Integration of Wireless Coil and Bluetooth Antenna for High Charging and Radiation Efficiencies

In this paper, a wireless charging coil integrated with a Bluetooth antenna is proposed. The equivalent circuit model is developed to analyze this multifunctional coil. The outermost turn of the coil is appropriately designed in order to improve the radiation efficiency of the antenna. In this way, the radiation frequency of the coil is determined by the length of the outermost coil. Meanwhile, the inductance of the coil is adjusted to achieve high charging efficiency. When the coil is taken as a Bluetooth antenna, the measured reflection coefficient has achieved a fractional bandwidth of 8.3%, covering a frequency range of 2.3-2.5 GHz, with |S₁₁| < -10 dB. The measured radiation efficiency of the antenna is about 62% at 2.4 GHz. When the coil is applied, the system charging efficiency is about 61%. In particular, the entire circuit module is fabricated on the low-cost FR4 printed circuit board. It can be expected that both the radiation and charging efficiencies can further be improved by using low-loss substrate.