Small-Size LTE/WWAN Printed Loop Antenna With an Inductively Coupled Branch Strip for Bandwidth Enhancement in the Tablet Computer

6144 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 61, NO. 12, DECEMBER 2013

Small-Size LTE/WWAN Printed Loop Antenna Withan Inductively Coupled Branch Strip for Bandwidth

Enhancement in the Tablet ComputerKin-Lu Wong, Fellow, IEEE, and Meng-Ting Chen, Student Member, IEEE

Abstract—The technique of using an inductively coupled branchstrip for bandwidth enhancement of a simple printed loop an-tenna to achieve small size yet multiband operation to cover theLTE/WWAN bands (704–960 and 1710–2690 MHz) in the tabletcomputer is presented. The antenna’s metal pattern occupies asmall area of 10 34.5mm and is printed on a thin FR4 substrate.The branch strip is coupled to the loop antenna through a chip in-ductor. In the antenna’s lower band, the branch strip can cause anadditional resonance excited near a resonant mode contributed bythe printed loop antenna and thereby greatly widens the antenna’slow-band bandwidth to cover the 704–960 MHz band for theLTE700/GSM850/900 operations. While in the antenna’s higherband, the chip inductor provides a high inductance and limits theexcitation of the branch strip, thereby making the branch striphave negligible effects on the high-band operation. In this case,with the printed loop antenna excited in the higher band, theantenna can generate two higher order resonant modes to form awide operating band for the GSM1800/1900/UMTS/LTE2300/2500operations (1710–2690 MHz). Details of the proposed antenna aredescribed, and the obtained results of the antenna are presentedand discussed.

Index Terms—Inductively coupled branch strip, LTE/WWANantennas, mobile antennas, small-size antennas, tablet computerantennas.

I. INTRODUCTION

O WING to the very limited space inside the slim or thin-profile tablet communication devices such as the tablet

computers, the embedded antennas having planar structure andsmall size for multiband operation [1]–[9] are generally de-manded. This requirement is especially challenging for the LTE/WWAN (long-term evolution/wireless wide area network) op-erations that require the antenna to cover a wide lower bandof 704–960 MHz and a wide higher band of 1710–2690 MHz.For such applications, some promising planar antennas suitableto be printed on a thin dielectric substrate for applications inthe slim tablet computers or notebook computers have been re-ported recently [4]–[9]. These promising LTE/WWAN antennasinclude using an inverted-F antenna of size about 11.2 96mm havingmultiple radiating strips [4], [5], a foldedmonopole

Manuscript received May 30, 2013; revised August 17, 2013; acceptedSeptember 11, 2013. Date of publication September 17, 2013; date of currentversion November 25, 2013.The authors are with the Department of Electrical Engineering, National Sun

Yat-sen University, Kaohsiung 80424, Taiwan (e-mail: [email protected]).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.2013.2282291

antenna of size 10 69 mm configured into a loop-like config-uration [6], a coupled-fed shorted strip antenna of size 15 40mm having an internal loop matching circuit [7], and an in-verted-F antenna of size 10 45 mm having a capacitivelycoupled feeding strip and a radiating strip having a coupled sec-tion [8], and a half-wavelength loop antenna of size 12 75mm having a feeding patch embedded therein to capacitivelyexcite the loop strip [9].Among these reported antennas, the inverted-F or folded

monopole or shorted strip antennas have been applied in [4]–[8],and the desired lower band of 704–960 MHz is mainly formedby the antenna’s quarter-wavelength resonant modes, therebymaking the antenna’s footprint generally occupying a smallerarea than that of the half-wavelength loop antenna reported in[9]. In this paper, we demonstrate that a quarter-wavelengthprinted loop antenna [10], [11] can be applied in achieving twowide operating bands for the LTE/WWAN operations. Owingto the quarter-wavelength resonant mode generated for the an-tenna’s lower band, the proposed loop antenna can have a muchsmaller footprint than that required for the half-wavelengthloop antenna in [9]. By including a new technique of applyingan inductively coupled branch strip for bandwidth enhancementof the antenna’s lower band to cover the LTE700/GSM850/900operations in the 704–960-MHz band, the antenna’s metalpattern occupies an area of 10 34.5 mm only, which isalso smaller than those of the quarter-wavelength inverted-For folded monopole or shorted strip antennas that have beenreported for the LTE/WWAN operations [4]–[8]. The compactsize obtained for the proposed antenna is owing to the antenna’ssimple structure, which is formed by a printed loop antennaconfigured into an inverted-L shape and an inductively coupledbranch strip connected to the loop strip for low-band bandwidthenhancement.In addition to the wide lower band obtained for the

