a wideband compact wlan/wimax mimo antenna based on...

2290 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 63, NO. 5, MAY 2015

[13] H.-J. Stöckmann, Quantum Chaos: An Introduction. Cambridge, U.K.:Cambridge Univ. Press, 2006.

[14] L. R. Arnaut and G. Gradoni, “Probability distributions of the quality fac-tor of a mode-stirred reverberation chamber,” IEEE Trans. Electromagn.Compat., vol. 55, no. 1, pp. 35–44, Feb. 2013.

[15] F. Monsef, “Why a reverberation chamber works at low modal overlap,”IEEE Trans. Electromagn. Compat., vol. 54, no. 6, pp. 1314–1317, Dec.2012.

[16] L. R. Arnaut, “Mode-stirred reverberation chambers: A paradigm forspatio-temporal complexity in dynamic electromagnetic environments,”Wave Motion, vol. 51, no. 4, pp. 673–684, Jun. 2014.

[17] L. R.Arnaut, “Measurement uncertainty in reverberation chambers–I.Sample statistics,” Natl. Phys. Lab., Teddington, U.K., NPL Report TQE2, 2nd ed., pp. 1–136, Dec. 2008.

[18] L. R.Arnaut, M. I. Andries, J. Sol, and P. Besnier, “Evaluation methodfor the probability distribution of the quality factor of reverberation cham-bers,” IEEE Trans. Antennas Propag., vol. 62, no. 8, pp. 4199–4208, Aug.2014.

[19] L. R. Arnaut, “Compound exponential distributions for undermodedreverberation chambers,” IEEE Trans. Electromagn. Compat., vol. 44,no. 3, pp. 442–457, Aug. 2002.

[20] L. R. Arnaut, “Evaluation of the NPL untuned stadium reverberationchamber using mechanical and electronic stirring techniques,” Natl.Phys. Lab., Teddington, U.K., NPL Report CEM 11, Aug. 1998.

[21] L. Bernadó, T. Zemen, F. Tufvesson, A. F. Molisch, andC. F. Mecklenbräuker, “Delay and Doppler spreads of nonstation-ary vehicular channels for safety-relevant scenarios,” IEEE Trans. Veh.Technol., vol. 63, no. 1, pp. 82–93, Jan. 2014.

[22] Y. Rissafi, L. Talbi, and M. Ghaddar, “Experimental characterization of anUWB propagation channel in underground mines,” IEEE Trans. AntennasPropag., vol. 60, no. 1, pp. 240–246, Jan. 2012.

[23] T. J. Willink, “Observation-based time-varying MIMO channel model,”IEEE Trans. Veh. Technol., vol. 59, no. 1, pp. 3–15, Jan. 2010.

[24] L. R. Arnaut, “Statistics of the quality factor of a rectangular rever-beration chamber,” IEEE Trans. Electromagn. Compat., vol. 45, no. 1,pp. 61–76, Feb. 2003.

A Wideband Compact WLAN/WiMAX MIMO AntennaBased on Dipole With V-shaped Ground Branch

Han Wang, Longsheng Liu, Zhijun Zhang, Yue Li, and Zhenghe Feng

Abstract—A wideband printed dipole with V-shaped ground branchesis proposed, which is designed for multiple-input multiple-output (MIMO)antennas. It is based on a dipole with an integrated balun, and V-shapedground branches are introduced to improve the impedance matching. Thebandwidth of this element reaches 62.3% (2.30–4.40 GHz), which covers allthree WiMAX bands (2.30, 2.50, and 3.30 GHz) and the 2.40 GHz WLANband. Based on this element, a quad-element MIMO antenna is designedand fabricated. By reusing the V-shaped ground structure between adja-cent elements, the size of this quad-element antenna is only 0.31λ ×0.31λ × 0.01λ. Meanwhile, a bandwidth of 60.6% (2.30–4.30 GHz) isachieved, in which the S11 < −10 dB, S12 < −10 dB, and S13 <−13 dB. Directional radiation patterns with 2.1 dBi average gain areattained, which are very stable throughout this band. This antenna systemcan be suitable for multielement MIMO devices such as wireless routersand adapters.

