Electrically small RF coil arrays for medical & wireless power applications

This paper analyzes the effect of close proximity for coil arrays and measures the S21 mutual coupling, resonant frequency, and quality factor (Q) of arrays with 2, 4, & 8 coils. It is demonstrated that for vertically separated coils, the mutual coupling effect decays asymptotically as a function of separation distance; whereas for horizontally separated coils, there are two mutual coupling minima with one minima occurring for overlapping coils and the second minima similar to the vertically separated coils. Results from this study can be used to optimize electrically small RF coil arrays used in low-field MRI, NFC, and wireless power transfer.