Design of UHF fractal antenna for localized near-field RFID application

Abstract

Abstract-In this paper, fractal structure is proposed for localized near-field UHF antenna of radio frequency identification (RFID) reader. In the design, traditional Koch fractal structure is modified by converting the direction of one fractal structure of each line segment. Parameters of the fractal structure are optimized to acquire strong and uniform magnetic field distribution within an interrogation zone at designated height over the antenna. Two Koch fractal near-field antennas fed from center or side are presented. And the side-fed fractal near-field antenna is fabricated and tested to demonstrate a localized coverage area with a perimeter of two wavelengths within a reading distance of 35mm above the antenna. For distance above 35mm, a sharp decrease of reading rate is observed, which demonstrates a satisfactory near-field UHF antenna. Index Terms-Antenna, Koch Fractal, RFID, UHF I INTRODUCTIONN quite some of RFID-based applications, UHF RFID tags on objects must be identified in near field of a UHF RFID reader antenna [1,2]. For example, UHF RFID reader antenna on a level of smart shelf should restrict its coverage without interrogating tags of objects on other levels. With a localized coverage of near-field reader antenna, area on the level can be further partitioned with several localized RFID antennas so that exact object position on the shelf level can be acquired by interrogating logically the localized RFID antennas. Ordinary directional UHF antenna may not be suitable for such applications because it is generally designed for far-field operation.In recent years, design of near-field UHF antennas for such RFID applications has received much attention, and several successful designs have been reported based on loop structure. magnetic field over an electrically-large interrogation zone. However, it's hard to realize the same magnetic field intensity at the centre of the loop as the magnetic field in the regions close to the loop lines, which will cause a sharp gap. Meanwhile, it's shown that designing electrically-large near-field UHF RFID reader antenna is a big challenge, especially when the perimeter of interrogation zone reaches three wavelengths or more [10].The majority of the ultrahigh frequency near-field RFID reader antenna designs are based on the theory that try to ensure the current along with the loop sharing same phase and direction by the means of adding various capacitors. For example, in 2007, Dobkin et al proposed a segmented loop antenna, with a diameter of 10 cm demonstrated desirable performance at 915 MHz, consists of a series of metal line sections and numbers of capacitors shaping a loop [3]. In[6], double-C shaped distributed capacitors. In 2013, Shi et al presented a dual loop antenna composed of a main and a parasitic loop which both are constructed using segmented lines with fork-shaped capacitors [11]. And then, they proposed a grid-array antenna composed of two segment loops to enlarge the interrogation zone [10].The major challenge with design of a near-field UHF ant...

A universal UHF near‐field RFID reader antenna with wide bandwidth and adjustable reading area

“…However, the tag detection has to be limited with an assigned and confined area near antenna surface in the applications, such as retail and library management systems 2 . So near‐field RFID systems have drawn wide attention, and high‐performance near‐field RFID reader antennas are essential 3–5 . Commonly, the operating principles of near‐field RFID reader antenna include magnetic and electrical coupling.…”

“…2 So near-field RFID systems have drawn wide attention, and high-performance near-field RFID reader antennas are essential. [3][4][5] Commonly, the operating principles of near-field RFID reader antenna include magnetic and electrical coupling. Nevertheless, magnetic coupling is preferred for the stable performance in the nearfield region.…”