proposed antenna, a wide higher band formed by the an-tenna’s two higher order resonant modes contributed bythe printed loop antenna alone is also obtained to coverthe GSM1800/1900/UMTS/LTE2300/2500 operations in the1710–2690-MHz band. Design considerations and operatingprinciple of the proposed antenna are described. A parametricstudy on the proposed antenna is also conducted, and the resultsof a fabricated antenna are presented and discussed.

II. PROPOSED ANTENNA

Fig. 1 shows the geometry of the proposed LTE/WWANtablet computer antenna. The antenna has a uniplanar structure

0018-926X © 2013 IEEE

WONG AND CHEN: SMALL-SIZE LTE/WWAN PRINTED LOOP ANTENNA WITH AN INDUCTIVELY COUPLED BRANCH STRIP 6145

Fig. 1. Geometry of the proposed LTE/WWAN tablet computer antenna.

and is printed on an FR4 substrate of thickness 0.8 mm, rel-ative permittivity 4.4, and loss tangent 0.024. The antenna ismounted along an edge and at a corner of the device groundplane of size 150 200 mm , which is a reasonable size for thetablet computer with a 10-in display panel. In the experimentaltesting, a copper plate of thickness 0.2 mm is used for thedevice ground plane (see Fig. 15).The antenna is formed by a loop strip configured into an in-

verted-L shape and a branch strip inductively coupled to theloop strip through a chip inductor of 22 nH. The loopstrip (section ADB) has a length of 64 mm (about at750 MHz) only. Note that the matching circuit including a se-ries chip capacitor (C) of 1.2 pF and a series chip inductorof 5.6 nH is disposed on a 0.8-mm-thick FR4 substrate of smallsize 7 3 mm , which is mounted on the device ground plane.Note that the matching circuit can also be disposed in any direc-tion on the device ground plane, such as the direction parallelto the edge where the antenna is mounted. Hence, for practicalapplications, the occupied size of the matching circuit on thedevice ground plane can generally be negligible.Aided by the series chip capacitor C and series chip inductor[10], [11], the loop antenna can generate a resonance at about

750 MHz, although the impedance matching thereof is not goodenough for applications (see the simulated return loss and inputimpedance results for Ant1 in Figs. 2 and 3). The simulated re-sults are obtained using the full-wave electromagnetic field sim-ulator HFSS version 14 [12], and the shaded frequency rangesat lower and higher frequencies in the figure are the desiredlower band (704–960MHz) and higher band (1710–2690MHz).The 750-MHz mode generation is mainly owing to the serieschip capacitor added to compensate for the large inductive re-actance around the lowest resonant mode or the quarter-wave-

Fig. 2. Simulated return loss for the proposed antenna and the case with theprinted loop antenna only (Ant1). Corresponding dimensions are the same asgiven in Fig. 1.

Fig. 3. Simulated input impedance for the proposed antenna and Ant1.

length mode of the printed loop antenna [10], [11]. Note thatin Fig. 2, although the return loss for part of the frequenciesin the lower band is less than 6 dB, the measured antenna effi-ciency (with mismatching loss included) is about 40%–62% inthe lower band (see Fig. 17 in Section IV), which is acceptablefor practical applications.In addition, it can be seen in Fig. 2 that two higher order res-

onant modes at about 1850 and 2550 MHz are generated withgood impedance matching, which is aided by the chip inductor[10], [11]. In addition, since there are current nulls that oc-

