4354 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 60, NO. 9, SEPTEMBER 2012

A Broadband LTE/WWAN AntennaDesign for Tablet PC

Shih-Hsun Chang, Student Member, IEEE, and Wen-Jiao Liao, Member, IEEE

Abstract—A broadband yet compact antenna design applicableto tablets and laptops is proposed. The antenna provides an ex-tensive coverage for existing and upcoming mobile communica-tion bands. Several band broadening and antenna miniaturizationtechniques were employed, including the use of a parasitic element,meandered structures, branched structures and a lump compo-nent. The proposed design is planar, compact and can be fabricatedvia printed circuit board technology. Measurement results exhibitsbroad resonant bandwidth. Nearly omni-directional patterns andreasonable radiation efficiency are observed.

Index Terms—Antenna miniaturization, broadband antenna,LTE, multiband antenna, portable device antenna.

I. INTRODUCTION

T HE long term evolution (LTE) release 10, which is alsoreferred as LTE-Advanced, is expected to be delivered in

2012 [1]. The so-called “4G” standard aims to provide a greaterbandwidth for mobile communications. It supports data ratesup to 100 Mbps for high mobility applications and 1 Gbps forlow mobility uses. The LTE standard supports both frequencydivision duplexing (FDD) and time division duplexing (TDD),which are of paired and unpaired spectra, respectively. TheLTE frequency band is rather extensive. It starts from 699 MHz(Band 12, FDD) and the highest band goes up to 3800 MHz(Band 43, TDD) [1].As a result, antenna design for portable LTE devices can

be very challenging. In addition to the small antenna footprintrequirement, the antenna has to provide a broad operation bandto meet LTE over-the-air (OTA) needs. In this work, we aim todevelop an LTE antenna design applicable to laptop and tabletcomputers. The objective is to maximize LTE band coveragewith uses of antenna structures in simple geometry such aslines and loops, so that the physical insights of applied antennaminiaturization and bandwidth broadening techniques can beidentified.Antenna bandwidth is a critical issue for electrically small

antennas. Conventionally, bandwidth performance is evaluatedvia the impedance bandwidth or the quality factor (Q). The frac-tional bandwidth is often used to appraise a broadband antenna.

Manuscript received December 11, 2011; revised January 31, 2012; acceptedApril 16, 2012. Date of publication July 03, 2012; date of current version August30, 2012. This work was supported by the National Science Council, Taiwan(R.O.C.), under Grant 100-2221-E-011-148.The authors are with the Department of Electrical Engineering, National

Taiwan University of Science and Technology, Taipei 10607, Taiwan, (R.O.C.)(e-mail: wjliao@mail.ntust.edu.tw).Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TAP.2012.2207075

In [2], the bandwidth is estimated by its quality factor in rela-tion to the Chu-based lower bound. Extensive works have beenmade for broadband antennas on mobile devices. In [3], [4],slots are applied to printed inverted-F antennas (PIFA). Thisapproach works in conjunction with the PIFA shorting strip toprovide a broader impedance bandwidth. Some designs employthe ground plane as part of the radiating structure to expand thebandwidth via additional resonance modes [5]. The PIFA de-sign provided in [6] contains three slots of different shapes andsizes on the ground to excite desired bands. In [7]–[9], externalmatching networks are used to achieve impedance matching fora broader band. However, matching networks may introduce ad-ditional losses and decrease the antenna efficiency. Recently,folded loop antennas are often found on mobile devices due tocompact size and insensitivity toward nearby structures [10].In [11], [12], parasitic shorted strips are used to provide extraresonance modes to enhance the bandwidth performance. Thistechnique is also used in the proposed design.Many methods have been developed for antenna miniaturiza-

tion on portable devises. On top of the use of meandered struc-tures, one can add structures such as disks, stubs or mini-coaxialcables as reactive-loading internal to the antenna to reduce theresonant path. In [13], [14], lump elements are placed on the ra-diating structure for this end.Lately, the attention on internal LTE antennas is picking up.

