Research Article Small-Size Wearable High-Efficiency TAG ...

Research ArticleSmall-Size Wearable High-Efficiency TAG Antenna for UHFRFID of People

Milan Svanda and Milan Polivka

Faculty of Electrical Engineering, Czech Technical University in Prague, Technicka 2, 166 27 Prague 6, Czech Republic

Correspondence should be addressed to Milan Svanda; [email protected]

Received 3 February 2014; Accepted 13 March 2014; Published 10 April 2014

Academic Editor: Yingsong Li

Copyright © 2014 M. Svanda and M. Polivka. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

This paper introduces a small-size, low-profile wearable radiator based on the coupled patches and vertically folded patchestechniques for application as a tag antenna for identification of people in the European UHF RFID band. The electric fielddistribution comes out dominantly from the central coupling slot, and thus the electric properties of the radiator are almostunaffected by the human body to which the antenna is intended to be attached. Accordingly, with the relative size 0.14 × 0.12 ×0.009 𝜆

0at 866MHz (50 × 40 × 3.04mm3), the antenna exhibits total efficiency better than 50%, even if it is attached directly to a

person.

1. Introduction

Modern body area network (BAN) communication systems[1–5] and also radiofrequency identification systems (RFID)[6–12] require small-size, low-weight, inexpensive radiators,which can be easily integrated into electronic devices or, forexample, on human clothes.

The coupled patches technique, introduced and employedin [13–15], developed especially for screening the influenceof the human body, enables the design of wearable antennaswith an extremely low profile (typically lower than 0.003𝜆0) and at the same time sufficient radiation efficiency

typically better than 50%. This is a significantly better valuethan the radiation efficiency of the common half or quarterwavelength patch antenna of the same size and height [16].However, this technique does not enable the resonant lengthof the antenna to be smaller than approximately 0.3 𝜆

0[14].

Thus, further miniaturization of antenna footprint size is achallenge for researchers.

Capacitive loading of a shorted patch antenna and itsgeneralization, a vertically folded patch technique, whichenables a half or quarter wavelength patch antenna to beminimized bymeans of repeat folding of the patch cavity, waspresented in [17, 18]; see Figure 1.

In this paper, we present a novel small-size high-efficientUHF RFID tag antenna of overall electrical size 𝑘𝑎 = 0.58combining both of the techniquesmentioned above.The cou-pled patches technique, which excites the maximum electricfield magnitude in the central coupling slot, enables highradiation efficiency to be achieved with a very low profile,together with good immunity from the human body asopposite sides are formed by metallic walls (see Figure 1(b)),which reduce the interaction of electric field with the basematerial. At the same time, the vertically folded patchestechnique enables a smaller footprint size of the structure tobe achieved together with an acceptable increase in antennaheight.

2. Design, Realization, and Measurement

Figure 2 depicts a sketch and a photograph of the manufac-tured vertically folded coupled patches UHF RFID antenna.The antenna is manufactured on a low-permittivity substrateTaconic RF-30 with 𝜀

𝑟= 3.0 and loss tangent tan 𝛿 =

0.002. The total size of the proposed antenna is 50 × 40 ×3.04mm3, which gives a relative size of 0.14 × 0.12 × 0.009𝜆0at 866MHz; that is, 𝑘𝑎 = 0.58, where 𝑎 is the diameter of

Hindawi Publishing CorporationInternational Journal of Antennas and PropagationVolume 2014, Article ID 509768, 5 pageshttp://dx.doi.org/10.1155/2014/509768

2 International Journal of Antennas and Propagation

Electric wall

(a) (b)

ResonatingcavityRadiating

slot

FeedingpointResonating

cavityFeeding

point

56388

1.15e5

(V/m

)

27433

13245

6294

2007

1218

401

0

∼𝜆0/8 ∼𝜆0/4

Figure 1:The side cross-section and electric field distribution of the vertically folded quarter wavelength patch antenna (a) and the proposedvertically folded coupled patches antenna (b).

1.52

1.52

2

3

40

50

44 Chip position

Z

Y

X

(a) (b)

Figure 2: Sketch (a) and photograph (b) of folded coupled patches RFID antenna (dimensions stated in mm).

the sphere completely circumscribing the antenna, includingthe mirror currents. The antenna is fed by the NXP G2X2RFID chip with input impedance 𝑍in = 22 − 𝑗195Ω andpower sensitivity −15 dBm. The weight of the antenna isapproximately 15 g.

The performance properties of the antenna were verifiedin a monopole-type arrangement [14] in order to avoid theuse of a balun situated between the antenna and the coaxialconnector; see Figure 3.Themonopole-type input impedancethen accounts for a half of the value compared to the dipole-type impedance. Consequently, 𝑍monopole = 𝑍dipole/2 is

considered for further evaluation (where 𝑍dipole = 𝑍chip∗=

22 + 𝑗195Ω).The transmission coefficient (see Figure 4) between the

antenna and the chip input impedance was evaluated fromthe standard reflection coefficient measurement. The mea-surement was performed with and without a human bodyphantom (manufactured from agar with 𝜀

𝑟∼ 55 and tan 𝛿 ∼

0.5 of 80 × 110 × 15 mm3 size) which was enclosed directly inthe back of the antenna.