curred at the two adjacent sections in the L-shaped loop strip(see the simulated surface current distributions on the antenna’smetal pattern at 1850 and 2550 MHz in Fig. 4), adjusting thestrip widths of the two adjacent sections and the gap there be-tween can improve the impedance matching of the two higherorder resonant modes. In the proposed design, the two resonantmodes can be formed into a very wide operating band of largerthan 1 GHz, thereby making the antenna’s higher band coverthe desired 1710–2690-MHz band. From the input impedanceresults of Ant1 shown in Fig. 3, it is also clear to see that theantenna’s higher band is mainly contributed by the loop strip.In order to achieve a wide lower band, a branch strip of length39 mm and width 0.3 mm is coupled to the loop strip through

a chip inductor of 22 nH. The chip inductor provides a high in-


Fig. 4. Simulated surface current distributions on the metal pattern of the pro-posed antenna at 750, 950, 1850, and 2550 MHz.

ductance at higher frequencies and can therefore limit the exci-tation of the coupled strip [13], which makes the added branchstrip have negligible effects on the antenna’s high-band oper-ation. On the other hand, owing to the additional inductancecontributed by the branch strip and the additional capacitancecontributed between the branch strip and the loop strip, a par-allel resonance [14], [15] controlled by the additional induc-tance and capacitance can occur at about 1150 MHz (see theinput impedance of the proposed antenna in Fig. 3). This par-allel resonance causes the resonant mode owing to the loop stripshifted to be at about 750MHz and having improved impedancematching. Further, an additional resonance at about 950 MHz isalso generated, which greatly widens the bandwidth of the an-tenna’s lower band to cover the desired LTE700/GSM850/900operations. Also note that by adjusting the distance betweenthe branch strip and the loop strip, the contributed capacitancebetween them can be adjusted, which will affect the generatedparallel resonance and the excited resonant mode in the an-tenna’s lower band.For the results of the proposed antenna shown in Fig. 2, the

two resonant modes at about 750 and 950 MHz are combinedinto a wide operating band to cover the desired 704–960-MHzband. From the simulated surface current distributions on the an-tenna’s metal pattern at 750 and 950MHz shown in Fig. 4, it canbe seen that strong surface currents are excited on the loop strip,indicating that the loop strip is excited in the desired lower band.For the branch strip, strong surface currents at 950MHz are alsoseen. This can explain that the branch strip helps generate theadditional resonant mode in the desired lower band. Fig. 5 alsoshows the simulated surface current distributions on the deviceground plane. Strong excited surface currents are seen, and the

Fig. 5. Simulated surface current distributions on the device ground plane at750, 950, 1850, and 2550 MHz.

device ground plane is expected to contribute to the radiation ofthe antenna.

III. PARAMETRIC STUDY

To further analyze the effects of the inductively coupledbranch strip, Fig. 6 shows the simulated return loss as a func-tion of the chip inductor. Results for the chip inductor with

0, 7, 14, 22 nH are shown. Note that for , the chipinductor is replaced by a short metal strip, and the branch stripis directly connected to the loop strip. For , the addedbranch strip makes the proposed antenna operates more like asimple shorted strip antenna having a radiating strip of lengthabout 49 mm (section ADE) and a shorting strip (section DB)of length about 57 mm. This explains the resonant mode excitedat about 1.5 GHz, which is mainly owing to the radiating stripbeing about a quarter-wavelength at 1.5 GHz. In this case, thesection ADB can still function as a loop antenna, and a resonantmode at about 750 MHz and two higher order resonant modesin the higher band are still generated, with the first mode in thehigher band being affected and shifted to higher frequencies.For 7 and 14 nH, the resonant mode owing to the branchstrip is shifted to lower frequencies at about 1.2 and 1.1 GHz,and the two higher order resonant modes in the higher band arealmost the same as those of the proposed antenna with22 nH. The results indicate that the added chip inductor withinductance larger than about 7 nH can cause small or negligibleeffects of the branch strip on the two higher order resonantmodes of the printed loop antenna.The simulated input impedance for the proposed antenna with

0, 7, and 14 nH is also shown in Fig. 7 for comparison.It is seen that for 7 and 14 nH, the resonances in the an-tenna’s higher band are almost not affected. This behavior alsoconfirms that the added inductor causes negligible effectson the two higher order resonant modes of the printed loop an-tenna. As for 0, however, some effects on the resonancearound 1900 MHz are seen. It can also be seen that the parallelresonances, which are contributed by the inductively coupledbranch strip, occur at about 1380 and 1250 MHz for 7 and


Fig. 6. Simulated return loss as a function of the chip inductor .

