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?