Compact Eight-Band Frequency Reconfigurable Antenna for LTE ...

IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 62, NO. 1, JANUARY 2014

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Compact Eight-Band Frequency Reconfigurable Antenna for LTE/WWAN Tablet Computer Applications Yong-Ling Ban, Si-Cheng Sun, Peng-Peng Li, Joshua Le-Wei Li, and Kai Kang

Abstract—A novel eight-band LTE/WWAN frequency reconfigurable antenna for tablet computer applications is proposed in this communication. , the proposed antenna comWith a small dimension of 40 12 4 prises a loop feeding strip and a shorting strip in which a single-pole fourthrow RF switch is embedded. The RF switch is used to change the resonant modes of lower band among four different working states, so that the antenna can provide a multiband operation of LTE700/GSM850 /900/1800/ 1900/UMTS2100/LTE2300/2500 with return loss better than 6 dB. Reasonably good radiating efficiency and antenna gain are also achieved for the practical tablet computer. Index Terms—Eight-band antenna, frequency reconfigurable antenna, LTE/WWAN antenna, small antenna, tablet computer antenna.

I. INTRODUCTION With the rapid development of wireless communication systems, there is a significant interest in providing more small size and multiband antennas in mobile devices. However, designing an internal compact antenna for multiple bands of LTE/WWAN services in small handsets is still a serious challenge, especially for the tablet computer. For common mobile communication applications, there are some wideband techniques to achieve multiband operating bands. For instance, an internal eight-band antenna with coupled-fed structure for tablet computer applications in [1] is demonstrated. Then a folded monopole/dipole/loop antenna in [2] with four resonances can also cover six operating bands. A novel tablet antenna in [3] applies a band-stop matching circuit structure to realize the multiband coverage. In addition, there are also some other wideband techniques, such as loading the parallel resonant structure in the antennas in [4], and using multi-branch structure in [5] and so on. However, these conventional wideband designs [1]–[8] usually cover all the bands of interest at once, and the antenna will occupy a larger size and the design is also difficult. Hence, some other techniques need to be investigated to design compact wideband antennas for the internal antenna applications. Given the very limited volume allocated for the antennas in a mobile terminal, the frequency reconfigurable technique [9]–[12] seems an attractive option. In this way a good antenna operation can be reached at several cellular frequency bands while having reasonably small antenna volume. The Manuscript received April 19, 2013; revised September 03, 2013; accepted October 07, 2013. Date of publication October 28, 2013; date of current version December 31, 2013. This work was supported by the National Higher-education Institution General Research and Development Project (No. ZYGX2013J013) and the National Natural Science Foundation of China (No. 61001002). Y.-L. Ban, P.-P. Li, and K. Kang are with Institute of Electromagnetics and School of Electronic Engineering, University of Electronic Science and Technology of China (UESTC), Chengdu 611731, China (e-mail: [email protected]. cn). J. L.-W. Li is with the Institute of Electromagnetics and School of Electronic Engineering, University of Electronic Science and Technology of China (UESTC), Chengdu 611731, China and also with the Advanced Engineering Platform and School of Engineering, Monash University, Selangor 46150, Malaysia and also with the Department of Electrical and Computer Systems Engineering, Monash University, Victoria 3800, Australia. Color versions of one or more of the figures in this communication are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TAP.2013.2287522

Fig. 1. (a) Geometry of the proposed basic antenna for tablet computer applications. (b) Detailed dimensions of the basic antenna and the structure of the RF switch. (units: mm).

reconfigurable technique for tablet applications can also reducing the dependence on chassis modes. In the tablet computer, the system ground plane is much larger than that of the mobile handset, and it generally cannot support the chassis mode as in the mobile handset to aid in enhancing the operating bandwidth of the antenna [3], [7], [8]. This is the biggest difference between the design of mobile phone antenna and tablet computer antenna. However, in the reconfigurable technique for tablet applications, the ground plane does not play the decisive role in designing the bandwidth of lower band such as GSM850/900 or LTE700. At present, some frequency reconfigurable terminal antennas are published, but most of them are for mobile phone applications [11], [12], and few for tablet applications. In this communication, a novel compact frequency reconfigurable antenna with a single-pole four-throw RF switch is proposed for the tablet computer. The RF switch has four states and controls four lumped inductances of different values. The obtained results show that the designed antenna can cover the entire LTE/WWAN bands when combining the four matching states with return loss better than 6 dB. The bandwidth is almost free from the impact of the ground plane size, so one designed antenna can be reused in some other tablet computers. The gain and radiation patterns are also measured, and the obtained results are presented in the following sections. II. PROPOSED ANTENNA DESIGN Fig. 1(a) shows a proposed basic antenna structure. In this study, the (9.7-inch) display is considtablet computer with a 150 200 ered. The designed antenna is printed on a 0.8 mm thick FR4 substrate . (relative permittivity 4.4 and loss tangent 0.02) of size 40 12 There is a shielding wall on the ground plane and the shielding wall

