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Journal of Electromagnetic Waves and

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Printed slot antenna for seven-band

GSM/UMTS/LTE operation in the

internal mobile phone

Y.-L. Ban a , J.-H. Chen a , J.L.-W. Li a & W. Hu b a

Institute of Electromagnetics, University of Electronic Science and Technology of China, 2006 Xi-Yuan Avenue, Western High-Tech District, Chengdu, Sichuan, 611731, China

b

System Planning Division, Potevio Institute of Technology Co. Ltd, Beijing, 100080, China

Version of record first published: 11 Sep 2012.

To cite this article: Y.-L. Ban , J.-H. Chen , J.L.-W. Li & W. Hu (2012): Printed slot antenna for

seven-band GSM/UMTS/LTE operation in the internal mobile phone, Journal of Electromagnetic Waves and Applications, 26:14-15, 2033-2042

To link to this article: http://dx.doi.org/10.1080/09205071.2012.724766

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Printed slot antenna for seven-band GSM/UMTS/LTE operation in the

internal mobile phone

Y.-L. Bana, J.-H. Chena*, J.L.-W. Liaand W. Hub

a

Institute of Electromagnetics, University of Electronic Science and Technology of China, 2006 Xi-Yuan Avenue, Western High-Tech District, Chengdu, Sichuan 611731, China;bSystem Planning Division,

Potevio Institute of Technology Co. Ltd, Beijing 100080, China (Received 21 May 2012; accepted 6 July 2012)

In this paper, a promising internal printed slot antenna for seven-band operation applied to the mobile phone has been proposed, covering all thefive WWAN (namely, Wireless Wide Area Network) bands of GSM850/900/DCS1800/PCS1900/UMTS2100 and two LTE (namely, Long Term Evolution) bands of LTE2300/2500. With the use of slot elements, the presented antenna can be easily printed on an inexpensive FR4 substrate and shows a pla-nar structure for low-profile mobile handsets. The antenna comprises an inverted L-shaped slot and three open-ended monopole slots, and these slot elements are integrated into a compact configuration to occupy a small area of 15 mm 40 mm in the back side of the system circuit board. A 50-X microstrip feed line is employed to excite the designed antenna, which is printed on the top side of the PCB (Printed Circuit Board). Details of the antenna design, simulated and measured results on the return loss and radiation characteris-tics of the proposed antenna are presented and discussed.

1. Introduction

The increasing need of multiple-band antennas with lightweight, low proﬁle, and ease of fab-rication has stimulated comprehensive researches to pursue cost-effective antenna designs. Recently, printed slot antennas become attractive because of their wide bandwidths, planar structures, and easy integration with active devices or MMICs (namely, monolithic microwave integrated circuit) in the mobile applications [1–8]. Besides, when a slot antenna is fed by a microstrip line, it does not add weight and size to the system, thus it is very suitable for the mobile phone. A few Wireless Wide Area Network (WWAN) printed slot antenna conﬁgura-tions have been reported in the open literature, including wide slot antenna [1–3,7], straight slot antenna [6,8], L-shaped slot antenna [4–6], inverted T-slot antenna [5], and fractal-shaped slot antenna [7]. As we know, the traditional slot antenna has about a half-wavelength reso-nance [4,5], which is a large size. Moreover, some slot antennas have limited operating band-width, making them not enough to use for more mobile terminals.

To reduce the overall size of a slot antenna, some quarter-wavelength open slots are designed in [4–6], which can realize a compact size and wide bandwidth. In [5], some T-shaped and E-shaped slots are used in the design of an antenna only for GSM900/ DCS1800/PCS1900/UMTS2100 operation, which is too limited to meet the demands for

*Corresponding author. Email: [email protected] Journal of Electromagnetic Waves and Applications Vol. 26, Nos. 14–15, October 2012, 2033–2042

ISSN 0920-5071 print/ISSN 1569-3937 online Ó 2012 Taylor & Francis

http://dx.doi.org/10.1080/09205071.2012.724766 http://www.tandfonline.com

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WWAN systems. The authors in [2,3] use different slot elements, including a wide slot and tapered slot, to enhance the impedance bandwidth of an UWB antenna. And a promising slot antenna for laptop or tablet computer [6] is proposed, but it is not suitable for mobile phone due to its large size (about 12 65 mm2or 12 75 mm2, etc.). Another design for internal mobile handset in [8] is a complex structure of standing closed slot and open slot, which has a size of 15 mm 60 mm 10 mm and cannot cover all the seven-band GSM850/900/ DCS1800/PCS1900/UMTS2100/LTE2300/2500 operation. Based on the above considerations, it is important to further reduce the size and enhance operating bandwidth of the antenna.