In [15], a single-layer PIFA antenna with multiple branches isproposed for ultra-thin laptop computers. The antenna is fab-ricated on a ceramic substrate with a high dielectric constant.Though the radiation efficiency is no less than 50%, the antennasize is somewhat large (97 11.2 ) and the cost may behigh. Hence, recent works are focusing on making small LTEantennas using the cost effective FR4 substrate. In [16]–[18],compact PIFA designs with gap-coupled feeds are proposed. Inthis work, we also target on developing thin, planar and compactdesigns compatible with the printed circuit board technology.The antenna is meant to be used on portable devices of mod-erate sizes such as laptops and tablets.The proposed antenna provides a rather comprehensive

coverage on most FDD LTE bands and existing mobilecommunication bands. According to its reflection coefficientspectrum, the antenna has two broad operation bands. The lowerband centers around 830 MHz and is applicable to LTE700and GSM850/900. The higher band covers GSM1800/1900,UMTS, and LTE2300/2500 bands. The design employs severalup-to-date techniques to broaden the bandwidth. Table I sum-marizes frequency bands of existing and projected applicationscovered by the proposed antenna.

0018-926X/$31.00 © 2012 IEEE

CHANG AND LIAO: A BROADBAND LTE/WWAN ANTENNA DESIGN FOR TABLET PC 4355

TABLE IFREQUENCY BANDS APPLICABLE TO THE PROPOSED ANTENNA

Fig. 1. Geometric details of the proposed antenna on a tablet platform.

II. ANTENNA DESIGN

The proposed design, which is based on a monopole antenna,exhibits a closed loop configuration. The antenna can be fab-ricated via etching on inexpensive fiberglass reinforced epoxy(FR4) substrates. The projected antenna platform is a tabletcomputer. Fig. 1 shows the geometry and detail dimensions ofthe developed multiband antenna. The FR4 substrate used is1.6 mm thick, and the loss tangent is 0.02. The overallantenna footprint is 69 10 . The antenna length is about0.16 wavelengths for the lowest 700 MHz band. As indicatedin Fig. 1, the closed-loop monopole is mounted on the top-rightcorner of a 220 130 ground plane, which emulates theLCD panel of a 10-inch tablet.Many broadband and multiband monopole antennas are

developed with branches and asymmetric radiating structures.Some designs employ simulation tools with numeric methodssuch as the genetic algorithm to adjust the antenna performanceto meet specifications. Though this approach may be effective,nevertheless, one may find difficult to appreciate the physicalinsights of the developed antenna. In this work, we attemptedto construct the antenna using simple structures such as strips

Fig. 2. Geometric evolution of the proposed antenna.

and loops. Antenna sections are added one at a time to prop-erly reveal their effects on impedance matching and radiationcharacteristics.The evolution of the proposed closed-loop monopole antenna

is illustrated in Fig. 2, which is divided into five stages. In laterstages, the antenna is divided into two separate parts. The leftpart is attached to the feed and performs as the driving ele-ment. The right part is connected to the ground panel and actsas a parasitic element. Both parts are in closed-loop shape andfacing each other in a rather symmetric way. The antenna res-onant modes are grouped into two broad bands to comply withLTE band allocations. The lower band spans from 698 to 960MHz, and the higher band extends from 1710 to 2700 MHz.Antenna simulations are conducted with HFSS, which is a fullwavelength numeric tool [19].The simple loop antenna labeled as “Ant 1” in Fig. 2 aims

to provide the lowest resonant band. The simulated reflectioncoefficient spectrum shown in Fig. 3 indicates that the first res-onance appears around 1.1 GHz. In order to further lower theoperation band without enlarging antenna sizes, “Ant 1” loopis broken into two as two closed-loop elements in “Ant 2”. Thetwo loops are coupled via a narrow vertical gap, which is labeledas coupling gap in Fig. 2. This approach reduces the lower bandresonance to approximately 750 MHz as shown in Fig. 3.To broaden the higher operation band, a pair of folded strips

is added in the middle to form “Ant 3”. This structure allowsdiversified current paths to permit multiple resonances for thehigher band. According to the solid line in Fig. 3, three apparentmodes appear at 1.8, 2.05 and 2.8 GHz, respectively.Fig. 4 exhibits the current distributions of “Ant 3” at 800 and

1860 MHz. For both cases, currents are mostly concentrated in

4356 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 60, NO. 9, SEPTEMBER 2012

Fig. 3. Simulated reflection coefficient spectra of “Ant 1”, “Ant 2” and “Ant3”.

