Compact Dual-Band Monopole Antenna for LTE Mobile Phones

Ibra Dioum#1, Aliou Diallo*2, Cyril Luxey*3, Sidi Mohamed Farsi#4 # Département Génie Informatique, Ecole Supérieure Polytechnique - Université Cheikh Anta Diop

Dakar - Sénégal dioum.ibra@yahoo.fr farsism@yahoo.com

*Laboratoire d’Electronique, Antennes et Télécommunications Université de Nice-Sophia Antipolis, CNRS UMR 6071, Bât.4, 250 rue Albert Einstein, 06560 Valbonne, France

Aliou.diallo@unice.fr cyril.luxey@unice.fr

Abstract— In this paper, we present a dual-band antenna for Long Term Evolution (LTE) handsets. The proposed antenna is composed of a meandered monopole operating in the 700 MHz band and a parasitic element which radiates in the 2.5 – 2.7 GHz band. Two identical antennas are then closely positioned on the same 120x50 mm2 ground plane (Printed Circuit Board) which represents a modern-size PDA-mobile phone. To enhance the port-to-port isolation of the antennas, a neutralization technique is implemented between them. Scattering parameters, radiations patterns and total efficiencies are presented to illustrate the performance of the antenna-system.

I. INTRODUCTION The LTE standard is schedule to operate in different

frequency bands from 400 MHz to 4 GHz [1]. This cellular communication technology known as the fourth mobile-phone generation has been drafted to be able to theoretically support Multi-Input Multi-Output (MIMO) operation [2]. However, modern handsets still experience miniaturization where thin and slim shapes are making difficult the integration of several antennas onto a small PCB [1-2]. Moreover, position closely-spaced antennas produces high coupling between them. To enhance their isolation, different methods have been adopted [3-4]. The neutralization technique is commonly used in our laboratory [5-10]. In addition, modern handsets must operate in different communication standards [11] and reach a sufficient antenna-to-antenna isolation in each of these frequency bands is not a trivial achievement.

In this paper, we propose to address these issues for a small communicating device. First, a single 700 MHz/2.5-2.7 GHz dual-band LTE antenna is presented. The antenna consists of a main meandered monopole and a connected parasitic element dedicated to enlarge the higher frequency band. Then, two antennas are closely positioned on the same 120x50mm2 PCB. To enhance their port-to-port isolation, a neutralization technique is implemented. Scattering parameters, radiations patterns and total efficiencies are presented to prove the usefulness of the method.

II. SINGLE ANTENNA ELEMENT The layout of the optimized single-band antenna is

presented in Fig. 1 (3-D view).

Fig. 1 3D-view of the single-band antenna

The monopole is meandered to fit in the upper corner of the PCB and its electrical length is optimized to make it resonant at 740 MHz. The antenna is printed on a duroid substrate. The permittivity εr of the dielectric is 2.2 and it has a thickness of 0.762 mm. The total size of the PCB is 120×50mm2. The antenna occupies an area of 38 × 50mm2. The 82 × 50mm2 ground plane acting as a counterpole is etched on the other side of the board. The simulated scattering parameters, using HFSS software, are presented in Fig. 2.

Fig. 2 Simulated reflection coefficient of the single-band antenna

The fundamental mode of the monopole and several

higher-order resonances (taken as minima of the |S11|) are easily seen. If the lower LTE band is sufficiently covered with

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a -6dB matching performance, the antenna does not satisfy this constraint at the higher LTE frequency-band. To be able to cover the 2.5-2.7GHz band, a parasitic element, connected to the main monopole in a low-impedance region of it, whose electrical length is around a quarter-wavelength at 2.6 GHz, has been optimized to create a new resonance and to increase the existing bandwidth (Fig. 3).

Fig. 3 3D-view of the dual-band antenna

The simulated reflection coefficient of the optimized antenna is presented in Figure 4. The higher band is not well matched and the minimum of the reflection coefficient is now -13.7 dB.

