A Passive RFID Tag Antenna Installed in a Recessed Cavity in a Metallic Platform

A passive RFID tag antenna in a recessed cavity in metallic objects such as vehicles, air craft, and metallic containers, and an impedance matching technique for this configuration are proposed. The proposed tag antenna can be considered as a cavity-backed antenna and it is comprised of a bowtie antenna installed in a recessed cubic volume in a large metallic object, and the input impedance of the bowtie antenna can be easily matched by adjusting the coupling effect between the tag antenna and the cavity. Main parameters affecting the coupling are the dimensions of the cavity and the location of the bowtie antenna inside the cavity. For more practical analysis of the impedance matching process, an equivalent-circuit model consisting of several lumped elements is introduced and each element is optimized using a genetic algorithm (GA). Experimental results show that the maximum reading distance of the proposed tag antenna with optimized dimensions can be improved about 3.2 times compared to a commercial RFID tag.

I. Introduction

As one of rapidly developing technologies in automatic identification and data capture (AIDC) industries, radio frequency identification (RFID) has been increasingly used in many applications such as supply chain management, inventory control, security management, and logistics [1]. To meet the requirements for these applications, RFID tags need to be attached on various objects with different shapes and material properties. Therefore, an RFID tag antenna attached on a target object plays a crucial role in determining the overall performance of an RFID system. A big challenge to design a tag antenna is how to alleviate the influence of material properties of objects and the presence of other object nearby the tag [2]–[4]. This effect is most severe in UHF band for metallic objects. Based on the boundary condition, electromagnetic wave reflects from metallic objects with reversed phase. Therefore, when we place antennas close to metallic bodies, electric field intensity is greatly reduced, which also decrease radiation impedance of the antennas. Consequently, radiation efficiency is dropped significantly, which yields overall degradation of antenna performance [5], [6]. On the other hand, there exist various types of recessed volumes in metallic objects such as vehicles, aircraft, heavy equipment, and metallic containers, and these recessed volumes can be considered as “cavities”, which can be used with common label-type RFID tag to form a cavity-backed antenna. We can make a long range RFID tag antenna with low-profile and moderate form factor when this type of configuration is used in metallic objects.