Fig. 4. Simulated current distributions of “Ant 3” on the ground panel andblow-up view around the antenna at 800 and 1860 MHz.

the antenna region, while some currents are observed on theground panel. Nevertheless, the ground currents are stronglycoupled to the antenna currents and no substantial current ex-ists outside the antenna neighborhood.At 800 MHz, the current distribution and current flow direc-

tions are largely symmetric with respect to the coupling gapin the middle indicating the left and right parts are stronglycoupled. Note the folded strips act as a current choke at thelower band and no substantial current is observed in this region.At 1860 MHz, the current distribution appears asymmetric andconcentrates around the folded strips. Judging from the currentdirections and magnitudes, the resonance is in mode.The antenna developed so far shows great potential in

providing a comprehensive LTE band coverage. However,matching conditions at certain bands are poor. In particular, theLTE 700/GSM 900 band bandwidth, which is about 200 MHz,should be made broader. In “Ant 4”, a wide metal strip is addedto the top-left corner. The added section is then bended over

Fig. 5. Simulated reflection coefficient spectra of “Ant 3”, “Ant 4” and “Ant5”.

Fig. 6. Simulated current distributions of “Ant 5” on the ground panel andblow-up view around the antenna at 750 and 900 MHz.

the FR4 substrate to the bottom surface. As shown in Fig. 5,this structure introduces another resonance around 1.05 GHz,which is close to the existing band. A broad band can be formedby slightly moving down the new resonance. Unfortunately,further increasing the strip length deteriorates the lower bandmatching performance and results in a single narrow band. Inorder to solve this problem, a surface-mount inductor is addedin the middle of the strip in attempt to lower the correspondingresonance frequency by compensating excessive capacitance.Various inductance values are trialed. With proper choice ofthe inductance, the added inductor and the metal strip help tobring down the resonance to approximately 900 MHz. Fig. 6shows current distributions at 750 and 900 MHz. Comparingto the 750 MHz and the 800 MHz current distribution shownin Fig. 4, the addition of the inductor-loaded strip attracts thecurrent to the top left corner and creates a distinctive band.According to the refection coefficient spectrum, the overall

CHANG AND LIAO: A BROADBAND LTE/WWAN ANTENNA DESIGN FOR TABLET PC 4357

Fig. 7. Simulated current distributions of “Ant 5” on the ground panel andblow-up view around the antenna at 2300 MHz.

lower band bandwidth, which merges three resonant modes, isincreased to 250 MHz.A further look into the reflection coefficient spectrum of the

proposed “Ant. 5” design, one finds that there are two resonantmodes above 2 GHz. One locates at 2.1 GHz and the other is at2.8 GHz. Since there is no apparent notch band between the tworesonant modes, one may take advantage of this feature to forma relatively broad band. Fig. 7 shows the current distribution at2300 MHz, which is different to the 1860 MHz result shown inFig. 4. We also examined distributions at different frequenciesand found that the pattern is invariant to frequency changes andits magnitude is proportional to the matching condition. Thischaracteristic in the high band can help provide a broad bandwith a consistent radiation pattern.

III. PARAMETRIC STUDIES OF ANTENNA CONFIGURATION

In order to satisfy the broad bandwidth need of LTE, the pro-posed antenna includes a closed-loop parasitic element, a pairof folded strips and an extended branch with a lump inductor toexcite multiple resonance modes. Since the radiating structure iscompact and complicated, its radiation characteristics are sensi-tive to changes in geometric parameters. Hence, in this section,we conducted several parametric studies on key geometric pa-rameters via simulations to derive the optimal design.The lower band operation is supported by the parasitic loop

structure placed next to the driving antenna. Fig. 8 examinesreflection coefficient spectra of different widths of the couplinggap, which is located between driving and parasitic elements.The matching of the lowest mode at 723 MHz is sensitive tothe changes in gap width. This result validates that the parasiticstructure helps to tweak thematching condition in an electricallysmall antenna.The purpose of the corner branch structure is to create an ad-

