Seamless exploitation of cell-phone antennas for near-field WPT by a frequency-diplexing approach

This paper introduces a novel solution for portable devices to exploit their existing communication antennas for bi-directional near-field wireless re-charging, without compromising their far-field properties. To demonstrate the concept, the radio-frequencies (RF) 900/1800 MHz and 433 MHz are adopted for the far-field communication and for the near-field wireless recharging, respectively. First, a pair of faced, dual-band, printed monopoles is characterized as a two-port network over the bands of interest at several distances. Then, each antenna is fed by a frequency-selective three-port network, designed to simultaneously ensure data communication at the higher frequencies and wireless re-charging at the lower one. In this way, the combined far-field and near-field operations of the resulting four-port network can be optimized. A proof-of-concept prototype has been characterized, showing ports isolation higher than 20 dB and RF-to-RF power transfer efficiency between 15% and 40% for variable faced antennas distance. When a rectifier is connected to the receiving power port, the measured RF-to-dc link efficiency varies between 9% and 20%, with portable devices located 1 and 2 centimeters apart, and the transmitted power is 10 dBm. The simultaneous high frequency communication is verified with slightly reduced performance.