In this article, considering these slot mobile handset antennas [4–6,8] and other mobile antennas [1–3,9–20], a new printed slot antenna formed by an inverted L-shaped closed slot and three open slots is proposed. The presented design has a compact printed size of 15 mm 40 mm, which can cover all the ﬁve WWAN bands of GSM850 (824–894 MHz)/900 (880–960 MHz)/DCS1800 (1710–1880 MHz)/PCS1900 (1850–1990 MHz)/UMTS2100 (1920– 2170 MHz) and two LTE bands of LTE2300 (2300–2400 MHz)/2500 (2500–2690 MHz). These slot elements are coupled-fed by a 50-X straight microstrip line printed on the top side of the system circuit board of the mobile phone. Design considerations, parametric studies, and experimental results are given and discussed in the following sections.

2. Proposed antenna design

The geometry of the proposed slot antenna printed on the system circuit board of the mobile phone is illustrated in Figure 1, where the detailed dimensions of the printed slot antenna are also shown after several simulations. In the study, the FR4 substrate of thickness 0.8 mm with relative permittivity 4.4 and loss tangent 0.02 was used. A compact size of 15 mm 40 mm on the top of the printed circuit board (PCB) is used to design the antenna, and 100 and

Figure 1. Geometry and dimensions of the proposed slot antenna.

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40 mm denote the length and the width of the ground plane, respectively. A 50-X microstrip line is printed on the top of the used FR4 substrate. The studied mobile phone is also enclosed by a 1-mm thick plastic housing of the relative permittivity 3.3, and its conductivity is 0.02 S/mm. Notice that the height of the plastic housing is 10 mm here, which is an attrac-tive height for thin mobile phones, and the proposed slot antenna is at the center of the plastic housing.

The antenna consists of a straight 50-X microstrip line with a width of 1.5 mm, an inverted L closed-ended slot with a length of s¼ 46:5 mm (slot 1), and three open-ended straight slots with different lengths (slot 2, g¼ 35 mm; slot 3, t ¼ 25:5 mm; slot 4, w¼ 13 mm). The distance d is used to describe the location of the microstrip line, which is connected to a 50-X SMA connector for testing the proposed slot antenna. The operating principle of the slot antenna can be described as the following. Firstly, the open-ended mono-pole slot 2 generates a quarter-wavelength resonant mode at around 900 MHz for the pro-posed slot antenna’s lower band to cover the desired GSM850/900 operation (824–960 MHz). Secondly, the antenna’s upper band of DCS1800/PCS1900/UMTS2100/LTE2300/2500 (1710– 2690 MHz) is formed by three resonant modes, theﬁrst being the quarter-wavelength resonant mode contributed by monopole slot 2 at 1700 MHz, the second being the monopole slot 3 at 2200 MHz (conventional quarter-wavelength resonant mode), and the third being the closed-ended slot 1 at 2700 MHz (half-wavelength higher-order resonant mode).

In addition, the system ground plane of the mobile phone is also helpful for the desired band’s achievement, especially for the lower band of GSM850/900. So, by designing dimen-sions of the slots and proper feeding structure, both the wide operating bands can be success-fully obtained, which is attractive for the WWAN/LTE operation in the thin internal mobile handsets. Simulated results from the simulation software HFSS 12.0 will be given to demonstrate the design.

3. Parametric study and discussion

To analyze the operating principle of the antenna, Figure 2 shows the simulated return loss as a function of the dimensions in slot 1. Results for the length s varied from 30 to 46.5 mm are presented in Figure 2(a). Relatively large effects on the fourth resonant mode at 2.7 GHz are seen, indicating that the resonant mode can be controlled by adjusting the length s. And the effects on the return loss of the width b are given in Figure 2(b). It can be found that the input impedance matching of the desired lower and upper bands is affected by the width of

Figure 2. Simulated return loss as a function of (a) the length s and (b) the width b of the slot 1. Other dimensions are the same as in Figure 1.

