compact multiband interdigital-coupled-fed planar antenna...

Research ArticleCompact Multiband Interdigital-Coupled-Fed Planar Antennawith Stepped-Impedance Structures for Mobile Handsets

Tao Zhou, Yazi Cao, and Zhiqun Cheng

Hangzhou Dianzi University, Hangzhou, Zhejiang, China

Correspondence should be addressed to Yazi Cao; [email protected]

Received 9 February 2017; Revised 3 April 2017; Accepted 19 April 2017; Published 20 June 2017

Academic Editor: Miguel Ferrando Bataller

Copyright © 2017 Tao Zhou et al.This is an open access article distributed under theCreative CommonsAttribution License, whichpermits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

A new compact multiband planar antenna combining an interdigital-coupled feeding line and one stepped-impedance structureis presented here. This antenna is capable of generating five resonant modes to cover the ISM 915/2400/5800MHz bands, GPSband, and IMT C-band, respectively. The five resonant frequencies covered by the proposed antenna can be adjusted individuallyby controlling the impedances and electrical lengths of the corresponding stepped-impedance sections. An additional advantageof the proposed stepped-impedance structure is its ability to suppress higher order resonance modes, thus filtering out unwantedinterference.The proposed antenna utilizes a simple planar compact structure and occupies a small area of only 12× 30mm2. Detailsof the antenna design and experimental results are presented and discussed.

1. Introduction

Mobile wireless communication handsets typically usemulti-band antennas to transmit and receive wireless signals tocover all required wireless communication frequency bands[1–4]. Because of its compact size and multiband perfor-mance, the planar inverted F-antenna (PIFA) is preferred formultiband antenna for wireless communication devices [5–7]. However, the ability to cover multiple bands while min-imizing the structure of slot antennas is still a challenge forantenna designers. Furthermore, the PIFA typically exhibitsperformance limitations related to the radiating branches, notonly generating the lower resonant modes but also excitingseveral higher order modes. These unexpected higher modeswill complicate frequency tuning of the multiband antenna.Additionally, these unexpected higher ordermodes will affectthe power amplifier or low-noise amplifier’s performancesand in turn degrademultiband antenna’s radiation properties.

Here we present a novel compact multiband planarantenna formed by the interdigital-coupled feeding line, onestepped-impedance line, and one shorted stripe line con-nected to the ground plane of the mobile handsets. Theantenna feeding line comprises an interdigital structure forenhancing signal coupling and to allow more flexibility forwideband impedance matching [8]. The stepped-impedance

line forms folded stripe lines with different impedancesand electrical lengths. This stepped-impedance structureis introduced to control five resonant modes to coverISM 915/2400/5800 bands, GPS band, and IMT C-bandoperations, respectively. An additional advantage of usingstepped-impedance structure includes the ability to suppresshigher order resonant frequencies thus filtering out unwantedinterference. Design consideration and experimental perfor-mances of the proposed antenna are studied and presented.

2. Antenna Design

Figure 1 shows the structure of the proposed interdigital-coupled-fed planar antenna. The proposed antenna mainlycomprises the FR4 substrate, a ground plane under thesubstrate, and a radiating metal portion on the top of thesubstrate. Under the upside of the substrate, there is oneground-clear area under the radiating metal portion. Theradiating metal portion comprises the interdigital-coupledfeeding line, the stepped-impedance line, and one shortedline.The interdigital-coupled feeding line connecting a signalsource forms a three-finger interdigital structure. One endof the interdigital-coupled-fed line is of the L-shape. Thestepped-impedance line includes six bending stepped lines

HindawiInternational Journal of Antennas and PropagationVolume 2017, Article ID 7435834, 8 pageshttps://doi.org/10.1155/2017/7435834

https://doi.org/10.1155/2017/7435834

2 International Journal of Antennas and Propagation

30

Feeding point Unit: mmShorting point

1211.323.5

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0.8Line 2

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W5 = 1.2

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W2 = 2.8W1 = 1

W3 = 3

(a)

85

54

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Y

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(b)

Figure 1: Configuration of the proposed antenna on a 1.6mm FR4 substrate. (a) Detailed dimensions of radiating element (front view). (b)Photo of the antenna prototype.

SimulationMeasured

IMT C-band 3200–3400

ISM 5725–5875

VSWR = 2.0 : 1

GPS 1575

ISM 2400–2500

ISM 902–928

1.5 2.5 3.5 4.5 5.5 6.50.5Frequency (GHz)

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Figure 2: Measured and simulated return loss for the proposed antenna.

with different impedances and electrical lengths.The shortedline is connected to ground plane through one via hole.