Index Terms—Multiple-input multiple-output (MIMO), V-shapeddipole, wireless local area network (WLAN).

I. INTRODUCTION

During the last decades, the growing demand for high-speed wire-less data access has promoted the development of broadband wire-less access techniques such as wireless local area network (WLAN)and world interoperability for microwave access (WiMAX). Morespectrum resources have been allocated, and multiple-input-multiple-output (MIMO) technology has widely been deployed in these systemsto further improve the spectrum efficiency. It cooperates with multi-ple independent spatial streams simultaneously to increase the channelcapacity, and these spatial streams are sent from an M -element arrayto an N -element array, with which the MIMO system is called anM ×N MIMO system. In mature wireless applications that are basedon 802.11n (WLAN) and 802.16e (WiMAX), two spatial streams(M,N ≤ 2) are typically deployed, and extensive studies have beenperformed on the dual-element MIMO antenna design. Elements withcompact size and wideband characteristics have been proposed in[1]–[3], and low-mutual coupling is achieved with various methodssuch as metamaterial-based isolator [4], decoupling network [5], andparasitic element [6].

However, with a growing demand for higher transmitting rates,new wireless standards appeared, such as 802.11ac (WLAN) [7] and802.11 m (WiMAX) [8] that support from four to eight spatial streams.MIMO systems with 4× 4 up to 8× 8 configurations have gradually

Manuscript received July 30, 2014; revised February 02, 2015; acceptedFebruary 08, 2015. Date of publication February 24, 2015; date of currentversion May 01, 2015. This work was supported in part by the NationalBasic Research Program of China under Contract 2013CB329002, in partby the National High Technology Research and Development Program ofChina (863 Program) under Contract 2011AA010202, in part by the NationalNatural Science Foundation of China under Contract 61271135, in part by theNational Science and Technology Major Project of the Ministry of Science andTechnology of China 2013ZX03003008-002, in part by the China PostdoctoralScience Foundation funded project 2013M530046.

The authors are with the State Key Laboratory of Microwave andCommunications, Tsinghua National Laboratory for Information Science andTechnology, Tsinghua University, Beijing, 100084, China (e-mail: [email protected]).

Color versions of one or more of the figures in this communication areavailable online at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TAP.2015.2405091

0018-926X © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 63, NO. 5, MAY 2015 2291

been deployed, and devices such as portable wireless routers and adap-tors appear, which demand wider bandwidth and size miniaturizationin their built-in MIMO system. This raises new challenges in MIMOantenna design, in that the elements are closely spaced, and tradeoffexists among size, bandwidth, and mutual coupling.

In existing literature, sectorial antenna [9], [10] is a good candi-date, in that its patterns are directional and shaped, which can providegood pattern diversity for a MIMO system. Nevertheless, these advan-tages are achieved by using large ground, director, or reflector, whichis not applicable in size limited portable applications. For multi-element MIMO antennas, most radiation elements are narrowband,and the designer may focus more on lowering the mutual couplingbetween closely spaced radiation elements. Representative designsinclude H-shaped elements with orthogonal arrangement [11], PlanarInverted F Antenna (PIFA)/slot elements with diagonal arrangement[12], patch/slot elements with modifying ground structure [13], Yagi-Uda elements with directional radiation patterns [14], among others.For wideband design, most reported structures, such as cavity backtapered slot [15] and self-ground monopole [16]-based multielementMIMO antenna, are nonplanar. They are difficult to be reproduced inmass production and the size is still too large for size-limited applica-tions. For wideband planar design, such as the ultra-wideband (UWB)element [17] and the F-shaped slot element [18]-based MIMO antenna,similar problem exists in that the elements are independent, whichcannot push the size into limit. Thus, developing a planar multiele-ment antenna with compact size and wideband characteristic is still indemand for both the industrial and academic areas.

In this work, a wideband printed dipole with V-shaped groundbranch is proposed, which is designed for multielement MIMOapplications. Comparing with traditional radiation element in MIMOantenna, its advantages lie in the following aspects.

