An Inductive Self-complementary Hilbert-curve Antenna for UHF RFID Tags

Recently there has been a rapidly growing interest in RFID systems and its applications. Operating frequencies including 125 KHz–134 KHz and 140 KHz–148.5 KHz LF band, 13.56 MHz HF band and 868 MHz–960 MHz UHF band were applied to various supply chains. 433 MHz band was decided for active reader and 2.45 GHz band was applied for WiFi reader. Besides the reader antennas, the requirements of tag antennas are necessary for applications. In which, due to the benefit of long read range and low cost, the UHF tag will be used as the system of distribution and logistics around the world [1–13], [29–41]. Meander line antennas were commonly for UHF tags, due to the characteristics of high gain, omni-directionality, planarity and relatively small surface size [5]. However, the length-towidth ratio limited as 5:1 was proposed [2]. Recently, the half-Sierpinski fractal antenna was introduced with a small length-to-width ratio (<2:1) [11]. Meanwhile, the inductive impedance of tag antenna was necessary for matching the capacitive terminations of chip IC, thus the tuning apparatus was proposed [4], [8]–[10]. H-shaped meandered-slot antennas with the performance of broadband and conjugate impedance matching were developed for on-body applications [14], [15]. On the other hand, the self-complementary dipoles were introduced for the performance of wideband, high impedance and balun [16]–[23]. The Hilbert-curve, proposed by Hilbert and introduced by Peano [24], was known as the space-filling curves. The structure of this shape can be made of a long metallic wire compacted within a patch. As the iteration order of the curve increases, the Hilbert-curve can be space-filling the patch. It has been used in fractal antenna with size reduction [25–28], [44–52]. The main aim of this paper is to merge the meander line and meandered-slot structure of the RFID tag antenna in order to obtain a good performance of compact, broadband and conjugate impedance matching. Meantime, demonstrating the performance with a selfcomplementary Hilbert-curve tag antenna is proposed. The self-complementary Hilbertcurve tag antenna is constructed with substrate, Hilbert-curve, Hilbert-curve slot and tuning pad. For circular polarization analysis, the current distribution and electric field are exhibited. The inductive and broadband characteristics of frequency responses and directivity feature of radiation patterns and polarization are studied and presented.