Fig. 7. Simulated input impedance for the proposed antenna with 0, 7,and 14 nH.

14 nH, respectively. The additional resonances caused by theparallel resonances are also shifted to lower frequencies withincreasing inductance. Furthermore, the lowest resonant modeof the printed loop antenna is also seen to be shifted to lower fre-quencies with the added having a larger inductance, and theimpedance variations thereof become much smoother, therebycausing good excitation of the resonant mode at about 750 MHzwhen the chip inductor of 22 nH is used (the proposed antenna).As seen in Fig. 6, the resonantmode at about 750MHz combineswith the resonant mode at about 950MHz caused by the parallelresonance to form a wide operating band for the antenna’s lowerband.Effects of the length of the branch strip on the formation

of the antenna’s lower band are also studied. Fig. 8 shows thesimulated return loss for the length varied from 25 to 38 mm.With an increase in the length , which corresponds to increasedinductance contributed by the branch strip, the resonant modeowing to the inductively coupled branch strip is shifted to thefrequencies in the desired lower band, and improved impedancematching of the lowest resonant mode contributed by the printedloop antenna is also obtained. The obtained results are similarto those observed in Fig. 6.Fig. 9 shows the simulated return loss as a function of the dis-

tance between the branch strip and the loop strip. Results ofthe simulated return loss for the distance varied from 1.2 to 2.2

Fig. 8. Simulated return loss as a function of the length of the branch strip.

Fig. 9. Simulated return loss as a function of the distance between the branchstrip and the loop strip.

mm are shown. Small effects on the first mode in the lower bandare seen. However, with a decrease in the distance , the secondmode, which is the resonant mode owing to the inductively cou-pled branch strip, is shifted to lower frequencies, hence causinga decrease in the antenna’s low-band bandwidth. The results canalso be confirmed from the simulated input impedance shown inFig. 10. The parallel resonance caused by the added branch stripis seen to be shifted to lower frequencies as the distance de-creases, which, in turn, makes the additional resonance causedby the parallel resonance shifted to lower frequencies as well.The antenna’s low-band bandwidth is hence decreased, as it isobserved in Fig. 9. This behavior is mainly because a smallerdistance will increase the contributed capacitance betweenthe branch strip and the loop strip, and hence affects the ad-ditional resonance caused by the added branch strip. Further-more, with a smaller distance , there is some degradation inthe impedance matching of the second mode in the antenna’shigher band, which causes a decrease in the high-band band-width. Hence, a proper distance is also important in achievingtwo wide operating bands for the proposed antenna.Ground plane effects on the antenna’s performances are

also studied. Fig. 11 shows the simulated return loss forthe device ground plane sizes of 200 150 mm (case A,antenna along the short edge and facing a ground length of200 mm), 150 150 mm (case B, square ground plane oflength 150 mm), and 150 200 mm (case C, antenna alongthe long edge as shown in Fig. 1 and facing a ground lengthof 150 mm). Effects of the ground plane size are seen mainlyon the antenna’s lower band. For example, when comparedto case C, the impedance matching of the first resonant modein the lower band is improved for the square ground plane of150 150 mm (case B) and is degraded when the antenna


Fig. 10. Simulated input impedance as a function of the distance between thebranch strip and the loop strip.

Fig. 11. Simulated return loss for different sizes of the device ground plane.

faces a ground length of 200 mm (case A). This can be ex-plained from the simulated input impedance shown in Fig. 12.The real part of the input impedance of the first resonant modein the lower band, which is mainly owing to the loop strip, be-comes larger than 200 for case A. The large input resistanceresults in some degradation in the impedance matching seen inFig. 11. The results indicate that the ground plane size affectsthe antenna’s performances.The simulated antenna efficiency (with mismatching loss in-

cluded) for the antenna’s lower and higher bands is also pre-sented in Fig. 13. Larger effects are seen in the lower band thanin the higher band. Different ground plane sizes can result invariations in the obtained antenna efficiency, especially for thefrequencies in the lower band. The simulated surface currentdistributions for the device ground planes of case A and B arealso plotted in Fig. 14 for comparison (the results for case C are

Fig. 12. Simulated input impedance for different sizes of the device groundplane of case A, B, and C in Fig. 11.

