Further, FIGS. 5A and 5B illustrate a comparison between two different design structures, one where the coil antenna 21 has (FIG. 5A) and one without (FIG. 5B) the pairs of small loop transmission coils 22 placed under the primary coil antenna 21. FIG. 5C shows the coupling efficiency for the primary coil antenna 21 with pairs of small loop transmission coils 22 can be achieved at 40% while the coupling efficiency is 5% only for the primary coil antenna 21 without the pairs of small loop transmission coils 22, given that the distance between the mobile (target) device second coil (PRU antenna) 25 and the primary coil antenna 21 is 10 cm.

As shown in FIG. 6A, the small loop transmission coils 22 can be in different shapes, such as a rectangle, a circle, a polygon, a square, a triangle or any combination of shapes. Of course, other shapes are possible and workable. D1 and D2 are the inner and outer diameters, respectively, of the differently shaped small loop transmission coils 22. As shown in FIG. 6B, according to an embodiment, the ones of the pairs of small loop transmission coils 22 with different shapes and/or sizes can be tiled one by one (side by side, and see FIG. 8) under the primary coil antenna 21 of the wireless charging device 20 in order to achieve an optimal coupling effect for the wireless charging application.