Based on generalized transmission line theory, the mul-timode property of this stepped-impedance line can be char-acterized and determined by the impedances and electricallengths of adjacent folded stripe lines [9–11]. In this proposedstructure, the folded stripe lines control the excitations of theantenna bands centered at 915MHz, 1575MHz, 2400MHz,3200MHz, and 5800MHz to cover ISM 915/2400/5800bands, GPS band, and IMT C-band operations, respectively.These five resonant modes generated by the correspondingrespective stepped sections can be controlled individually.A ground plane with a length of 73mm and a width of54mm is printed on the 1.6mm thick FR4 substrate of relativepermittivity of 4.4 and loss tangent of 0.02. The total sizeof FR4 corresponded to the width and length of 85mm and54mm, respectively. The signal source feeds the interdigital-coupled lines and is subsequently coupled into the stepped-impedance lines. Both measurement and simulated results

including the peak gain, radiation efficiency, and radiationpattern are presented next to validate our proposed structure.

3. Experimental Results and Discussion

Figure 2 shows the measured and simulated return loss ofthe proposed antenna with the dimensions given in Figure 1.Themeasured data agrees very well with the simulated resultsobtained from the Ansoft simulation software HFSS [12].The low band exhibits a measured 2.0 : 1 VSWR (−10 dBreturn loss) bandwidth covering ISM 915/2400/5800 bands(902∼928MHz, 2400–2500MHz, and 5725–5875MHz), GPS1575MHz band, and IMT C-band 3200–3400MHz opera-tion.

The measured gain radiation patterns of the constructedprototype are shown for the 915, 1575, 2450, 3200, and5875MHz frequency bands in Figure 3. At 915MHz, theomnidirectional dipole-like radiation patterns can be seen,

International Journal of Antennas and Propagation 3

G𝜃

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Freq = 1575MHz Freq = 1575MHzFreq = 1575MHz

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Figure 3: Continued.


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Figure 3: Measured gain radiation patterns at (a) 915MHz; (b) 1575MHz; (c) 2450MHz; (d) 3200MHz; and (e) 5875MHz.

and the radiation patterns for higher frequencies show similaragreement with expected/simulated results. For the 1575,2450, 3200, and 5875MHz frequency bands, the measuredgain radiation patterns are comparable to those observed inconventional internal mobile phone antennas.

For the lower band at ISM 915MHz, the measured gainvaried from about 0.4 to 0.7 dBi, and the antenna radiationefficiency is better than 50% over 902∼928MHz. For theupper bands including GPS 1575MHz, ISM 2450/5800MHz,and IMT C-band 3200MHz, the antenna gain varies from1.0 to 3.0 dBi, with the radiation efficiency better than 60%.Themeasured peak gain and radiation efficiency results of theproposed antenna are acceptable for practical application.

In order to show the resonant modes of the proposedantenna, the simulated surface current distributions on theradiating metal portion and the ground plane at 915, 1575,2450, 3200, and 5875MHz are shown in Figure 4. As shown

in Figure 4(a), the strong excited surface currents at 915MHzare flowing along the folded stripe line 6 as defined in Figure 1.As shown in Figure 4(b), the strong excited surface currentsat 1575MHz are flowing along the folded stripe line 4. Asshown in Figure 4(c), the strong excited surface currents at2450MHz are flowing along the folded stripe lines 2, 3, and6.As shown in Figure 4(d), the strong excited surface currentsat 3300MHz are flowing along the folded stripe lines 2, 4,and 5. As shown in Figure 4(e), the strong excited surfacecurrents at 5875MHz are flowing along the folded stripe lines1 and 2. These current distribution characteristics indicatethat these five resonant modes can be tuned and controlledby the corresponding impedances and electrical lengths offolded stripe lines 1–6.

Furthermore, Figure 5 depicts the simulated return lossvariationswith the differentwidths𝑊

2,𝑊

3,𝑊

4,𝑊

5, and𝑊

6of

folded stripe lines 2, 3, 4, 5, and 6, respectively. The obtained


Jsurf (A_per_m)1.18E + 002

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7.90E + 001

6.58E + 001

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2.64E + 001

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1.18E − 001

(a)


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5.87E + 001

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(b)


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1.61E − 002

(c)

Figure 4: Continued.