Journal of Electromagnetic Waves and Applications 2035

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slot 1. When b¼ 2 or 6, either the lower or the upper band of the impedance matching goes bad. Considering the results, the length and the width of the closed-ended slot 1 are chosen as 46.5 and 4 mm, respectively.

Figure 3(a) shows the simulated return loss as a function of the length g of the open-ended slot 2. Other dimensions are the same as in Figure 1. Results for the length g varied from 30 to 37 mm indicate that the ﬁrst resonant mode at about 900 MHz shifts down with the increasing length g, which leads to poor impedance matching in the GSM850/900 bands. As seen from Figure 3(b) showing the effects for the length t (varied from 34.5 to 19.5 mm) of the slot 3, the occurrence of the fundamental resonant mode in the lower band and other three resonant modes in the upper band can be adjusted properly. In the study, by selecting the length t to be 25.5 mm (the length of w¼ 13 mm), the desired 824–960 MHz and 1710–2690 MHz can be obtained.

Moreover, Figure 4(a) and (b) shows the simulated return loss as a function of the location d and the length f of the microstrip feed line. With other dimensionsﬁxed as given in Figure 1, results for the location d varied from 13 to 23 mm are plotted in Figure 4(a). Since the varia-tions in the location change the coupling strength between the feeding line and the slots, all the resonant modes and impedance matching are affected. Similar effects can be clearly observed in Figure 4(b), where results for the length of the feeding line are shown. With the increase of the length f from 8 to 15 mm, the impedance matching of the lower and upper bands can be improved, resulting in achieving the desired 824–960 MHz and 1710–2690 MHz, because a proper location and the length of the feeding line are helpful for adjusting the input impedance of the proposed slot antenna.

Finally, Figure 5 shows the comparison of the simulated return loss and input imped-ance for the proposed antenna, the case of the slot 1 opened (Ref 1) and the case of the slot 2 closed (Ref 2). Configurations of the two cases are shown in the insets of the figure and corresponding dimensions of the two referenced antennas are the same as those of the proposed antenna given in Figure 1. It is first seen that there are four resonant modes alike with the proposed design for Ref 1 although the impedance matching at about 950 and 1800 MHz is poor, where the GSM900/DCS1800 (880–960 MHz, 1710– 1880 MHz) cannot be covered. The variations about the input impedance verify the trend which are also shown in Figure 5(b). The narrow bandwidth is mainly due to the rela-tively high input impedance level of the excited resonant modes at 900 and 1800 MHz in the desired lower and upper bands.

Figure 3. Simulated return loss as a function of (a) the length g of the slot 2 and (b) the length t of the slot 3. Other dimensions are the same as in Figure 1.

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Then, for Ref 2, the lower resonant mode shifts up to 1100 MHz, which worsens the desired lower bandwidth, meaning that the band of 824–960 MHz cannot be obtained. And the impedance matching at about 1700 MHz is poor. In fact, the behavior can be resulted from that slot 2 closed which cannot provide an effective resonant path for the frequency of 1700 MHz. In addition, for the proposed antenna and Ref 2, the impedance matching for frequencies over the desired 824–960 MHz and 1710–2690 MHz bands are presented in Figure 5(c). Compared with Ref 2, the proposed antenna has a proper input impedance (about 50 X 50 X, espe-cially for resistance) over frequencies of the desired bandwidth.

Figure 5. Comparison of the simulated (a) return loss of the proposed antenna and the cases (Ref 1: slot 1 opened, Ref 2: slot 2 closed), the simulated input impedance of the (b) proposed antenna and Ref 1, and the (c) proposed antenna and Ref 2. Other dimensions are the same as in Figure 1.

Figure 4. Simulated return loss as a function of (a) the location d and (b) the length f of the feeding line. Other dimensions are the same as in Figure 1.

