A reader device includes an array of antenna coils configured to electromagnetically couple with devices implanted beneath or within skin of a human body. An implanted device can include a loop antenna or other means configured to couple with at least one antenna coil of the reader device to receive radio frequency energy from the reader device. The antenna coil array is configured to mount to the skin surface to improve the coupling between the implanted device and coils of the array. Further, the reader device is configured to select two or more antenna coils of the array and to operate the selected antenna coils to emit radio frequency power at respective amplitudes and relative phases to provide radio frequency power to the implanted device while increasing efficiency of the power transfer and reducing the exposure of the skin to radio frequency energy.