A Line-Array Technique for Wireless Power Transfer Toward a 100 $\mu$ m $\times100$ $\mu$ m Coil Antenna

Wireless power transfer (WPT) has grown rapidly in the past decade to power up devices such as radio frequency identification (RFID) tags, sensors, and medical implants. In recent years, miniaturization has become a new key requirement for some emerging applications such as the Internet of Things (IoT) and biomedical devices. Targeting at a miniaturized receiving (Rx) coil, conventional-inductive WPT (IWPT) has insufficient power transfer gain. The multi-coil transmitting (Tx) array techniques mainly aim at improving the misalignment tolerance, but the power gain (or efficiency) is significantly limited by the mutual coupling among the Tx elements. In this article, we propose a new Tx array structure formed by lines instead of coils. The line-array technique keeps the antimisalignment character of the conventional multi-coil arrays, while the power gain is enhanced in two ways: 1) the lossy adjacent sides are removed in the line array, and 2) the mutual coupling among the array elements is alleviated. To validate the proposed technique, we demonstrate a Ku-band WPT prototype, including a CMOS Rx rectenna and a printed circuit board (PCB) Tx line array. The rectenna is fabricated in a 65-nm CMOS technology, with a coil antenna measuring 100 μm × 100 μm. At varying power transfer ranges, the power transfer gain and misalignment tolerance are measured for both the single-coil and line-array systems, respectively. In comparison to the conventional single-Tx-coil IWPT, the overall power gain can be improved by 17.3 dB at a typical power transfer range of 2.5 mm, while a 1-mm misalignment only reduces the gain by 1.3 dB.