1) Around 62.3% (2.30–4.40 GHz) bandwidth is achieved by intro-ducing the V-shaped ground branch, which can cover all threeWiMAX bands (2.30–2.36 GHz, 2.50–2.90 GHz, and 3.30–3.80 GHz) and the 2.40 GHz WLAN band (2.40–2.48 GHz).

2) Small ground plane and stable radiation patterns are achieved byadopting the integrated balance-to-unbalance transition.

3) Directional radiation patterns are obtained throughout the band,which can provide good pattern diversity in a MIMO antennadesign.

To investigate its performance in an array, a tri-port MIMO antennais proposed [19], in which the bandwidth is not fully optimizedand the operating principle is not explained. In this work, a quad-element MIMO antenna based on this element is proposed, which isdesigned for portable wireless routers and adapters with 9% band-width improvement. These four elements are arranged rotationallyand symmetrically with an interval of 90◦, and their V-shaped groundstructures are reused by adjacent elements. Thus, the size of the pro-posed MIMO antenna is only 0.31λ× 0.31λ× 0.01λ, whereas itsbandwidth reaches 60.6%. Meanwhile, the orthogonal arrangement ofelements reduces the mutual coupling effectively, and the V-shapedground branches also block the spatial coupling. As a result, the cou-pling level is reduced to −10 and −13 dB between the adjacent andopposite elements, respectively, and four directional patterns aiming at0◦, 90◦, 180◦, and 270◦ in the azimuth plane are achieved throughoutthe band. Their 3-dB beamwidth is above 84◦ throughout the band,which can provide nearly full azimuth coverage with low envelopecorrelation coefficient (ECC).

This communication is organized as follows. Section II presents theelement design and analyzes the effect of adding the V-shape groundbranches. Section III demonstrates a quad-element MIMO antennadesign based on this element and its performance is evaluated andcompared with other MIMO antenna designs in Section IV. Finally, the

Fig. 1. Geometry and dimensions (in millimeters) of the proposed radiationelement.

Fig. 2. Impedance matching with/without V-shaped ground branches.

diversity performance of the quad-element MIMO antenna is discussedin Section V.

II. ELEMENT DESIGN

The proposed antenna element is printed on a 1.6-mm thick FR-4substrate (εr = 4.4, tan δ = 0.02). Its top, bottom, and side viewsare shown in Fig. 1. On the bottom side of the substrate, a round-shaped central ground is placed at the center, and is connected to adipole with two cogrounded L-shaped arms. The feeding line of thisdipole is placed right above one arm, and is connected to the other armthrough shorting vias at the corner of the L-shaped arm. As the dis-tance between shorting vias and the central ground is close to λ/4, abalun is formed, with which the currents on the arms are balanced.Thus, the size of the central ground is small in that no additionalunbalance-to-balance transition is needed.

However, with such a compact size, the matching of the elementat lower frequency (2.30–3.30 GHz, which covers the lower band ofWiMAX and WLAN), is not good. Thus, V-shaped ground branchesare introduced, with which the impedance matching is ameliorated asshown in Fig. 2.

In this figure, the loci of the element with/without ground branchesare plotted with different colors and symbols. It can be observed thatthe locus with the V-shaped branches is shrunk, and a loop is formed atlower frequency. This extends the matching range at the lower bound,and fits the whole locus into −10 dB matching circle. About 62.3%


Fig. 3. Current distribution (a) with and (b) without V-shaped ground branchesat 2.35 GHz.

Fig. 4. Effect of tuning the length of the dipole arms.

Fig. 5. Effect of tuning the length of the ground branches.

bandwidth (2.30–4.40 GHz) is achieved, which can cover all threeWiMAX bands and the 2.40-GHz WLAN band.