Fig. 13. Simulated antenna efficiency (with mismatching loss included) for dif-ferent sizes of the device ground plane of case A, B, and C in Fig. 11.

shown in Fig. 5). Although there are some variations in the de-vice ground plane sizes, similar surface current distributions areexcited, which are expected to contribute to the radiation of theantenna.

IV. EXPERIMENTAL RESULTS AND DISCUSSION

The proposed antenna was fabricated and tested. A photo ofthe fabricated antenna mounted along an edge and at a cornerof a 0.2-mm-thick copper plate to simulate the device groundplane in Fig. 1 is shown in Fig. 15. Measured and HFSS simu-lated return loss of the proposed antenna is presented in Fig. 16.Good agreement between the measurement and simulation isseen. The measured antenna efficiency of the fabricated antennais shown in Fig. 17. The measured antenna efficiency includesthe mismatching loss and was measured in a far-field anechoicchamber. The measured antenna efficiency is about 40%–62%and 64%–92% in the lower and higher bands, respectively. Theobtained antenna efficiency is acceptable for mobile communi-cations [16], [17].In Fig. 17, the simulated radiation efficiency (perfect

matching condition) and simulated antenna efficiency (mis-matching loss included) are also shown for comparison. Themarked curves show the results for the higher band, while the


Fig. 14. Simulated surface current distributions on the device ground planes ofcase A and B in Fig. 11. (a) Case A. (b) Case B.

Fig. 15. Photo of the fabricated antenna.

unmarked curves are for the lower band. The differences in theantenna efficiency and radiation efficiency are mainly owingto the mismatching loss. The measured antenna efficiency in

Fig. 16. Measured and simulated return loss of the proposed antenna.

Fig. 17. Measured antenna efficiency of the fabricated antenna.

general agrees with the simulated antenna efficiency. In theexperiment, high- lumped elements with low loss (MurataEKLRQW15J) were used. In addition, probably because thechip inductor in the matching circuit has a small inductance(5.6 nH) and the chip inductor (22 nH) with the connectedbranch strip does not work as a radiating arm or a resonantpath for both the antenna’s lower and higher bands, acceptableantenna efficiency is obtained for the proposed antenna.Fig. 18 shows the measured radiation patterns at 900, 1900,

and 2500 MHz for the fabricated antenna. In the – plane,near-omnidirectional radiation is seen for the three represen-tative frequencies, which provides good coverage for commu-nications. The simulated realized antenna gain is also shownin Fig. 19, and the realized gain in the lower band is about0.6–2.0 dBi. At higher frequencies, comparable and ra-

diation is also seen, which is especially advantageous for prac-tical applications where the propagation environment is com-plex. In the – and – planes (vertical planes), by consid-ering the and radiation in whole (that is, the total radiatedpower), there are generally no large dips in all directions, and thesimulated realized gain in the higher band is about 2.1–5.5 dBi(see Fig. 19). The obtained radiation characteristics are accept-able for practical applications.

V. CONCLUSION

A small-size, planar LTE/WWAN tablet computer antennahas been proposed. The antenna’s metal pattern can be printedon a thin FR4 substrate of size 10 34.5 mm , and the antennacan provide twowide operating bands to cover the 704–960- and1710–2690-MHz bands for the LTE/WWAN operations. Theantenna’s higher band is mainly formed by two higher order


Fig. 18. Measured radiation patterns at 900, 1900 and 2500 MHz of the fabri-cated antenna.

Fig. 19. Simulated realized antenna gain of the antenna.

resonant modes contributed by a small printed loop antenna,while the lower band is formed by the fundamental resonantmode of the printed loop antenna and an additional resonantmode caused by a branch strip inductively coupled to the loopantenna. The operating principle of the proposed antenna, espe-cially the effects of the inductively coupled branch strip on thelow-band bandwidth enhancement has been described. Exper-imental results of the fabricated antenna have been presentedand discussed. Acceptable radiation characteristics for mobilecommunications have been obtained. The proposed antenna isespecially suitable for the LTE/WWAN operation in the slimtablet computer applications.

