While, in principle, more than two flat spiral coils could be provided in respective layers of a PCB, due to thermal conduction the outer layers of a PCB have two to three times greater current carrying capacity than any inner layers of the PCB. Accordingly, a double-coil structure such as that described above provides a balance between performance and complexity. Further, in this embodiment, each of the coils 31, 32 is a round or circular flat spiral coil. In other embodiments, one or each of the coils 31, 32 could instead be a rectangular (e.g., square) flat spiral coil. Whilst rectangular profile coils have a slightly higher inductance for a given profile, circular coils can be more easily interleaved and/or can have components packed between them, leading to an overall increase in PCB area utilization. A rectangular profile also required a longer track length for a given strength of magnetic field along the coil axis, which increases the resistance and reduces the Q value as compared to a circular coil of similar width.

The magnetic field generator 130 of this embodiment comprises first to sixth induction coil arrangements 132, each of which is identical to the induction coil arrangement 132 shown in FIG. 5 and is attached or connected to the retainer 131. In this embodiment, the retainer 131 is 3D printed SLS (selective laser sintering) nylon. In other embodiments, the retainer 131 may be formed in any other suitable way, such as from a PCB, or from any other suitable material. In some embodiments, the retainer 131 comprises a base 131 and the induction coil arrangements 132 extend away from the base 131 in a direction orthogonal or normal to a surface of the base 131.