To understand this phenomenon, the current distribution with/without the V-shaped branches at 2.35 GHz is shown in Fig. 3. Itcan be observed that the current density on the branches is close tothe one on the arms. This indicates that the ground branches alsobecome one part of the radiator at lower frequency, thus improv-ing the impedance matching and extending the bandwidth coverageat lower frequency. Moreover, the effects of the design parametersare presented in Figs. 4–6. It can be observed that the length ofthe dipole arms affects the impedance matching as a whole, and theimpedance matching at lower and higher frequency can be tuned sep-arately with the length of the V-shaped branches and the feeding line.In all, the wideband characteristic is achieved by properly combingthe V-shaped branches, feeding structure, and dipole together, and

Fig. 6. Effect of tuning the length of the feeding line.

Fig. 7. Geometry (a) and dimensions (b) and (c) in millimeters of the proposedquad-element MIMO antenna.

the impedance matching is improved significantly by introducing theV-shaped ground branch as the S-parameter comparison (with/withoutground branch) shown in the Fig. 5. In addition, the V-shaped groundstructure also acts as a reflector, with which the radiation patternbecomes directional.

III. QUAD-ELEMENT MIMO ANTENNA DESIGN

Using the element proposed in Section II, a quad-element MIMOantenna is built and shown in Fig. 7. This antenna is fabricated on a1.6-mm thick FR-4 substrate, and the elements are placed rotationallysymmetrically with an interval of 90◦.

Since the central ground and the V-shaped ground branches arereused by adjacent elements, the overall size of the antenna is40 (0.31λ)× 40 (0.31λ)× 1.6mm (0.01λ), and the area per ele-ment is only 0.024λ2. The impedance matching is not much affected,and around 60.7% bandwidth is achieved with the optimized dimen-sions as shown in Fig. 7(b) and (c). By adopting the orthogonalarrangement of elements, the mutual coupling between adjacent ele-ments is well controlled. Meanwhile, the V-shaped ground branchesalso reduce the spatial coupling, with which the coupling level betweenadjacent and opposite elements is below −10 dB and −13 dB at lower


Fig. 8. Photos of the fabricated prototype.

Fig. 9. Measured and simulated S-parameters of the proposed MIMO antenna.

TABLE ISIZE, BANDWIDTH, AND MUTUAL COUPLING COMPARISON BETWEEN

THE PROPOSED ANTENNA AND OTHER PLANAR MULTIELEMENT

ANTENNAS

frequencies, and is further reduced to −12 dB and −25 dB at higherfrequencies.

Moreover, with the orthogonal arrangement of elements, the radia-tion patterns are also orthogonal. Directional radiation patterns can beobserved, which are aiming at 0◦, 90◦, 180◦, and 270◦ respectively.The average gain reaches 2.1 dBi, and the 3-dB beamwidth is above84◦ throughout the operating band. Consequently, the pattern corre-lation between elements is low, and nearly full azimuth coverage isachieved with this proposed antenna.

IV. PROTOTYPE AND MEASUREMENT RESULT

To validate the performance of the proposed MIMO antenna, aprototype is built and measured, whose photos are shown in Fig. 8.Four semi-rigid coaxial cables are connected to perform the measure-ment, and its S-parameters, radiation patterns, gain, and efficiencyare investigated. Considering that the structure of this antenna is

rotationally symmetric and the results are similar among all ports, theresults of port 1 are provided.

A. S-Parameters

The S-parameters are measured with Agilent VNA E5071B, and theresults are shown in Fig. 9. It can be observed that the measured resultsfit the simulation results well, and the −10 dB bandwidth covers from2.30 to 4.30 GHz. Meanwhile, S12 and S13 are around −10 dB and−13 dB at lower frequency and are gradually decreased to −14 dBand −20 dB at higher frequencies. Thus, this design is capable ofproviding good impedance matching and low-mutual coupling forWiMAX and WLAN MIMO applications.

Table I shows comparison between the proposed antenna and otherplanar multielement antennas in terms of size, bandwidth, and mutualcoupling. It can be observed that the proposed antenna is the mostcompact planar multielement antenna with wideband characteristicand low-mutual coupling.

