.

[bigSteve]

Don't get caught up on efficiency. It makes it more complicated than it is.

Inductors store energy in the magnetic fields that surround them. If these fields interacts with another nearby inductor, a current is induced in the second inductor, and this can be detected. See here:

http://www.antenna-theory.com/tutorial/electromagnetics/magneticfield.php#inductance

Also, note that a capacitor is just two parallel plates. Even though the capacitor has no "efficiency", energy transfers right through the plates, via the electric field. It is really the same thing.