ditional mode locating around the existing band to broaden theoperation bandwidth. Since the branch length can’t be too long,we added a surface-mount inductor to adjust the resonance fre-quency. In Fig. 9, we observed that the resonance appears at0.98 GHz for a 3.3 nH inductor. When the inductance is in-creased, the resonance moves down and merges with the ex-isting lower band. Note the matching in the higher band is some-what perturbed, but the resonances are invariant to the changesin inductance.Next, we looked into the antenna impedance sensitivity to-

ward test bench sizes and the antenna location. Simulationswere

Fig. 8. Reflection coefficient spectra of the proposed antenna of various cou-pling gap widths.

Fig. 9. Reflection coefficient spectra of the proposed antenna of various sur-face-mount inductor values.

Fig. 10. Fabricated prototype antenna.

conducted with the antenna placed on the top-right corner of dif-ferent ground plane sizes, which vary from 100 200 to185 335 . No substantial change in reflection coefficientspectra is observed. Next, we placed the antenna at different po-sitions on the top edge of a 130 220 ground plane. Thematching is slightly perturbed within resonance bands, but stillmeets LTE specifications. In general, the proposed design canbe applied to laptops as well as tablets without restrictions onLCD panel sizes and mounting positions.

IV. PERFORMANCE VERIFICATION

The prototype antenna is fabricated via the printed circuitboard etching process. Fig. 10 shows the front and back of theantenna on a 130 220 test board. The inductors used are

4358 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 60, NO. 9, SEPTEMBER 2012

Fig. 11. Comparison of simulated and measured reflection coefficient spectraof the proposed antenna.

MURATA 0402, which are of 1.0 0.5 package. Simu-lated and measured reflection coefficient spectra are illustratedin Fig. 11. The 6-dB impedance bandwidthmeasured is 250 MHz at the lower band, which amounts to 30%with respect to the 831 MHz center frequency. At the higherband, the available bandwidth measured is 560 MHz, which isapproximately 29% with respect to the 1920 MHz center fre-quency. The measured spectrum largely agrees with the simu-lated one. Note some discrepancies are found in higher bandresonances. This can be partly attributed to the use of the sur-face-mount inductor. Results from Fig. 9 suggest the higherband impedance matching is sensitive to the added inductanceand we performed simulations with an ideal inductor model.Another cause for the differences may be the feeding cable,which is not included in the simulation model. To test if there isany substantial return current on it, we performed S11 measure-ments under various setups including hanging the cable freelyas shown in Fig. 10, pressing the cable to the ground via coppertape, attaching extra copper tape strips on ground panel edgesand the cable, and touching different ground and cable positionswith finger tips. All show similar spectra indicating the feedingstructure does not perturb the current distribution. These resultsverify the proposed design is fairly invariant to changes on theground panel.Next, we examined the radiation features of the proposed an-

tenna. Radiation patterns were measured in a 7.0 4.9 4.63-D near-field spherical anechoic chamber. The near-field

probe samples radiated fields every 3 degrees on conical andgreat-circle cuts. The antenna-under-test includes the antennaand its test board to account for radiation from the ground panelsurface. Figs. 12 and 13 show transformed far-field andpatterns on -, -, and -planes at 0.8 and 1.86 GHz, re-spectively. Corresponding 3D simulated patterns are shown inthe bottom-right corner. The obtained radiation patterns exhibitvery few nulls, which make it an appropriate candidate for mostwireless portable devices.The 800MHz patterns shown in Fig. 12 exhibit a rather omni-

directional coverage on the -plane. The amplitude vari-ation is less than 3.7 dB on that plane. Note nulls appearon the -axis, indicating that the antenna behaves as a dipoleplaced along the -axis. As to the 1860 MHz results shown in

Fig. 12. Measured radiation patterns at 800 MHz on principle planes and sim-ulated 3D pattern.

Fig. 13. Measured radiation patterns at 1860MHz on principle planes and sim-ulated 3D pattern.