B. Radiation Patterns

The radiation patterns are measured in an ETS-Lingren AMS-8500anechoic chamber, and the normalized radiation patterns, including themeasured and simulated co-pol and cross-pol patterns, in its E-plane(X–O–Y) and H-plane (Y–O–Z) at 2.40, 3.00, 3.60, and 4.20 GHz areshown in Fig. 10. It can be observed that directional radiation patternsare achieved, which are very stable throughout the band. In addition,the patterns of different ports in azimuth plane (X–O–Y) are also pro-vided. Good consistency can be observed among all ports, and the3-dB beamwidth is above 84◦ throughout the operating band. Thus,it is capable of providing good pattern diversity along with nearly fullazimuth coverage with its four beams.

C. Gain and Efficiency

The gain and efficiency are also measured in the AMS8500 cham-ber, and the results are plotted in Fig. 11. The gain and efficiencyreach 2.8 dBi and 82% in peak, and the average gain and efficiencyare 2.1 dBi and 68%, respectively. The lower efficiency is due to thehigher coupling between radiation elements at lower frequency, whichis an inevitable tradeoff between size and performance.

V. DIVERSITY PERFORMANCE

In this section, the diversity performance of the proposed quad-element MIMO antenna is evaluated by its ECC. This parameterquantifies the correlation between the branch signals received by dif-ferent elements, and a lower ECC means higher pattern diversity ingeneral. A widely adopted criteria for a MIMO antenna is ECC < 0.5,and it can be calculated with the far field results as shown as [20]

ρe,ij ≈

∣∣∣∣∣∣∣∣∣∣

�Aij(Ω)dΩ√�

Aii(Ω)dΩ

√�Ajj(Ω)dΩ

∣∣∣∣∣∣∣∣∣∣

2

Aij = ΓEθ,i (Ω)E∗θ,j (Ω) · pθ (Ω) + Eϕ,i (Ω)E

∗ϕ,j (Ω) · pϕ(Ω)

(1)

where Eθ and Eϕ denote the two orthogonal components of the com-plex electrical field of the antenna patterns; pθ(Ω) and pϕ(Ω) are themultipath angular density function of the θ and ϕ polarization, and theΓ, namely the average cross-polarization ratio, reflects the polarizationenvironment applied in the calculation.


Fig. 10. Measured and simulated radiation patterns of the proposed antenna at (a) 2.40 GHz; (b) 3.00 GHz; (c) 3.60 GHz; and (d) 4.20 GHz.

Fig. 11. Measured and simulated gain and efficiency.

Considering that the proposed antenna is potentially applied in anindoor WiMAX/WLAN system, the Γ is selected as 0 dB, whichsimulates a rich multipath environment whose vertical and horizon-tal polarization component are evenly distributed. Benefitting fromits directional radiation patterns, low ECC level is achieved in boththe simulation and measurements. Fig. 12 shows the results, in whichthe ECC between adjacent and opposite elements is below 0.045and 0.266, respectively. They are much smaller than 0.5, which indi-cates that this proposed antenna is capable of providing good patterndiversity for a MIMO system.

Fig. 12. Measured and simulated ECC.

VI. CONCLUSION

In this communication, a wideband radiation element for multi-element MIMO application is proposed, which is based on printeddipoles with integrated baluns. By introducing the V-shaped groundbranches, its bandwidth reaches 62.3% (2.30–4.40 GHz), and stabledirectional patterns are achieved throughout this band. An orthogo-nal arrangement of elements is adopted to lower the coupling. Itssize is only 0.31λ× 0.31λ× 0.01λ, which is achieved by reusingthe V-shaped ground structure. The measured results show that 60.6%(2.30–4.30 GHz) bandwidth is achieved with acceptable isolation


levels. Meanwhile, four directional beams with 2.1 dBi average gainare obtained, which can realize nearly full coverage in azimuth planeand provides low ECC values.

REFERENCES

[1] J. F. Li, Q. X. Chu, and T. G. Huang, “A compact wideband MIMOantenna with two novel bent slits,” IEEE Trans. Antennas Propag.,vol. 60, no. 2, pp. 482–489, Feb. 2012.

[2] L. Liu, S. W. Cheung, and T. I. Yuk, “Compact MIMO antenna forportable devices in UWB applications,” IEEE Trans. Antennas Propag.,vol. 61, no. 8, pp. 4257–4264, Aug. 2013.

