A Metamaterial-Based Series Connected Rectangular Patch Antenna Array for UHF RFID Readers Benjamin D. Braaten, Sayan Roy, Sanjay Nariyal, Masud A. Aziz, Bilal Ijaz and Muhammad M. Masud Department of Electrical and Computer Engineering North Dakota State University Fargo, North Dakota 58108-6050 Email: benbraaten@ieee.org Abstract—In this paper, a new metamaterial-based printed antenna for UHF Radio Frequency Identification (RFID) Readers is presented. This antenna is designed such that several can be connected in series to provide local RFID coverage. In this work, simulation and measurements are shown to agree very well and that confined UHF communication can be achieved with this antenna. Furthermore, the geometry of the new metamaterial- based UHF RFID antenna is altered and the resonant frequency is computed using simulations. It is shown that by varying certain geometrical features of the array, the operating frequency can be increased and decreased in a predictable manner. I. I NTRODUCTION Over the last few decades, researchers and engineers have been finding ever more applications for Radio Frequency Identification (RFID) systems. Because of this wide spread use, the need for low cost, small size and better performance is becoming more of a requirement. Applications such as product authentication and document management using smart shelves is one new sector that RFID systems are being applied [1]-[2]. In particular, some smart shelf applications require a leaky- wave antenna; however one drawback of a leaky wave antenna is the difficulty in manufacturing such an antenna. One way to mitigate this manufacturing drawback is to design an antenna on a printed circuit board substrate that uses metamaterial- based elements in the layout [3]-[5]. II. THE METAMATERIAL-BASED ANTENNA ARRAY The antenna introduced in this paper is a metamaterial- based rectangular patch antenna array for UHF RFID read- ers. A drawing of the antenna array is shown in Fig. 1. Each rectangular patch array can be connected in series with other array elements to form a travelling wave antenna. In each array element, each rectangular patch is connected to a metamaterial-based host transmission line between ports 1 and 2. A tuning stub is also defined between the two rectangular elements. The design in Fig. 1 is based on the design introduced in [3]. For the new layout presented here, rectangular patches and a tuning stub have been introduced to reduce the operating frequency of the antenna. These two elements are very useful for an antenna designer for the reason that the stub and patch size can be chosen in a manner such Port 1 Port 2 d b a c e q p n m f h W H g k j x y Fig. 1. The proposed metamaterial-based UHF RFID reader antenna. that radiation efficiency can be improved and that the overall array resonates at a particular frequency. III. MEASUREMENT AND SIMULATION RESULTS The antenna in Fig. 1 was designed and simulated on a grounded FR-4 substrate (thickness = 1.58 mm and ε r =4.3) in Momentum [6]. The layout was then manufactured using a milling machine and the final antenna is shown in Fig. 2a. Next, the S-parameters of a single metamaterial-based rectangular patch array were measured from 800 MHz to 1.0 GHz using an Agilent ENA series network analyzer and compared to computations in Momentum. The results shown in Fig. 3 compare very well and confirm a resonant frequency of 920MHz. To determine the performance of the array, two elements were connected in series to an Alien Technologies 9780 reader [7] (with a maximum output power of 1 W) and placed in an anechoic chamber (Fig. 2b). The edge-to-edge separation between the antennas in Fig. 2b was 10.16 cm. The EPC Gen2 UHF passive tag chosen to test this antenna array was manufactured by Texas Instruments [8]. A picture of the tag is shown in Fig. 2c. During testing, the passive tag was attached (vertically) to a piece of Styrofoam block and placed broadside 6th European Conference on Antennas and Propagation (EUCAP) 978-1-4577-0919-7/12/$26.00 ©2011 IEEE 3164