1

A Novel Compact UHF RFID Tag Antenna Designedwith Series Connected Open Complementary Split Ring

Resonator (OCSRR) Particles

Benjamin D. Braaten

Abstract—A novel compact planar antenna for passive UHFradio frequency identification (RFID) tags is presented. Instead ofusing meander-line sections, much smaller open complementarysplit ring resonator (OCSRR) particles are connected in seriesto create a small dipole with a conjugate match to the powerharvesting circuit on the passive RFID tag. The manufactured(prototype) OCSRR RFID tag presented in this paper has anantenna input impedance of 15.8+j142.5 Ω at a frequency of920 MHz and a max read range of 5.48 m. This performance isachieved with overall tag dimensions of .036λ0× .17λ0 where λ0

is the free space wavelength of 920 MHz.

Index Terms—Open Complementary Split Ring Resonators,RFID, meander-line, dipole and passive tag.

I. INTRODUCTION

The antenna is a major component in the design of passiveUHF radio frequency identification (RFID) tags. Previously,meander-line antennas have been popular designs for compactpassive UHF RFID tags [1]-[5]. Meander-line antennas aredesigned by bending the arms of a printed dipole at rightangles to create an electrically larger antenna in a smallerarea [6]. The resulting antenna is a dipole with square-shapedsections along each dipole arm (Fig. 1 (a)). The current oneach vertical section (i.e., y-directed trace) of the dipole is inthe opposite direction of the current on each adjacent verticalsection. This results in a canceled far-field, thus, the radiationof the meander-line antenna is mainly due to the horizontalsections. The behavior of the meander-line antenna in Fig.1 (a) can be described from a parallel circuit point-of-view.An equivalent capacitance exists between the longer parallelvertical traces, while the horizontal traces form an equivalentinductance. This results in a parallel LC-circuit representationof each meander-line section [7]. Thus, each meander-line polecan be thought of as several series connected parallel LC-circuits. Therefore, to achieve an inductive input impedance,the inductive part of each meander-line section needs to bedominant. This is especially important for electrically smallantennas, because the input impedance can be capacitive [8]-[9].

There are two major advantages of using a meander-lineantenna. First, an inductive input impedance can be achievedwith overall dimensions that are a fraction of the sourcewavelength. Second, the meander-line antenna only requiresa single conducting layer. These two characteristics of anRFID antenna are very useful because it is usually desirableto have a small RFID tag printed on a single substrate (i.e.,adhesive materials) with a conjugate match to the passiveRFID integrated circuit (IC). Even though the meander-lineantenna has resulted in small printed antenna designs, asmaller antenna is still very desirable. This is because a smallerRFID antenna can be used in many more applications and areless costly to manufacture (less material).

Recently, a novel band pass filter (BPF) using an opencomplementary split ring resonator (OCSRR) particle was

Port 2 Port 1

(a)

(b)

(c)

Port 2

Locsrr

Cocsrr

Port 1

ro

gh c

m

n

fd

v

y

dipole arm

Port

x

meander-line section

Fig. 1. (a) A meander-line antenna [7]; (b) the OCSRR particle; (c)the equivalent circuit of the OCSRR particle.

presented in [10]. This OCSRR BPF was implemented incoplanar waveguide (CPW) technology. The OCSRR particleis shown in Fig. 1 (b) with the equivalent circuit shown inFig. 1 (c). The equivalent circuit of the OCSRR particle hasthe same equivalent circuit as each meander-line section inFig. 1 (a). Thus, series connected OCSRR particles have thesame equivalent circuit as the series connected meander-linesections.

The work in this paper presents the application of usingthese OCSRRs in a manner similar to the series connectedmeander-line sections in Fig. 1 (a). In particular, the particlesof OCSRRs in Fig. 1 (b) are connected in series to form theprinted dipole shown in Fig. 2. The newly proposed designshown in Fig. 2 performs well as an RFID tag antenna andhas the following very useful characteristics: 1) very simple todesign; 2) very easy to manufacture; 3) only one conductinglayer is required for normal operation thus avoiding compli-cated structures to enhance the performance; 4) each OCSRRparticle is much smaller than the meander-line sections inpreviously published designs; 5) this antenna has a gain that islarger than other published meander-line antennas; and finally6) the design presented in this paper is up to 50% smaller thancommercially available tags with comparable read ranges.