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Kin-Lu Wong (M’91–SM’97–F’07) received theB.S. degree in electrical engineering from NationalTaiwan University, Taipei, Taiwan, and the M.S. andPh.D. degrees in electrical engineering from TexasTech University, Lubbock, TX, USA, in 1981, 1984,and 1986, respectively.From 1986 to 1987, he was a Visiting Scientist

with the Max-Planck-Institute for Plasma Physics,Munich, Germany. Since 1987, he has been withElectrical Engineering Department, National SunYat-Sen University (NSYSU), Kaohsiung, Taiwan,

where he became a Professor in 1991. From 1998 to 1999, he was a VisitingScholar with the ElectroScience Laboratory, The Ohio State University,Columbus, OH, USA. In 2005, he was elected to be the Sun Yat-sen ChairProfessor of NSYSU. He also served as Chairman of the Electrical EngineeringDepartment from 1994 to 1997, Vice-President for Research Affairs from2005 to 2008, and Senior Vice-President, NSYSU, from 2007 to 2012. He haspublished more than 500 refereed journal papers and 250 conference articles


and has personally supervised 52 graduated Ph.D.s. He holds more than 200patents, including U.S., Taiwan, China, EU patents. He is the author of Designof Nonplanar Microstrip Antennas and Transmission Lines (Wiley, 1999),Compact and Broadband Microstrip Antennas (Wiley, 2002), and PlanarAntennas for Wireless Communications (Wiley, 2003).Dr. Wong is a member of the IEEE AP-S Best Papers Awards Committee

(2011–2013), and an Associate Editor of the IEEE AP-S TRANSACTIONS (2013to present). He received the Outstanding Research Award three times from theNational Science Council of Taiwan in 1995, 2000, and 2002. He also receivedthe Outstanding Electrical Engineering Professor Award from the Institute ofElectrical Engineers of Taiwan in 2003, and the Outstanding Engineering Pro-fessor Award from Institute of Engineers of Taiwan in 2004. In 2008, the re-search achievements on handheld device antennas of NSYSU Antenna Lab ledby Dr. Wong was selected to be among the top 50 scientific achievements ofNational Science Council of Taiwan in past 50 years (1959–2009). He wasawarded the 2010 Outstanding Research Award of Pan Wen Yuan Foundationand selected as top 100 honor of Taiwan by Global Views Monthly in August2010 for his contribution to mobile antenna research. In 2012, he was awardedthe Academic Award from Ministry of Education, Taiwan, and the OutstandingDistinguished Researcher Award from the National Science Council, Taiwan, in2013. Hewas awarded the Best Paper Award (APMCPrize) in the 2008Asia-Pa-cificMicrowave Conference. His graduate students were the winners of the Best

Student Paper Award/Young Scientist Award in 2007 ISAP, 2008 APMC, 2009ISAP, 2010 ISAP, and 2012 ISAP. His graduate students also won the first prizeof the 2007 and 2009 Taiwan NationalMobile Handset Antenna Design Compe-tition. He is General Chair of the 2012 APMC at Kaohsiung, Taiwan, and 2014ISAP at Kaohsiung as well. He has been an International Steering Committeemember for many international conferences. His published articles have beencited over 15 000 times in Google Scholar and have an h-index of 40 in ISI Webof Science.

Meng-Ting Chen (S’13) was born in Taichung,Taiwan. He received the B.S. degree in communica-tion engineering from Yuan Ze University, Taoyuan,Taiwan, in 2010. He is currently working towardthe M.S. degree at National Sun Yat-sen University,Kaohsiung, Taiwan.His main research interests are in antenna design

for wireless communications, especially for thesmall-size WWAN/LTE multiband antennas formobile devices.

Documents

Small-Size LTE/WWAN Printed Loop Antenna With an Inductively Coupled Branch Strip for Bandwidth Enhancement in the Tablet Computer