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Key Laboratory of Radar Imaging and Microwave Photonics，Ministry of Education，College of Electronic and Information Engineering，Nanjing University of Aeronautics and Astronautics，Nanjing 211106，P.R.China

Abstract: A compact antenna formed by three concentric split rings for ultra-high frequency（UHF）radio frequency identification（RFID）tag is proposed in this paper. The antenna is composed of two parts，an outer short-circuited ring modified from a traditional split-ring resonator（SRR）antenna and an inner SRR load，so the antenna can be regarded as a short-circuited ring loaded with SRR. According to the transmission line theory，to conjugate match with the capacitive input-impedance of a tag chip，the length of the short-circuited ring is λg/4 shorter than that of an open-circuited dipole of a traditional SRR antenna，where λg is the wavelengh of the operating frequency. Hence，the size of the proposed antenna is more compact than that of the traditional SRR antenna. Thereafter，the proposed antenna is simulated and optimized by ANSYS high-frequency structure simulator（HFSS）. The impedance，efficiency，and mutual coupling of the fabricated antenna are tested in a reverberation chamber（RC）. The results show that the size of the presented antenna is 83% smaller than that of the traditional SRR antenna and the proposed antenna can cover the whole UHF RFID operating frequency band worldwide（840—960 MHz）. The measured read range of the tag exhibits maximum values of 45 cm in free space and 37 cm under dense tag environment.

Key words：radio frequency identification（RFID）；compact antenna；split-ring resonator（SRR）；reverberation chamber（RC）；dense tag environment

0 Introduction

Radio frequency identification（RFID）is a de?noting technology that uses electromagnetic waves to uniquely identify tagged objects. An RFID tag generally contains an integrated circuit chip for stor?ing the information about the tagged item and an an?tenna for receiving and transmitting radio signals.Passive RFID tags can collect electrical energy from a reader to activate the chip and response a modulat?ed wave back to the reader. Due to the small dimen?sion and long read range，passive ultra-high frequen?cy（UHF）RFID tags have become the focus of in?tense research.

One of the most challenging aspects of RFID tag design is the reduction in antenna size. In practi?cal applications，the dimensions of tag antenna are commonly required to be much smaller than the wavelength for the frequency of operation. Several traditional methods have been reported to reduce the antenna size，such as meandering technique and ca?pacitive tip-loading structure［1-2］. With the flexibility and new properties provided by metamaterials，new types of miniaturized antennas have been con?ceived［3-4］. Among these，the split-ring resonator（SRR）structure gives an effective technique for the electrically small RFID tag design［5-8］. Zamora et al.［7］presented a compact planar passive UHF RFID tag（0.23λ0× 0.23λ0）based on an SRR an?tenna. Zuffanelli et al.［8］proposed an edge coupled SRR tag antenna with size ofλ0/11 ×λ0/11. Apart from the miniaturization， the mutual coupling among tags under dense tag environment needs to be investigated. The reading performance of the RFID system may be degraded when RFID tags are closely stacked together［9-10］. Hence，it is urgently required to develop a compact and reliably tag anten?na in short-range reading application.

In this paper，a compact antenna formed by three concentric split rings for UHF RFID tag is pre?sented. The antenna can be regarded as a short-cir?cuited ring loaded with SRR. According to the trans?mission line theory and the equivalent circuit mod?el，the principles of the traditional SRR antenna and the presented antenna are researched. Then，the presented tag antenna is simulated and optimized by high-frequency structure simulator （HFSS）. By analogy with the multipath environment in a metal cabinet，a reverberation chamber（RC）with a simi?lar size is employed to evaluate the radiation perfor?mance of the antenna［11-13］. The simulated and mea?sured results show that the antenna has an 83%smaller area than the traditional SRR antenna and can cover all the band of the UHF RFID（840—960 MHz）. The measured maximum read range of the presented tag agrees well with the theory values.

1 Antenna Principle

1.1 Miniaturization principle

A traditional SRR tag antenna shown in Fig.1 has been presented and analyzed in detail in Ref.［14］. In the traditional view，an SRR structure is composed of two circular rings with splits at oppo?site edges. When employing it as a tag antenna，its excitation port is placed across an additional gap in the outer ring. In this circumstance，the antenna can be considered as a curved dipole loaded with a split ring in another view. In the theory of transmission line，the dipole can be regarded as an open-circuited transmission line. To obtain a conjugate match with the capacitive input-impedance of a RFID chip，the minimum length of an inductive open-circuited trans?mission line isλg/4 longer than that of an inductive short-circuited one，whereλgis the wavelengh of the operating frequency. As shown in Fig.2，a Smith chart is employed to show the procedure of a conjugate match，in which a solid blue star and a hollow red star represent the chip impedance and its conjugate impedance， respectively. As is well known，clockwise rotation in the Smith chart repre?sents a movement towards the signal source and each revolution on the Smith chart corresponds to moving half wavelength along the transmission line.Obviously，the distance of clockwise rotation from the open-end to the hollow red star isλg/4 longer than that of from the short-end. Hence，if the end of the curved dipole is shorted，a modified antenna with smaller size than that of traditional SRR can be obtained.