II. UHF RFID ANTENNA WITH SERIESCONNECTED OPEN COMPLEMENTARY SPLIT

RING RESONATOR PARTICLES

Other than the equivalent circuit in Fig. 1 (c), the OCSRRpresented in [10] was chosen for the design in Fig. 2 for

2

w

h

p

kq

s

port

Fig. 2. The proposed UHF RFID antenna using series connectedOCSRR particles.

two major reasons. First, each particle can be printed ona single conducting plane. This is very useful because, asmentioned before, antenna designs for passive UHF RFID tagsare usually printed on a single adhesive dielectric substrate.For example, the design presented in this paper is muchless complex than the metamaterial-based designs presentedin [11]-[18]. Even though the antenna designs presented in[11]-[18] have promising results, the designs require complexground structures, lumped elements and different materials toperform well. Using these techniques could result in a costlyand complicated RFID tag. Second, the inductance of theOCSRR particle is four times than that of the complementarysplit ring resonator (CSRR) [10]. This makes the OCSRR par-ticularly useful for compact UHF RFID antennas because theinput impedance of an electrically small antenna is capacitive.By using several of the OCSRR particles connected in series,the input impedance of an electrically small resonant antennacould be made inductive, which will be a conjugate matchto the input impedance of the power harvesting circuit in thepassive IC on the RFID tag.

The inductance of each particle is created by the loop ofconducting material that has a width of h in Fig. 1 (b) (i.e.,the conducting loop between the two inner and outer ringslots). This conducting loop creates an electrical short betweenports A and B. The capacitance in the particle is betweenthe inner conducting disk with radius ro and the surroundingouter conducting planes. These two structural features in theparticle result in an equivalent inductance and capacitanceconnected in parallel. This resulted in the equivalent circuit inFig. 1 (c) [10]. By connecting the OCSRR particles in seriesand choosing a size of each particle were the inductance isdominant; a designer is able to produce a layout that resemblesa thick printed dipole with periodic ring slots along the lengthof the dipole. Having the layout resembling a thick printeddipole has the added advantage of increasing the radiationresistance and bandwidth [9]. This is very useful for matchingthe real part of the input impedance of the antenna to the powerharvesting circuit, especially because the input resistance of anelectrically small antenna is very low.

A. Experimental Results and Validation

The first step was to design, manufacture and test a proto-type passive RFID tag with the antenna in Fig. 2. Momentum

Higgs-2

Fig. 3. The printed OCSRR RFID antenna on the prototype passiveRFID tag (c = .59 mm, d = .55 mm, f = .66 mm, g = .63 mm, h =.65 mm, k = .77 mm, m = 13.37 mm, n = 11.91 mm, p = 1.23 mm,q = .61 mm, ro = 3 mm, s = 4.11 mm, w = 55.54 mm, h = 11.91mm and v = .7 mm).

800 850 900 950 10000

50

100

150

f (MHz)

Re(

Zin

) (Ω

)

SimulationMeasured

800 850 900 950 1000

0

200

400

f (MHz)

Im(Z

in)

(Ω)

Fig. 4. Measured input impedance of the printed OCSRR antenna.

[19] was used to design the printed antenna on 1.36 mm ofFR-4 (ε = 3.7 measured and tan δ = .0011 measured). Thepassive IC on the prototype RFID tag chosen was the Higgs-2 by Alien Technologies [20]. For a center frequency of 920MHz, the Higgs-2 IC has an input impedance of approximatelyZIC = 13.6-j142.8 Ω [20]. Thus, the printed antenna designshould be close to the conjugate of ZIC at 920 MHz. Thisconjugate match requirement resulted in the printed designshown in Fig. 3. The size of each particle was determinedexperimentally in Momentum. The simulated and measured1

input impedance results agreed well and are shown in Fig. 4.The next step was to determine how well the prototype

RFID tag in Fig. 3 performed. The max read range of the tagwas determined by attaching the tag to a piece of styrofoam.

1To measure the input impedance, the printed antenna was cut symmetri-cally down the middle, placed vertically above a ground plane and fed withan SMA connector through the ground plane. The input impedance was thenmeasured using a calibrated network analyzer.

3

OCSRR RFID tag

Fig. 5. Measuring the max read range of the prototype OCSRR RFIDtag in Fig. 3.

Then an Alien 9780 reader [20] and the prototype tag on thestyrofoam were placed in an anechoic chamber. The experi-mental test is shown in Fig. 5. The prototype tag was movedaway from the reader antenna and a max read range of 5.48 mwas determined. This max read range is much greater than thephysically larger meander-line antenna presented in [5]. Then,the prototype tag was placed on a plastic container with a lowvalue of permittivity. This last test was performed to illustratethe performance of the prototype tag on a manufactured item.Again, the Alien 9780 reader and the plastic container wereplaced in the anechoic chamber (Fig. 6). The max read rangewas then determined to be 5.33 m.