Fig. 4 Simulated reflection coefficient of the dual-band antenna

III. INITIAL TWO-ANTENNA SYSTEM In a first attempt, two dual-band antennas have been

simply positioned at the same top-edge of the PCB (Fig. 5).

Fig. 5 Top view of the initial two-antenna system (zoomed view of the

antennas)

The feeding line of each antenna has been extended for practical purpose. Simulated scattering parameters are shown in Fig. 6.

Fig. 6 Scattering parameters of the initial two-antenna system (|S11| and |S22| in blue; |S21| in red)

In the lower band, a low isolation (|S21| parameter) is

obtained between the two monopoles: a minimum of 2.5 dB and a maximum of 6 dB. In the higher band, the isolation seems to be more suitable with a lower level of 8.5 dB and higher level of 11.5 dB. So it is necessary to enhance this performance with an appropriate technique.

IV. IMPLEMENTATION OF THE NEUTRALIZATION TECHNIQUE In order to improve the port-to-port isolation, a

neutralization line was simply inserted between both antennas, in a low impedance area where the amplitude of the current is high and the electric field is less important (Fig. 7).

Fig. 7 Neutralized two-antenna system (zoomed view of the antennas)

The simulated |S21| parameter is showing an increase of the

isolation in the lower band which seems to be an interesting result but at the opposite, a decreased isolation in the higher band (Fig. 8). A parametric study was therefore conducted on the width, the length and the position of the neutralization line to achieve the best isolation values.

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Fig. 8 Scattering parameters of the neutralized two-antenna system (|S21| in red)

First the position of the line was varied from 2mm to

10mm (see Fig. 7 for the connection point to be varied and Fig. 9 for the simulated results). The optimum value was found to be 4mm, both for matching and isolation performance.

At this optimum position, a parametric study was also conducted by varying the width of the neutralisation line, from 0.5 to 2mm and its length. The obtained scattering parameters are not presented here for sake of brevity. The layout of the final optimized structure is presented in Fig. 10.

(a)

(b)

(c)

Fig. 9 Scattering parameters of the neutralized two-antenna system (|S21| in red). The neutralization line is placed at 2mm (a), 8mm (b) and 10mm (c).

Fig. 10 Top-view of the optimized system with detailed dimensions. The simulated scattering parameters of the antenna in the 700 MHz and the 2.5 – 2.7 GHz band are shown in Fig. 11 (a-b).

(a)

(b)

Fig. 11 Simulated S-parameters of the optimized two-antenna system

(a) lower band (b) higher band.

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In the lower band, if we consider a -6 dB matching criterion, we have a 40 MHz bandwidth. In the higher band, the antenna has a larger bandwidth: it covers the 2.5 GHz – 2.7 GHz band with a return loss always higher than 12.5 dB. The isolation value is found to be higher than 6 dB in the whole lower LTE band and the same in the higher band. From the author’s knowledge, these values are state-of-the art performance for dual-band LTE antennas, so closely packed in a small communicating device. They can be still improved as deep |S21| nulls are indeed created by the neutralization technique but not fully frequency-optimized (Fig. 11).

The radiation patterns at the centre frequency in each band (740 MHz and 2.6 GHz) are presented in Fig. 12 (φ=0° and φ= 90° planes). In the lower band, a dipole-like pattern with omnidirectionnal characteristics is observed while in the higher band, some directivity occurs. The maximum total efficiency of both antennas has been computed using the radiation efficiencies simulated with HFSS and with the simulated scattering parameters (equation (1)).

)1( 2

212

11 SSraytot (1) They are presented in Table 1. It can be seen that the efficiency is extremely high in the higher band while really interesting in the lower band where the whole structure is indeed a small antenna (740 MHz).