Fig.1 Configuration of the traditional SRR tag

Fig.2 Impedances (normalized to 50 Ω)in a Smith chart

1.2 Equivalent circuit model

The traditional SRR tag is analyzed by an equivalent circuit model［14］，as shown in Fig.3. The equivalent resistance and capacitance of the chip port are denoted byRCandCC，respectively. The antenna radiation resistance is ignored in the model as its value is small. The equivalent circuit of the SRR structure can be considered as a series of induc?tances and capacitances［15］. The equivalent induc?tances consist of the self-inductanceL0of the ring and mutual inductanceLmbetween the rings.Crrep?resents the mutual capacitance between the inner and outer ring，which is divided into two equal parts by splits of two rings. The equivalent capacitances between two splits are denoted asCs. Both of them are paralleled to the corresponding half ofCr. The circumference of the SRR and the gap widthg3（Fig.1）between two split rings have great effect on the equivalent inductance and capacitance of the SRR structure.

To make the antenna compact，the outer ring of the traditional SRR antenna is replaced by a shortcircuited ring. At this time，the inner ring will be se?verely detuned and deviated from the resonant fre?quency，which acts as a capacitive load to adjust the resonant frequency of the antenna.

2 Antenna Design and Simulation

2.1 Proposed tag antenna based on compact SRR

The proposed tag antenna is printed on a polytetrafluoroethylene （PTFE） substrate with a relative permittivity of 2.55 and a thickness of 0.5 mm，as shown in Fig.4. A Monza 4 chip with an input-impedance of（11-j143）Ω at 920 MHz is used for the proposed tag. To introduce an addition?al capacitance，a smaller split ring with a radius ofR3is loaded onto the proposed antenna. An SRR is formed by these two split rings with radii ofR2andR3. Therefore，the proposed tag antenna is consid?ered to be formed by the short-circuited ring loaded with the compact SRR，where the short-circuited ring is modified from the traditional SRR antenna.

Fig.4 Configuration of the proposed compact passive UHF RFID tag antenna based on SRR structure

An equivalent circuit model of the presented tag antenna is shown in Fig.5. The equivalent induc?tances of the proposed antenna consist of the self-in?ductanceL'0of each split ring and the mutual induc?tanceL'mbetween split rings.CLandCLrrepresent the self-capacitance of the short-circuited ring and the mutual capacitance between the short-circuited ring and SRR，respectively. It is observed that two equivalent circuit parametersCLandCLrare newly introduced in this circuit. They make a great contri?bution to the reduction of antenna size.

Fig.5 Equivalent circuit model of the proposed compact tag antenna

The simulated vector current density distribu?tion of the proposed tag antenna is depicted in Fig.6（a）. It can be seen that the current is mainly concentrated on the outer square loop. The currenti0on the left and right sides of the outer square loop is opposite to that on the inner ring. Hence，the an?tenna obtains a bidirectional pattern，as shown in Fig.6（b）. Due to the large reduction of antenna size，the split ring loads are too small to resonate at 920 MHz. They are loaded onto the proposed anten?na to provide an additional capacitance to adjust the resonant frequency of the antenna slightly.

Fig.6 Simulated vector current density distribution and 3D gain pattern of the proposed tag antenna

2.2 Parametric analysis

A parametric study is conducted to examine the effects of the parameters of the short-circuited ring and the SRR at the antenna resonant frequency. In this section，parametric analysis is carried out forW1，R1，andR3. ParameterWvaries withW1and the other parameters remain unchanged as follows：L= 12 mm，R2= 2.8 mm，Wr= 0.8 mm，Ws=0.9 mm，andh= 0.5 mm. The antenna is analyzed by HFSS.

The dimension of the outer square loop has a great effect on the resonant frequency of the anten?na. An increase of the length of the tagW1can re?sult in a lower resonant frequency，as shown in Fig.7（a）. Once the dimensions of the outer square loop are fixed，the key parameter affecting the reso?nant frequency of the antenna is the radius of the in?ner folded ring. By decreasingR1to increase the gap widthg2，the capacitance of the presented antenna can be reduced. This leads to the decrease of the res?onant frequency of the antenna，as shown in Fig.7（b）. For the loaded compact SRR，the radius of the inner split ring is the primary variables for the shift of the antenna resonant frequency. Keep the value ofR2unchanged，an increase ofR3leads to a decrease ofg3，which may increase the capacitance of the SRR structure. Hence，the antenna resonant fre?quency can be decreased with the increase ofR3，as shown in Fig.7（c）.

Fig.7 Simulated reflection coefficients of the tag antenna with different values

The optimized parameters of the tag antenna are listed in Table 1. The final optimized dimension of the presented antenna is about 12 mm×11.5 mm.Compared with that of the traditional SRR antenna，as shown in Fig.1，it can be seen that the size of the presented antenna is reduced by 83%.