[3] T. S. P. See and Z. N. Chen, “An ultrawideband diversity antenna,” IEEETrans. Antennas Propag., vol. 57, no. 6, pp. 1597–1605, Jun. 2009.

[4] C. C. Hsu, K. H. Lin, and H. L. Su, “Implementation of broadbandisolator using metamaterial-inspired resonators and a T-shaped branchfor MIMO antennas,” IEEE Trans. Antennas Propag., vol. 59, no. 10,pp. 3936–3939, Oct. 2011.

[5] C. H. See, R. A. Abd-Alhameed, Z. Z. Abidin, N. J. McEwan, andP. S. Excell, “Wideband printed MIMO/diversity monopole antenna forWiFi/WiMAX applications,” IEEE Trans. Antennas Propag., vol. 60,no. 4, pp. 2028–2035, Apr. 2012.

[6] S. Zhang, Z. N. Ying, J. Xiong, and S. L. He, “UltrawidebandMIMO/diversity antennas with a tree-like structure to enhance widebandisolation,” IEEE Antennas Wireless Propag. Lett., vol. 8, pp. 1279–1282,Nov. 2009.

[7] O. Bejarano, E. W. Knightly, and M. Y. Park, “IEEE 802.11ac: Fromchannelization to multi-user MIMO,” IEEE Commun. Mag., vol. 51,no. 10, pp. 84–90, Oct. 2013.

[8] Q. H. Li et al., “MIMO techniques in WiMAX and LTE: A featureoverview,” IEEE Commun. Mag., vol. 48, no. 5, pp. 86–92, May 2010.

[9] M. S. Sharawi, F. Sultan, and D. N. Aloi, “An 8-element printed V-shapedcircular antenna array for power-based vehicular localization,” IEEEAntennas Wireless Propag. Lett., vol. 11, pp. 1133–1136, Sep. 2012.

[10] N. Honma, T. Seki, and K. Nishikawa, “Compact planar four-sectorantenna comprising microstrip yagi-uda arrays in a square configuration,”IEEE Antennas Wireless Propag. Lett., vol. 7, pp. 596–598, May 2008.

[11] Y. Luo, Q. Chu, J. Li, and Y. Wu, “A planar H-shaped directive antennaand its application in compact MIMO antenna,” IEEE Trans. AntennasPropag., vol. 61, no. 9, pp. 4810–4814, Sep. 2013.

[12] C. Y. Chiu and R. D. Murch, “Compact four-port antenna suitable forportable MIMO devices,” IEEE Antennas Wireless Propag. Lett., vol. 7,pp. 142–144, Feb. 2008.

[13] H. Li, J. Xiong, and S. He, “A compact planar MIMO antenna systemof four elements with similar radiation characteristics and isolation struc-ture,” IEEE Antennas Wireless Propag. Lett., vol. 8, pp. 1107–1110, Oct.2009.

[14] A. D. Capobianco et al., “A compact MIMO array of planar end-fire antennas for WLAN applications,” IEEE Trans. Antennas Propag.,vol. 59, no. 9, pp. 3462–3465, Sep. 2011.

[15] J. R. Costa, E. B. Lima, C. R. Medeiros, and C. A. Fernandes, “Evaluationof a new wideband slot array for MIMO performance enhancement inindoor WLANs,” IEEE Trans. Antennas Propag., vol. 59, no. 4, pp. 1200–1206, Apr. 2011.

[16] A. Al-Rawi et al., “A new compact wideband MIMO antenna—The double-sided tapered self-grounded monopole array,” IEEE Trans.Antennas Propag., vol. 62, no. 6, pp. 3365–3369, Jun. 2014.

[17] S. Y. Lin and H. R. Huang, “Ultra-wideband MIMO antenna withenhanced isolation,” Microw. Opt. Technol. Lett., vol. 51, no. 2,pp. 570–573, 2009.

