Design and Fabrication of a Compact, Low-Cost UHF-RFID Repeater, Exploiting Circular Cross-Polarization

In this article we aim to increase the range of commercial passive UHF RFID technology. We propose a prototype forward-link repeater, which consists of a pair of antennas, a band pass filter, an RF power limiter and a low-noise amplifier (LNA). Initially, we focus on the design of the two antennas with inverse direction of circular polarization (CP), so that sufficient decoupling is ensured, while adhering to the following constraints: i) the input antenna is circularly polarized to maximize the gain towards the reader's antenna, ii) the output antenna is circularly polarized to improve the probability of successful tag reception, given the expected randomness in the tag's orientation, iii) the volume of the entire structure is kept small. Then, we present the achieved range improvement of the proposed repeater in two applications: a) a fixed installation, where the reader successfully identified passive RFID tags from 60m and b) by placing the repeater on top of an autonomous robot, to provide power in distant battery-less RFID tags, while the robot randomly moves inside a large area. The robotic-repeater prototype achieved remarkable performance, inventorying distant passive RFID tags, under Non-Line-Of-Sight (NLOS) conditions with the reader, exploiting the robot's mobility for fading-mitigation, combined with increased incident power from the repeater.