Table 1 Tag antenna dimensions mm

3 Measurement Results and Analy?sis

3.1 Impedance measurement

Because of the demand of balanced feeding，a test fixture［16-17］，as shown in Fig.8，is employed to measure the antenna impedance with an experimen?tal methodology proposed in Ref.［16］. The length of each coaxial line is 100 mm. The balanced anten?na and test fixture are considered as two-port net?works. The simulated and measured impedance of the presented tag antenna are shown in Fig.9. The measured real and imaginary parts of the impedance agree with the simulated results. But due to the ex?perimental error caused by the fixture，the test re?sults are not exactly the same as the simulation re?sults.

Fig.8 Photograph of the test fixture made up of two coaxial lines of common ground connection

Fig.9 Simulated and measured impedance of the proposed tag antenna

3.2 Measurement of antenna gain and read rang

The antenna gain is measured by using two identical tag antennas，which is a basic method for measuring antenna gain. Assuming the gain of two antennas are the same，the gain can be expressed as

wherePtandPrare the transmit power and receive power respectively，andRis the distance between two antennas that needs to subject to the far field condition. In this test，a homemadeλ/4 coaxial bal?un［17-18］is employed to feed the tag antenna. The bal?un is the same as that used in Ref.［17］. For the con?venience of testing，the gain values are measured ev?ery 30° inxy-plane. The gain patterns of the present?ed tag antenna are demonstrated in Fig.10. The sim?ulated and measured gains inxy-plane are 0.69 dBi and 0.2 dBi，respectively.As expected，a weak bidi?rectional characteristic of the antenna is obtained.

Fig.10 Simulated and measured gain patterns of the pro?posed antenna in xy-plane and xz-plane at 920 MHz

Thereafter，the read range of the presented tag is measured by a handheld reader in an anechoic chamber，as shown in Fig.11. The tested maximum read range values are agreed well with the theoreti?cal values calculated by

Fig.11 Scenario of read range measurement

where the equivalent isotropically radiated power（EIRP）of the handheld reader is 4 W，Grthe gain of the antenna，Pththe minimum threshold power to activate the chip，andτthe power transmission coef?ficient that can be expressed asτ=（1-|Γ|2），hereΓis the reflection coefficient of the tag antenna. The sensitivity of Monza 4 chip is -17.4 dBm. The test?ed results and theoretical values of the maximum read ranges in free space and the dense tag environ?ment are shown in Table 2. Multiple tags are stacked to simulate the dense tag environment with a stacking interval of 5 mm. From the results，the read ranges of the proposed tag in dense tag environ?ment are close to that in free space. This signifies that the mutual coupling has little effect on the tag and the proposed antenna can be employed reliably for RFID tag in dense tag environment.

Table 2 Maximum read range in free space and dense tag environment cm

3.3 Measurement of efficiency and coupling

By analogy with the multipath environment in a metal cabinet，an RC with a similar size is em?ployed to test the antenna. The dimension of the RC is 1.2 m×0.8 m×1.2 m，as shown in Fig.12. Theλ/4 coaxial balun is used to connect the tag antenna and the coaxial cable. The antenna radiation efficien?cy is tested by the reference antenna（Ref Ant）method. The antenna under test（AUT）is the pro?posed tag antenna and the Ref Ant is a wide band Vivaldi antenna with known radiation efficiency of 85%. The measurement method and the scenario can be found in Refs.［13，17］and the measured re?sults are shown in Fig.13. It is observed that the ra?diation efficiency of the antenna can over 75% at 920—925 MHz and the matching of the antenna port is fairly good since the total efficiency is closest to the radiation efficiency at 920—925 MHz.

Fig.12 Scenario of the reverberation chamber

Fig.13 Measured radiation efficiency and total efficiency of the manufactured tag antenna in the RC

The mutual coupling effect between tag anten?nas is also investigated in this paper. The coupling values between two identical proposed tags are test?ed in the anechoic chamber and RC，respectively.The tags are stacked at 5 mm intervals and the test method can be found in Ref.［17］. The simulated mutual coupling values between the presented tag antennas are compared with that of the traditional SRR antennas in free space. The results in Fig.14 show that the simulated coupling between two pro?posed tag antennas is lower than that of two tradi?tional SRR antennas. Besides，the mutual coupling of the proposed antenna is less than -10 dB when two tags stacked at 5 mm intervals，which indicates the metal cabinet environment has no obvious effect on the tag and the tag is well isolated from the oth?ers.

Fig.14 Mutual coupling between two identical tag antennas stacked at 5 mm intervals measured in free space and RC

4 Conclusions

A compact antenna formed by three concentric split rings for UHF RFID tag is presented. The principles of the traditional SRR antenna and the proposed antenna are explained. The size of the pre?sented antenna is 83% smaller than that of the tradi?tional SRR antenna while the gain remains moder?ate. The impedance，gain，maximum read range，efficiency，and mutual coupling of the proposed an?tenna are investigated. The simulated and measured results show that the presented antenna can cover the whole UHF RFID operating frequency band.Moreover，the tested maximum read range of the tag is in good agreement with the simulated results and can reach 45 cm in free space and 37 cm under dense tag environment. The mutual coupling be?tween two stacked tag antennas is relatively low，which makes the tag operate reliably in practical ap?plications.