Fig. 13, nulls are found at -axis on the -plane; a broadbeam is pointing toward the -axis on the -plane; and a

broad beam is pointing toward the -axis on the -plane.These results suggest the antenna behaves like a current sourceplaced along the -axis, which is in parallel to the top edge ofthe ground plane. This conclusion agrees with the strong hori-zontal currents observed in the bottom plot of Fig. 4. Note thereare also some vertical current components, which result in the

beam pointing toward axis direction on the-plane.Radiation measurements were taken at various frequency

points in the lower and higher bands to plot the antenna peakgain and radiation efficiency spectra shown in Fig. 14. In theLTE 700 and GSM 850/900 bands, the antenna peak gain variesfrom 0.58 dBi to 1.42 dBi and the antenna efficiency is largerthan 56%. In the GSM1700/1800 and UMTS bands, whichextends from 1700 to 2170 MHz, the antenna peak gain variesfrom to 3.03 dBi and the antenna efficiency is between43% and 84%. In the WLAN2400 and LTE2300/2500 bands,

CHANG AND LIAO: A BROADBAND LTE/WWAN ANTENNA DESIGN FOR TABLET PC 4359

Fig. 14. Measured antenna peak gain and radiation efficiency in applicablebands.

the antenna peak gain changes from 1.48 to 3.43 dBi and theradiation efficiency is above 56% across the band.

V. CONCLUSION

In this work, a novel planar closed-loop monopole antenna isproposed for upcoming 4G mobile communication. The com-pact antenna configuration achieves broadband performanceusing a mirror-imaged parasitic element, which induces sub-stantial coupling to accommodate additional resonance modes.A short metal strip with a surface-mount inductor attached toits end is added to the corner of the antenna to further expandthe lower band. A pair of folded strips is added in the middleto provide more resonance modes in the higher band withoutperturbing the lower band operation. Note the proposed designnot only achieves large bandwidth in terms of impedancematching, measured data show sufficient radiation efficiencyvalues are achieved in operation bands.On top of the broadband, small footprint, and good radiation

efficiency, the proposed antenna is planar and can be easily in-tegrated within the LCD panel of tablets and laptops. Its fabri-cation procedure is compatible with printed circuit board tech-nology. Its radiation patterns are broad and consistent withinthe lower and higher bands. Above features make the proposeddesign an attractive candidate for LTE antennas on portabledevices.

REFERENCES[1] LTE Advanced The 3rd Generation Partnership Project, 2011 [Online].

Available: http://www.3gpp.org/LTE-Advanced[2] L. J. Chu, “Physical limitations in omnidirectional antennas,” J. Appl.

Phys., vol. 19, pp. 1163–1175, 1948.[3] A. S. Hussaini, R. A. Abd-Alhameed, C. H. See, H. I. Hraga, M. S.

Bin-Melha, P. S. Excell, and J. Rodriguez, “A dual-band frequencytunable planar inverted F antenna,” in Proc. 5th Eur. Conf. AntennasPropagation (EUCAP), Apr. 2011, pp. 223–227.

[4] Q. D. Li, Q. Y. Feng, and J. Yan, “Design and analysis of ferrite loadedimproved slot PIFA antenna,” in Proc. 7th Int. Conf. Wireless and Opt.Communication Networks, Sep. 2010, pp. 1–3.

[5] P. Vainikainen, J. Ollikainen, O. Kivekäs, and I. Kelander, “Res-onator-based analysis of the combination of mobile handset antennaand chassis,” IEEE Trans. Antennas Propag., vol. 50, pp. 1433–1444,Oct. 2002.

[6] A. Cabedo, J. Anguera, C. Picher, M. Ribó, and C. Puente, “Multibandhandset antenna combining a PIFA, slots, and ground plane modes,”IEEE Trans. Antennas Propag., vol. 57, pp. 2526–2533, Sep. 2009.

[7] M. Selvanayagam and G. V. Eleftheriades, “A compact printedantenna with an embedded double-tuned metamaterial matchingnetwork,” IEEE Trans. Antennas Propag., vol. 58, pp. 2354–2361,Jul. 2010.

