Analysis and design of a UHF RFID tag antenna with a Split ... - KAIST

Proceedings of iWAT2008, Chiba, JapanP308Analysis and design of a UHF RFID tag antennawith a Split Ring Resonator# Sang-Ho Lim 1 , Young-Cheol Oh 2 , Ho Lim 1 , Young-Seung Lee 1 , and Noh-Hoon Myung 11School of Electrical Engineering and Computer ScienceKorea Advanced Institute of Science and Technology373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Korea, limsangho@kaist.ac.kr1. Introduction2Mobile Internet Development Department, Infra Laboratory, KT17 Woomyeon-dong, Seocho-gu, Seoul 137-792, Korea, ycoh@kt.co.krRadio Frequency Identification (RFID) systems are one of the best ways to realizeUbiquitous society. RFID systems have plenty of advantages like long readable range, multipleaccess, high data rate, easy read/write functions, and so on. In these reasons, the Bar code systemswhich are currently widely used in the whole world would be replaced with RFID systems. RFIDsystems for fixed objects can be grouped by the size of objects (i.e. Pallet level, Case level, andItem level) and the recognition rates of each level are nearly 95%, 85%, and 66%, respectively. So itis important to improve the item level recognition rate which is strongly affected by tag antennas.Because the requirements of small object identifications are drastically increased, the compact tagantenna design and analysis become compulsory. For UHF ranges, the dipole-like tags which aremostly used in many applications are too big to attach to a small object. So there are lots of studiesabout small tag antennas with meander type line. However most of the tags are suffer from a narrowbandwidth, low gain, low radiation efficiency, and hard control of the tag antenna impedance.To improve these problems, a split ring resonator (SRR) as a radiating part, whose radius isλ/12 (the circumference of the SRR is nearly λ/2) and inductively coupled feeding method in orderto obtain wide bandwidth and simple matching technique of the tag antenna are used. The proposedstructure is analyzed and the equivalent circuit model for the tag antenna is presented. Finally wechecked the validation of the model through the comparison between the simulation and calculationof equivalent circuit.2. Proposed tag antenna2.1 Structure and Principle of operationFig. 1 shows the proposed antenna structure which consists of a loop-shaped feeding partwith a tag chip and SRR-shaped radiating part. The circumference of the radiating part should beclose to λ/2 for the purpose of effective radiation into free space. The principle of operationdepicted in Fig. 2 can be explained as follows. It is possible to assume that constant current flowson the feeding part because the feeding part is much smaller than the wavelength of UHF frequencyranges. If the current flows in counter clockwise directions, it produces the magnetic fields whichgo out at inside of the feeding loop and go into at outside of the feeding loop. In order to cancel themagnetic fields, new magnetic fields are induced near the radiating part. Therefore the currentwhich flows in clockwise directions also is induced on the radiating part. It is the source to radiateinto free space.2.2 Equivalent circuit modelThe proposed tag antenna can be expressed as an equivalent circuit in Fig. 3 [1]. Thecapacitance (C couple ) between the feeding part and the radiating part is negligible because thedistance between two parts is much larger than the area of feeding and radiating part. Therefore,only inductively coupling survives. The input resistance and reactance of the inductively coupledfeeding structure can be expressed as (1) and (2) near the resonance frequency, where M is the978-1-4244-1523-3/08/$25.00 © 2008 IEEE 446

mutual inductance between the two parts, R rp,o and Q rp are the radiation resistance and the qualityfactor of the radiating part at resonance frequency, respectively. L fp is the self inductance of thefeeding part [1]. The unknown variables (L fp , M, R rp,o , and Q rp ) of the equivalent circuit areanalyzed and designed next section.2(2 πfM) 1Ra=(1)2Rrp,o 1 + ⎡⎣Q ( / − / ) ⎤rpf fo fof ⎦2(2 πfM) Qrp ( f / fo − fo/ f )Xa= 2 πfLfp−(2)2Rrp,o 1 + ⎡Q f / f − f / f ⎤3. Analysis and design3.1 Inductance of the feeding part (L fp )⎣( )rp o oThe inductance of a circular loop which has radius a and wire radius b can be calculated as(3) [2]. However, the feeding part of the proposed antenna is a planar type which has radius a andfeeding width 2b’ as shown in Fig. 1, so (3) needs to be modified. If most currents flow on thesurface of the wire, from (4), the circumference of the wire is equal to the feeding part width of theproposed antenna. Substituting (4) in (3), we obtain the inductance of the feeding part as (5).⎡ ⎛8a⎞ ⎤Lloop=μ0a⎢ln ⎜ ⎟−2 ⎥(3)⎣ ⎝ b ⎠ ⎦2πb≈2 b' → b≈b'/ π(4)3.2 Mutual inductance (M)Lfp⎡ ⎛8aπ⎞⎤≈μ0a⎢ln ⎜ ⎟−2 (5)'⎥⎣ ⎝ b ⎠ ⎦The solution to the mutual inductance about the proposed structure is not known. So weanalyzed that and checked the validation. In general, a mutual inductance can be expressed using (6),where I fp is a current on the feeding part, B fp is the magnetic flux density which is created by I fp , andS rp is the area of the radiating part.1M = ∫ B fp ⋅dsrp( H) (6)Is rpfpIf the feeding part which has radius a is placed in the x-y plane, centred at the origin, andcarrying a constant current I fp , then the current density has only a φ component. The magneticvector potential A which can be derived from J could be expressed by using complete ellipticintegral of first kind and second kind in (8) [3][4].δ( r' − a)Jφ= Ifpsin θ' δ(cos θ') (7)a2μ 4 I0fpa ⎡(2 −k ) K( k) −2 E( k)⎤Aφ(, r θ ) = (8)2 2 1/2 24 π ( a + r + 2arsin θ)⎢ ⎥⎣ k ⎦2 4arsinθwhere k =2 2a + r + 2arsinθThe magnetic flux density B can be calculated using (9). Since the loop lies in the x-y plane(at θ=90°), B r becomes zero and only B θ component survives. Finally, substituting (9.b) in (6), wecan get the mutual inductance M between the feeding part and radiating part as shown in (10).Because the current flow direction of the feeding part is opposite to that of the radiating part, themutual inductance has minus sign.1 ∂Br= (sin θAφ) (9. a)r sin θ∂θ⎦447