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In literature it is stated that in reactive near field using high-Q coils in resonance the efficiency does not depend on coupling (ie. distance) and that coupling only has influence on rate of energy transfer. The reason is that energy stays inside the resonating system and the only radiative and resistive losses have influence on efficiency (and they are small because of high Q).

How is that possible? I have done analysis of S21 parameter of such resonant system and it clearly degrades even with relatively small increase in distance. Isn't S21 proportional to efficiency?

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  • $\begingroup$ S21 is linked to "that coupling only has influence on rate of energy transfer", but it can be totally uncorrelated with the efficiency $\endgroup$ – agemO Jan 7 '15 at 8:53
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It is counterintuitive, isn't it? I think the easiest way to understand what's going on is to imagine a perfect transmitter coil/resonant circuit and a lossy receiver coil. If the transmitter keeps supplying energy to the transmitter coil the current in the transmitter LC will keep building up, and so will the amplitude of the oscillating magnetic field. For infinite Q this is an unlimited process. When we introduce a loaded/lossy receiver coil, energy will be taken out of the magnetic field and the effective Q of the transmitter circuit will decrease to a finite value, which will depend on the amount of power that is being transmitted. The higher the Q is, the longer it will take to build up enough energy in the field to achieve the necessary power transfer.

For a transmitter with limited Q this process will lead to a limited transmission distance because the field can not build up to arbitrary amplitude. It is limited by both the Q and by the current or voltage limit of the transmitter circuit.

So, yes, I believe the statement is correct, although it's not totally obvious that this happens.

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