From equation (81), to achieve a high figure of merit U (i.e. to get maximum efficiency), one should use resonant coils with a high quality factor (or with a low intrinsic loss rate Г). Another way to increase efficiency is to make a tight coupling between two resonators. However, in practical applications there is always a geometrical limit of object size, and it applies restriction to KU and Qn magnification. In the following sections, ways of calculating Qn and KU will be presented together with ways to increase the figure of merit U. 3.1