The good results in Fig. 4 and the large read range valuesof the prototype tag indicate that Momentum is an excellentand precise tool for modeling the antenna in Fig. 2. Therefore,because of the numerous designs in the next sections, Momen-tum will be used solely to calculate the input impedance andgain of a variety of printed OCSRR antennas. In particular,Momentum will be used to calculate the input impedance andgain of the OCSRR antenna in Fig. 2 on various substratepermittivities, substrate thicknesses and with several valuesof ro (i.e., disk radius values). These results will then befollowed with Figures showing the surface current on theOCSRR antenna and the far-field patterns. Investigating theseproblems extensively will demonstrate the behavior of theOCSRR antenna presented in this paper. This could be veryuseful for a designer because a short review of the followingsections may determine if the OCSRR antenna presented inthis paper is suited for a particular application of a passiveUHF RFID tag.

B. Results for various values of permittivity

First, the permittivity ε of the substrate was varied. Thisillustrated how the input impedance and gain were affectedby different values of ε. The results from these simulationsare shown in Figs. 7 - 9. The results in Fig. 7 show the rateat which the input resistance increases for larger values of εwhile Fig. 8 shows how the antenna becomes more inductive

OCSRR RFID tag

plastic

container

Fig. 6. Measuring the max read range of the prototype OCSRR RFIDtag in Fig. 3 on a plastic container.

for larger values of ε. The results in Fig. 9 show a max gainof 1.93 dBi and that the gain reduces slightly for larger valuesof ε.

C. Results for various values of substrate thickness

Second, the thickness d of the substrate was changed. Thisillustrated how the input impedance and gain were affected bydifferent values of d. The results from these simulations areshown in Figs. 10 - 12. The results in Fig. 10 show how theinput resistance can increase substantially for larger values ofd and Fig. 11 shows that the antenna becomes more inductive(i.e., resonates at a lower frequency) for larger values of d.The results in Fig. 12 show a max gain of 1.86 dBi and thatthe gain can be significantly reduced for larger values of d.

D. Results for various values of inner disk radius

Next, the radius of the inner disk ro was varied. Thisillustrated how the input impedance and gain were affectedby different values of ro. The results from these simulationsare shown in Figs. 13 - 15. The results in Fig. 13 show that theinput resistance varies less than an ohm for various values ofro, while Fig. 14 shows that the input reactance of the antennaonly varies by a few ohms for ro approximately less than 2.6mm. Then, the capacitance between the inner disk and outerconducting plane becomes more significant for ro ≥ 2.6 mm.After this value, the input reactance begins to reduce muchmore rapidly. The results in Fig. 15 show a max gain of 1.89dBi and that the gain is mostly unaffected by the radius of theinner disk.

E. Surface currents and far-field patterns.

Finally, the surface currents and far-field patterns weredetermined. The surface currents on the OCSRR antenna areshown in Fig. 16. Notice the currents concentrated on the innerconducting ring. The normalized far-field patterns for the x-zand y-z planes are shown in Figs. 17 and 18, respectively. Thefar-field patterns are similar to the patterns of a small dipole.

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f (MHz)

Re(

Zin

) (Ω

)

ε = 1.0, d = .787 mm

ε = 2.2, d = .787 mm

ε = 4.25, d = .787 mm

ε = 5.8, d = .787 mm

Fig. 7. Input resistance for various values of ε.

800 850 900 950 1000−500

0

500

1000

1500

2000

f (MHz)

Im(Z

in)

(Ω)

ε = 1.0, d = .787 mm

ε = 2.2, d = .787 mm

ε = 4.25, d = .787 mm

ε = 5.8, d = .787 mm

Fig. 8. Input reactance for various values of ε.

800 850 900 950 10000

0.2

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1

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f (MHz)

Gai

n (d

Bi)

ε = 1.0, d = .787 mm

ε = 2.2, d = .787 mm

ε = 4.25, d = .787 mm

ε = 5.8, d = .787 mm

Fig. 9. Gain for various values of ε.

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f (MHz)

Re(

Zin

) (Ω

)

ε = 4.25, d = .127 mm

ε = 4.25, d = .787 mm

ε = 4.25, d = 1.57 mm

ε = 4.25, d = 3.14 mm

Fig. 10. Input resistance for various values of d.