The above-mentioned monopole-type arrangementenables us to measure the radiation and the total efficiencies

International Journal of Antennas and Propagation 3

Table 1: Impedance and radiation properties of the vertically folded coupled patches RFID antenna at frequency 866MHz.

Transmission coefficient (—) Radiation efficiency (%) Total efficiency (%)Measurement, free space 0.9 75 66Measurement, agar phantom 0.8 65 51Simulation, free space 1.0 66 51

Figure 3: Photograph of a manufactured prototype of foldedcoupled patches RFID antenna in the monopole-type arrangementwith the ground plane 130 × 130mm2.

800 810 820 830 840 850 860 870 880 890 9000

0.2

0.4

0.6

0.8

1

Frequency (MHz)

Measured, freeMeasured, agarSimulated, free

Tran

smiss

ion

coeffi

cien

t (—

)

Figure 4: Simulation and measurement of the transmission coeffi-cient of the vertically folded coupled patches tag antenna.

by the Wheeler cap method [19]. A cap size of 122 × 122 ×122mm3 was used. The simulation was performed in a fullarrangement, according to Figure 2(a). The measurementwas performed with and without the human body phantom;see Figures 5, 6 and Table 1. Very good immunity from thephantom as well as sufficient radiation and total efficiencycan be observed at operation frequency 866 MHz.

3. Read Range and Identification Tests

In order to evaluate the performance of the TAG antenna inreal operational conditions, read range tests were performedwith transmitted power of 30 dBm and standard 8 dBi readerantennas, which gives 6.3W of effective isotropic radiatedpower (EIRP). The tag antenna with the chip was fixed at a

0

20

40

60

80

100

Radi

atio

n effi

cien

cy (%

)

800 810 820 830 840 850 860 870 880 890 900Frequency (MHz)

Measured, Wheeler method, freeMeasured, Wheeler method, agarSimulated, free

Figure 5: Simulation and measurement of the radiation efficiencyof the vertically folded coupled patches tag antenna.

0

10

20

30

40

50

60

70

80

Tota

l effi

cien

cy (%

)

800 810 820 830 840 850 860 870 880 890 900Frequency (MHz)

Measured, Wheeler method, freeMeasured, Wheeler method, agarSimulated, free

Figure 6: Simulation and measurement of the total efficiency of thevertically folded coupled patches tag antenna.

height of 1.3m in free space and on a person’s chest over about2mm thin shirt. The standard commercial RFID system (seeTable 2) was used for the evaluation of the read distance aswell as the reliability of person identification in corridors.

The read range evaluated in 4m width corridor in a freespace is 7.5m, and for the antenna attached to a human chestthe read range is 7.0m; see Figure 7.The read range evaluatedin 2m width corridor in a free space is 11m, and for theantenna attached to a human chest the read range is 9.7m;see Table 3.

4 International Journal of Antennas and Propagation

(a) (b)

Figure 7: Photograph of a test configuration: general view (a) and details (b) of a person with the chest-fixed TAG in 4m wide corridor.

Table 2: Standard used UHF RFID system parameters.

Systemcomponents Parameter Values

Operating frequency(Europe) 866MHz

Reader

Transmitted power 30 dBmReceiver sensitivity −64 dBmReader antenna gain 8.0 dBiReader antenna polarization Linear

Transponder(TAG)

Chip sensitivity −15 dBmChip impedance 22 − 𝑗195Ω

TAG conversion loss Approximately 6 dB

Table 3: Identification tests of the folded coupled patches antennain free space as well as on the human chest in buildings corridors.

Corridor width Read range,free space (m)

Read range,human chest

(m)4m 7.5 72m 11 9.7

4. Conclusion

A novel small footprint size and extremely low profilewearable antenna based on a combination of coupled patchesand vertically folded patches techniques has been introduced,and a sample has been developed for European UHF RFIDband. The size of the tag antenna without a chip was 50 × 40× 3.04mm3, which is 0.14 × 0.12 × 0.009 𝜆

0at 866MHz. The

antenna exhibits total efficiency better than 50%, irrespectiveof whether it is placed in a free space or enclosed on a humanbody phantom. The read range of the antenna placed ona person’s chest tested was better than 7m, while showingnegligible influence of the human body to which the antennawas attached.

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper.

Acknowledgments

This research was undertaken at the Department of Electro-magnetic Field at the Czech Technical University in Prague.It was jointly supported by the Czech Science Foundation(Project no. P102/12/P863: “Electromagnetic Properties ofRadiating Structures and Artificial Screening Surfaces in theClose Vicinity of the Human Body”) and a COST Project (no.LD 12055 AMTAS: “Advanced Modelling and Technologiesfor Antennas and Sensors”), which forms a subpart of COSTProject no. IC 1102 VISTA: “Versatile, Integrated, and Signal-Aware Technologies for Antennas.”

References

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[16] K. F. Lee and W. Chen, Advances in Microstrip and PrintedAntennas, chapter 5, John Wiley & Sons, New York, NY, USA,1997.

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