TABLE I

TOTAL EFFICIENCY OF THE ANTENNA

Frequencies 0.74 GHz 2.6 GHz Total efficiency (%) 72.9 97.2

φ= 0° φ= 90°

(a) 740 MHz

φ= 0° φ= 90°

(b) 2.6 GHz

Fig. 12 Radiation patterns of the optimized two-antenna system (a) at 740 MHz (b) at 2.6 GHz in two different planes

V. CONCLUSIONS In this letter, we have presented a dual-band antenna for

LTE handsets. The antenna is based on a meandered monopole with a connected parasitic element to enhance its bandwidth in the higher band. Two antennas of the same kind have been closely positioned on the same 120x50mm2 PCB. To enhance their port-to-port isolation, a neutralization technique was implemented. It was especially revealed that the total efficiencies of the antennas are extremely satisfactory. Several antenna-systems are currently being fabricated and measurements will be given at the conference.

REFERENCES [1] R. A. Bhatti, S. Yi, and S. Park, “Compact antenna array with port

decoupling for LTE-Standardized Mobile Phones, ” IEEE Antennas and Wireless Propagation Letters, vol. 8, pp. 1430-1433, 2009.

[2] P. Tornatta, “Overcoming the LTE handset antenna design problem,” EE Times-Asia, http://www.eetasia.com/STATIC/PDF/200908/EEOL_2009AUG13_RFD_TA_01.pdf?SOURCES=DOWNLOAD, 13th August 2009.

[3] C. Luxey, “Design of multi-antenna systems for UMTS mobile phones,” Proc. Loughborough Antennas & Prop. Conf. (LAPC 2009), Loughborough, UK, November 16-17, 2009.

[4] C. Luxey, D. Manteuffel, “Highly-efficient Multiple antennas for MIMO-systems,” IEEE International Workshop on Antenna Technology, iWAT2010, Lisbon, Portugal, 1-3 March 2010.

[5] A. Diallo, C. Luxey, Ph. Le Thuc, R. Staraj, G. Kossiavas, “Study and Reduction of the Mutual Coupling between Two Mobile Phone PIFAs Operating in the DCS1800 and UMTS Bands,” IEEE Trans. Antennas Propagat., Part.1, vol.54, no. 11, pp. 3063-3074, November 2006.

[6] A. Diallo, C. Luxey, Ph. Le Thuc, R. Staraj, G. Kossiavas, “An efficient two-port antenna-system for GSM/DCS/UMTS multi-mode mobile phones,” Elec. Lett., vol.43, no.7, pp. 369-370, 29th March 2007.

[7] A. Diallo, C. Luxey, Ph. Le Thuc, R. Staraj, G. Kossiavas, “Diversity performance of multi-antenna systems for UMTS cellular phones in different propagation environments,” International Journal on Antennas and Propagation (IJAP), vol. 2008, Article ID 836050, 10 pages, 2008. doi:10.1155/2008/836050,http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2008/836050. 2008

[8] A. Chebihi, C. Luxey, A. Diallo, P. Le Thuc, R. Staraj, “A Novel Isolation Technique for Closely Spaced PIFAs for UMTS Mobile Phones,” IEEE Antennas & Wir. Prop. Lett., vol.7, pp.665-668, 2008.

[9] A. Diallo, C. Luxey, Ph. Le Thuc, R. Staraj, G. Kossiavas, M. Franzen, P.-S. Kildal, “Diversity characterization of optimized two-antenna systems for UMTS handsets,” EURASIP Journal on Wireless Communications and Networking, vol. 2007, Article ID 37574, 9 pages, doi:10.1155/2007/37574, http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2007/37574. 2007.

[10] A. Diallo, C. Luxey, Ph. Le Thuc, R. Staraj, G. Kossiavas, “Enhanced two-antenna structures for universal mobile telecommunications system diversity terminals,” IET Microwaves, Antennas and Propagation, vol. 2, no. 1, pp. 93-101, February 2008.

[11] P. Ciais, C. Luxey, A. Diallo, R. Staraj, G. Kossiavas, “Pentaband internal antenna for handset communication devices,” Microwave Optical Technology Letters, vol. 48, no. 8, pp. 1509-1512, August 2006.

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