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To analyze the excited resonant modes of the proposed printed slot antenna, Figure 6 shows the simulated vector electric-field distributions at 900, 2200, and 2700 MHz in the proposed slots. At the lower fundamental resonant mode of 900 MHz in Figure 6(a), the strong vector electric-field distributions are seen on slots 1 and 2, which indicate that the lower resonant mode at about 900 MHz is mainly contributed by the two slots (slots 1 and 2). It is also obviously observed from Figure 6(b) that there are relatively strong vector elec-tric-field distributions on the slots 1–3, which suggest that the resonant mode at 2200 MHz is provided by the slots 1–3. While at 2700 MHz in Figure 6(c), relatively strong vector elec-tric-field distributions are seen mainly on the slots 2 and 3. Of course, since the proposed printed antenna is an entire structure formed by the slots and system ground plane, the whole antenna structure comprises an effective radiating system to cover the two wide bands of the 824–960 MHz and 1710–2690 MHz.

4. Measured results

The proposed antenna was fabricated and tested in Figure 7. Figure 8 plots the measured and simulated return losses of the proposed slot antenna with the dimensions given in Figure 1. The desired lower band with a measured 3 : 1 VSWR (6 dB return loss) bandwidth of 355 MHz (810–1165 MHz) is obtained, which covers the GSM850/900 operation. The upper band is also obtained, which has a wide bandwidth of 1105 MHz (1710–2815 MHz) and cov-ers the DCS1800/PCS1900/UMTS2100/LTE2300/2500 operation. There are some disagree-ments between the measurement and simulation of return loss, mainly because of errors of fabrication (properties of used FR4 substrate, size errors of fabrication) and testing (effects of SMA coaxial connector introduced for testing). Notice that the impedance matching of 3: 1 VSWR is generally used as the design speciﬁcation for the WWAN operation in the practical internal mobile antenna terminals [4–6,8,14–20].

Figure 6. Simulated vector electric-ﬁeld distributions at (a) 900 MHz, (b) 2200 MHz, and (c) 2700 MHz for the proposed slot antenna.

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The fabricated slot antenna is tested in a SATIMO anechoic chamber, and the measured 2-D radiation patterns are plotted in Figure 9. Results at three typical frequencies of 880, 1960, and 2520 MHz are shown. At lower frequency of 880 MHz, dipole-like radiation patterns with omnidirectional radiation in the azimuthal plane (xy-plane) are observed. At higher frequencies of 1960 and 2520 MHz, some dips near the azimuthal plane of the radia-tion patterns are seen, especially for 2520 MHz, which is related to the surface current nulls on the main ground plane of the mobile phone at higher frequencies.

The measured peak gain and radiation efficiency results of the proposed slot antenna, which are promising for practical mobile phones, are shown in Figure 10. For the lower band of GSM850/900 (824–960 MHz), the antenna gain varies from 0:6 to 1.8 dBi, and the radia-tion efficiency is larger than 48%. For the upper band including DCS1800/PCS1900, UMTS2100, and LTE2300/2500, the antenna gain varies from 1.2 to 4.6 dBi, and the radiation efficiency is larger than 50%.

Figure 8. Measured and simulated return loss for the proposed slot antenna. Figure 7. Photos of the fabricated slot antenna for the GSM/UMTS/LTE operation.

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Figure 9. Measured 2-D radiation patterns at (a) 880 MHz, (b) 1960 MHz, and (c) 2520 MHz for the proposed slot antenna (dotted line is Eu, solid line is EhÞ.

Figure 10. Measured antenna gain and radiation efﬁciency for the proposed slot antenna. (a) The lower band of GSM850/900 and (b) the upper band of DCS1800/PCS1900/UMTS2100/LTE2300/2500.

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5. Conclusion

A novel multiple band slot antenna is proposed in the article. The antenna, comprising a closed slot and three open slots, is capable of generating multiple separate resonant modes with good impedance matching conditions. Prototype of the presented design has been fabri-cated and experimentally studied, where obtained results show good agreement with both the measured and simulated results and good multiband operation, with 3 : 1 VSWR impedance matching bandwidths of 30 and 55% at the central frequencies of 850 and 2350 MHz, respec-tively, suitable for mobile phones to the GSM850/900/DCS1800/PCS1900/UMTS2100/ LTE2300/2500 operation.

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