[18] R. Karimian, H. Oraizi, S. Fakhte, and M. Farahani, “Novel F-shapedquad-band printed slot antenna for WLAN and WiMAX MIMO systems,”IEEE Antennas Wireless Propag. Lett., vol. 12, pp. 405–408, Mar. 2013.

[19] H. Wang, L. S. Liu, Z. J. Zhang, and Z. H. Feng, “Wideband tri-portMIMO antenna with compact size and directional radiation pattern,”Electron. Lett., vol. 50, no. 18, pp. 1261–1262, 2014.

[20] R. G. Vaughan and J. B. Andersen, “Antenna diversity in mobile commu-nications,” IEEE Trans. Veh. Technol., vol. 36, no. 4, pp. 149–172, Nov.1987.

Matching Technique for an On-Body Low-ProfileCoupled-Patches UHF RFID Tag and for Sensor Antennas

Milan Svanda and Milan Polivka

Abstract—This paper introduces a novel impedance matching techniquefor extremely low-profile on-body UHF RFID tag antennas based on cou-pled shorted-patch antennas. The approach employs a novel arrangementof comb-notches perpendicular to the central radiation slot that excitesthe close higher order mode that affects the field distribution of the fun-damental mode and sets the input impedance to the required complexvalues of UHF RFID chips over the range of 5−50 Ω for the real partand 100−200 Ω for the imaginary part, or directly to 50 Ω impedance.A set of parametric studies shows the flexibility of the proposed techniquefor achieving complex input impedances. To verify the proposed technique,we have developed and measured two antenna samples of relative size0.3 × 0.17 × 0.0022λ0. A first antenna is matched to 50 Ω, and isintended to be used as an on-body antenna sensor for mapping the receivedsignal strength in applications of the European UHF RFID band. The sec-ond antenna operates as an RFID tag antenna with input impedance inZin = 22 + j195 Ω, and reaches a read range of 7.3 m.

Index Terms—Body centric communication, coupled-patches,impedance matching, low-profile antenna, radiofrequency identification(RFID), tag antenna.

I. INTRODUCTION

Small and very low-profile antennas for on-body applications are inhigh demand in the field of body area network (BAN) communication[1], [2], and also in radiofrequency identification (RFID) of people inthe UHF band (860–960 MHz) [3]–[5]. Proper input impedance andsufficient radiation efficiency are the main parameters for assessingthe quality of the radiator. The coupled-patches technique, introducedand applied in [6]–[8], enables the design of low-profile antennas withgood immunity from the influence of a human body. The radiation effi-ciency of these structures is satisfactory—typically better than 50%,even if an extremely low-profile substrate is used, i.e., lower than0.01λ0, when the radiation efficiency of a typical half wavelengthpatch antenna is significantly lower [9].

Two feeding techniques using an excitation dipole and tuning slotsparallel to the coupling slot have been developed. They enable theinput impedance to be tuned for complex values [7]. This is necessarywhen the antenna is fed by a UHF RFID chip. However, it is difficult oreven impossible to achieve 50 Ω input impedance across the couplingslot in the original simple coupled shorted-patches arrangement if thistype of antenna is to be used, e.g., for measurement or communicationpurposes in 50 Ω systems.

Manuscript received April 29, 2014; revised February 03, 2015; acceptedFebruary 06, 2015. Date of publication February 12, 2015; date of currentversion May 01, 2015. This work was supported in part by Czech ScienceFoundation Projects P102/12/P863 “Electromagnetic Properties of RadiatingStructures and Artificial Screening Surfaces in the Close Vicinity of the HumanBody” and in part by COST Project LD 12055 AMTAS: “Advanced Modellingand Technologies for Antennas and Sensors”, which forms a subpart of COSTProject IC 1102 VISTA: “Versatile, Integrated, and Signalaware Technologiesfor Antennas.”

The authors are with the Department of Electromagnetic Field, CzechTechnical University in Prague, 166 27 Prague, Czech Republic (e-mail:[email protected]).

Color versions of one or more of the figures in this communication areavailable online at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TAP.2015.2403399

0018-926X © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

a wideband compact wlan/wimax mimo antenna based on...

Documents