800 850 900 950 1000−500

0

500

1000

1500

2000

f (MHz)

Im(Z

in)

(Ω)

ε = 4.25, d = .127 mm

ε = 4.25, d = .787 mm

ε = 4.25, d = 1.57 mm

ε = 4.25, d = 3.14 mm

Fig. 11. Input reactance for various values of d.

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Gai

n (d

Bi)

ε = 4.25, d = .127 mm

ε = 4.25, d = .787 mm

ε = 4.25, d = 1.57 mm

ε = 4.25, d = 3.14 mm

Fig. 12. Gain for various values of d.

5

1 1.5 2 2.5 313

13.2

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13.6

13.8

14

14.2

14.4

14.6

14.8

15

ro (mm)

Re(

Zin

) (Ω

)

Fig. 13. Input resistance for various values of ro with ε = 4.25 andd = .787 mm.

1 1.5 2 2.5 3100

105

110

115

120

125

130

135

140

ro (mm)

Im(Z

in)

(Ω)

Fig. 14. Input reactance for various values of ro with ε = 4.25 andd = .787 mm.

1 1.5 2 2.5 30

0.2

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1

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ri (mm)

Gai

n (d

Bi)

Fig. 15. Gain for various values of ro with ε = 4.25 and d = .787mm.

0 A/m 50 A/m

Fig. 16. Surface currents on the printed OCSRR antenna in Fig. 3with ε = 3.7 and d = 1.36 mm..

0.2

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0.8

1

30

210

60

240

90

270

120

300

150

330

180 0

x−z plane

|Eθ|

|Eφ|

Fig. 17. Normalized pattern in the x-z plane of the printed OCSRRantenna in Fig. 3 with ε = 3.7 and d = 1.36 mm.

0.2

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0.8

1

30

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60

240

90

270

120

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180 0

y−z plane

|Eθ|

|Eφ|

Fig. 18. Normalized pattern in the y-z plane of the printed OCSRRantenna in Fig. 3 with ε = 3.7 and d = 1.36 mm..

6

III. DISCUSSION

Many noteworthy comments can be made about the newresults in Fig. 3, Fig. 5 and Figs. 7 - 18.

1) The dimensions in Fig. 3 show that the OCSRR UHFRFID antenna presented in this paper is much smaller (55.54mm X 11.91 mm or .036λ0 × .17λ0) than commerciallyavailable tags.

2) The measurements shown in Fig. 5 demonstrate thatthe prototype tag in Fig. 3 has a max read range (5.48 m)comparable to other published and commercially availablepassive RFID tags with twice the overall size.

3) In Figs. 7 and 10, it is shown how the radiation resistancecan be controlled with values of ε and d, respectively.

4) In Figs. 8 and 11, it is shown how the antenna canbe designed to resonate at a lower frequency with largervalues of ε and d, respectively. These results are similar tothe metamaterial-based antennas presented in [4].

5) From the results in Figs. 9 and 12, it can be concludedthat the gain of the antenna can be increased using smallervalues of ε and d, respectively.

6) The results in Figs. 13 and 14 show that the particleinductance can be used effectively to match the input reactanceof the antenna with the reactance of the tag.

7) The patterns shown in Figs. 17 and 18 are similar to asmall dipole and illustrate that the tag in Fig. 3 can be readfrom many different angles.

IV. CONCLUSIONS

A new compact UHF RFID tag antenna designed usingseries connected OCSRR particles was presented. The seriesconnected OCSRR particles were modeled in Momentumand the simulated input impedance values were successfullyvalidated with measurements. The result was a compact UHFRFID tag with a read range of 5.48 m. This read rangewas accomplished with an overall antenna dimension up to50% smaller than commercially available RFID tags withcomparable read ranges. Next, the properties of the substrateand dimensions of the inner disk were varied. Each of thesechanges were modeled numerically in Momentum and theinput impedance and gain was determined for the differentsubstrate and inner disk values. These simulations were doneto illustrate the behavior of the novel OCSRR RFID antennapresented in this paper. Finally, the surface currents and far-field patterns were determined. The OCSRR antenna presentedin this paper has a pattern similar to a small dipole.

ACKNOWLEDGEMENTS

The authors would like to thank Aaron Reinholz andMichael Reich, at the Center for Nanoscale Science and Engi-neering (CNSE), North Dakota State University, for supportingvarious aspects of this project.

REFERENCES

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[19] Advanced Design System-ADS 2009, Agilent Technologies.[20] Alien Technologies, [online], www.alientechnologies.com.