1.69E + 001

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7.55E + 000

5.67E + 000

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(d)


1.58E + 001

1.38E + 001

9.90E + 000

5.95E + 000

3.97E + 000

1.95E + 000

1.78E − 002

1.18E + 001

7.92E + 000

(e)

Figure 4: Simulated current distributions on the radiating metal portion and the ground plane at (a) 915MHz; (b) 1575MHz; (c) 2450MHz;(d) 3300MHz; and (e) 5875MHz.

result indicates that these five resonant frequencies can beadjusted effectively by the corresponding folded stripe lines.It agrees with the current distributions as shown in Figure 4.

4. Conclusion

In this letter we report the design and fabrication of a novelmultiband printed antenna formed by interdigital-coupledfeeding line, one stepped-impedance line, and one shortedstripe line connected to the ground plane of the mobilehandsets. A prototype of the proposed antenna has been

successfully realized and experimental validation matchesexpected results. The proposed antenna utilizes a simpleplanar structure with a small area of only 12 × 30mm2.This antenna is able to generate five resonant modes tocover the ISM 915/2400/5800 frequency bands, GPS band,and IMT C-band operation. These five resonant modes canbe controlled individually by the corresponding stepped-impedance lines. This merit would be attractive for antennadesigners since it enables optimization and tuning of theantenna by adjusting the geometrical parameters of eachindividual corresponding stepped-impedance lines withoutdisturbing adjacent sections.


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1.5 2.5 3.5 4.5 5.5 6.50.5Frequency (GHz)

2.8 mmW2 =

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turn

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0.7 mmW6 =

1mmW6 =

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(e)

Figure 5: Simulated return loss of the proposed antenna with variations of widths𝑊2(a),𝑊

3(b),𝑊

4(c),𝑊

5(d), and𝑊

6(e).


Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This work was supported by the National Nature ScienceFoundation of China under Grants 61411136003 and 61331007,the Zhejiang Provincial Natural Science Foundation of Chinaunder Grant LZ14F040001, and the Zhejiang ProvincialScience and Technology Program under Grant 2017C31066.

References

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[2] L. Hu and W. Hua, “Wide dual-band CPW-fed slot antenna,”Electronics Letters, vol. 47, no. 14, pp. 789-790, 2011.

[3] B. Yuan, Y. Cao, and G. Wang, “A miniaturized printed slotantenna for six-band operation of mobile handsets,” IEEEAntennas and Wireless Propagation Letters, vol. 10, pp. 854–857,2011.

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[5] Y. Cao, B. Yuan, and G. Wang, “A compact multiband open-ended slot antenna for mobile handsets,” IEEE Antennas andWireless Propagation Letters, vol. 10, pp. 911–914, 2011.

[6] Y.-L. Ban, Y.-F. Qiang, Z. Chen, K. Kang, and J.-H. Guo, “Adual-loop antenna design for hepta-band WWAN/LTE metal-rimmed smartphone applications,” Institute of Electrical andElectronics Engineers. Transactions on Antennas and Propaga-tion, vol. 63, no. 1, pp. 48–58, 2015.

[7] D. Wu, S. W. Cheung, and T. I. Yuk, “A compact and low-profile loop antenna with multiband operation for ultra-thinsmartphones,” Institute of Electrical and Electronics Engineers.Transactions on Antennas and Propagation, vol. 63, no. 6, pp.2745–2750, 2015.

[8] J.-X. Liu and W.-Y. Yin, “A compact interdigital capacitor-inserted multiband antenna for wireless communication appli-cations,” IEEE Antennas andWireless Propagation Letters, vol. 9,pp. 922–925, 2010.

[9] B. Yu, B. Jia, and Z. Zhu, “Compact tri-band bandpass filter withstub-loaded stepped-impedance resonator,” Electronics Letters,vol. 51, no. 9, pp. 701–703, 2015.

[10] C.-J. Wang and L.-T. Chen, “Modeling of stepped-impedanceslot antenna,” IEEE Transactions on Antennas and Propagation,vol. 62, no. 2, pp. 955–959, 2014.

[11] Y. Sung, “UWB monopole antenna with two notched bandsbased on the folded stepped impedance resonator,” IEEE Anten-nas and Wireless Propagation Letters, vol. 11, pp. 500–502, 2012.

[12] HFSS Version 17, ANSYS, Canonsburg, Pa, USA, 2016.

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