[8] M. A. Antoniades and G. V. Eleftheriades, “A multiband monopoleantenna using a double-tuned wheeler matching network,” in Proc. 4thEur. Conf. Antennas Propag. (EUCAP), Apr. 2010, pp. 1–4.

[9] J. W. Yoon, D. G. Kim, and C. D. Park, “Implementation of UWBantenna with bandpass filter using microstrip-to-CPW transitionmatching,” in Proc. Asia Pacific Microw. Conf., Dec. 2009, pp.2553–2556.

[10] C. W. Chiu, C. H. Chang, and Y. J. Chi, “A compact folded loop an-tenna for LTE/GSM band mobile phone applications,” in Proc. Int.Electromagnetics in Advanced Applications (ICEAA), Sep. 2010, pp.382–385.

[11] W. C. Liu, M. Ghavami, and W. C. Chung, “Triple-frequency mean-dered monopole antenna with shorted parasitic strips for wireless ap-plication,” IET Microw. Antennas Propag., vol. 3, pp. 1110–1117, Oct.2009.

[12] K. L. Wong, W. J. Chen, and T. W. Kang, “Small-size loop antennawith a parasitic shorted strip monopole for internal WWAN notebookcomputer antenna,” IEEE Trans. Antennas Propag., vol. 59, pp.1733–1738, May 2011.

[13] Y. G. Xia, J. Luo, and H. Ye, “A standard shielded loop antenna withload resistor,” in Proc. IEEE 3rd Int. Symp. on Microwave Antennas,Propagation and EMC Technologies for Wireless Communications,Oct. 2009, pp. 405–407.

[14] M. C. Scardelletti, G. E. Ponchak, S. Merritt, J. S. Minor, and C. A.Zorman, “Electrically small folded slot antenna utilizing capacitiveloaded slot lines,” in Proc. IEEE Radio and Wireless Symp., Jan. 2008,pp. 731–734.

[15] C. L. Hu, W. F. Lee, Y. E. Wu, C. F. Yang, and S. T. Lin, “A compactmultiband inverted-F antenna for LTE/WWAN/GPS/WiMAX/WLANoperations in the laptop computer,” IEEE Antennas Wireless Propag.Lett., vol. 9, pp. 1169–1173, 2010.

[16] K. L.Wong and P. J. Ma, “Small-size internal antenna for LTE/WWANoperation in the laptop computer,” in Proc. Int. Applications of Elec-tromagnetism and Student Innovation Competition Award. Conf.(AEM2C), Aug. 2010, pp. 152–156.

[17] S. C. Chen and K. L.Wong, “Study of a hearing aid-compatible internalLTE/WWAN bar-type mobile phone antenna,” in Proc. Asia PacificMicrow. Conf., Dec. 2010, pp. 1793–1796.

[18] T. W. Kang and K. L. Wong, “Coupled-fed PIFA with a loop feed for8-band internal LTE/WWAN laptop computer antenna,” in Proc. IEEEAntennas Propagation Soc. Int. Symp., Jul. 2010, pp. 1–4.

[19] ANSYS HFSS ANSYS, 2011 [Online]. Available: http://www.ansoft.com/

Shih-Hsun Chang (S’10) was born in Changhua,Taiwan, in 1986. He received B.S. degree from theNational Formosa University, Yunlin, Taiwan, in2008. He is currently working toward the Ph.D.degree at the National Taiwan University of Scienceand Technology, Taipei.His main research interests are miniaturized an-

tennas for wireless communications, especially formulti-input and multi-output (MIMO) antenna sys-tems on mobile devices.

Wen-Jiao Liao (S’98–M’04) received B.S. degreein electrical engineering from the National TaiwanUniversity, Taipei, Taiwan, in 1995 and the M.S.and Ph.D. degrees in electrical engineering from theOhio State University, Columbus, in 1999 and 2003,respectively.Previously, he was an Assistant Professor in

the Department of Communications Engineering,Yuan-Ze University. Since 2007, he has been anAssistant Professor in the Department of ElectricalEngineering, National Taiwan University of Science

and Technology. His main interests include miniaturized antennas, electron-ically reconfigurable antennas, leaky wave antennas, phased arrays, antennameasurement, and wave propagation.