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TABLE I THE TRUTH THE RF SWITCH

is perpendicular to the ground plane. The antenna is perpendicularly fixed on the shielding wall with a distance of 5 mm from the left edge. at the top edge of the display The shielding wall of size 5 200 ground can be used to accommodate the embedded antennas and may also provide some isolation between the antennas and the display, but it usually results in some degrading effects on the impedance matching of the internal antenna. By including the shielding wall, the proposed antenna can still cover the desired eight-band LTE/WWAN operation. The detailed dimensions of the metal pattern of the basic antenna and the structure of the RF switch are given in Fig. 1(b). Seen from the figure, the antenna comprises a shorting strip and a loop feeding strip. The shorting strip consists of two radiating strips of different lengths: strip 1 directly short-circuited to the shielding metal wall through the ground behind the RF switch and strip 2 grounded through the integrated RF switch. The two strips are capacitively excited by the loop feeding strip which has a loop structure formed at its front section. Moreover, it can be seen that the RF switch is placed within the dotted line in Fig. 1(b), and the GND pins of the switch are connected with the ground on the back via holes. The four lumped inductors are controlled by the bias voltages on the four ports (V1, V2, V3, V4), and in one state of the RF switch, there is only one lumped inductance used and the other three lumped inductances don’t work. Taking RF3 as the example, when the two AA batteries supply a high bias voltage to RF3 and then other three ports (RF1, RF2 and RF4) are in the low level, then the RF3 turns ON and other three ports turns OFF. The truth table of the RF switch can be seen in Table I. In the proposed antenna, the loop feeding strip not only serves as a capacitive feed to couple the energy to the shorting strip, but also functions as an effective radiator to contribute a resonant mode at 2450 MHz for the antenna. This characteristic in functioning as a feed and a radiator is an advantage for the proposed antenna to form a wide bandwidth. Besides, strip 1 of the shorting strip is a T-shape structure and can generate a resonant mode at about 1750 MHz. Then, combining the two modes contributed by the loop feeding strip and strip 1 of the shorting strip, a wide upper band covering 1710–2690 MHz for the antenna is achieved. Specifically, a four-state RF switch is used to adjust the impedance matching of strip 2 to produce resonant modes at different frequencies in the lower band. In the proposed procedure, the goal of a reconfigurable matching is to match different portions of the input impedance curve in the antenna’s lower band, which can realize different frequency bands coverage. This might sound simple, but in practice the matching must be carefully designed to achieve the desired performance. Fortunately, the single-pole four-throw RF switch can control four inductances via ON and OFF states to realize the frequency reconfigurability and form a wide lower band of 698–960 MHz for the antenna. The simulated surface current distributions for the proposed antenna are shown in Fig. 2 at 900, 1750 and 2450 MHz. In order to show it is ON) is chosen. It more clearly, only one state (the state is obviously observed from Fig. 2(a), at 900 MHz strong surface current distributions on strip 2 of the shorting strip, which confirm that the proposed antenna’s lower resonant mode at about 900 MHz is mainly contributed by strip 2. Also the surface current distributions are same for the other three resonant modes of lower band corresponding to the other three states. In Fig. 2(b), it is seen that strong current distributions

Fig. 2. Simulated current distributions on the radiators and system ground of the tablet antenna at (a) 900 MHz, (b) 1750 MHz, and (c) 2450 MHz.

Fig. 3. Photo of the manufactured antenna for tablet computer applications.

flow along with strip 1 of the shorting strip, which proves strip1 can provide a resonant path for the resonant mode at around 1750 MHz. Moreover, from Fig. 2(c), it can be seen that there are relatively strong current distributions on the loop feeding strip, which suggests that the resonant modes at 2450 MHz is provided by the loop feeding strip. All of the simulated surface current distributions in the resonant modes comply with the structure studied before very well. The photograph of the manufactured eight-band frequency reconfigurable tablet computer antenna is displayed in Fig. 3. The control voltage for the switch is supplied by two AA batteries. In the experiment, a 50 mini coaxial line is used to feed the antenna as in the practical tablet computer applications. The center conductor of the coaxial line is connected to point A which is at the loop feeding strip and the outer grounding sheath is welded on point B which is at the shielding wall, as shown in Fig. 1. The other end of the coaxial line has a SMA connector, which is used to connect to the test instrument. When the antenna is under test, the coaxial line is placed parallel to the ground plane. III. RESULTS AND DISCUSSION The proposed antenna was fabricated and tested. Results of the measured and simulated return loss for the fabricated antenna are shown in Fig. 4(a) and (b), respectively. The simulated results are obtained using full-wave electromagnetic field simulation software HFSS version 12.0 and the experimental results obtained on an Agilent N5247A vector network analyzer. As seen in the two figures, the upper band from 1700 to 2705 MHz can cover the desired GSM1800/1900/UMTS2100/ LTE2300/2500 bands. The lower band of the reconfigurable antenna including five frequencies from 690 to 980 MHz can cover the whole required LTE700/GSM850/900 bands with impedance matching better than 6 dB return loss (3:1 VSWR). Notice that the 3:1 VSWR bandwidth definition is widely used in the internal mobile device antenna for LTE/WWAN operation [1]–[4], [7]–[13]. It can be seen when changing


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Fig. 4. Measured and simulated return loss for the proposed antenna against the operating frequency.

the states of the switch, only the lower band is affected. That is because the resonant modes of the lower band are generated by the strip 2 where the RF switch is embedded and the resonant modes of the upper band are mainly contributed by the strip 1 and the loop feeding strip. When changing the states of the switch, the impedance of the strip 1 and the loop feeding strip changes little and the upper band is almost no effect. Several main design parameters of the presented antenna are studied and discussed in the following sections. To analyze the operating principle of the proposed antenna, some comparisons are provided. Fig. 5(a) shows the simulated return loss for the case with the loop feeding strip only (Ref1), and the case with the loop feeding strip and strip 1 of the shorting strip only (Ref2). For the Ref1 case, there is only one resonant mode at about 2700 MHz (although the impedance matching is not good), which coincides with the condition of a resonance (zero reactance) occurred in the upper band in Fig. 5(b). Fig. 5(b) shows the input impedance of Ref 1 and Ref 2. Then when strip 1 is added to form Ref2, an additional resonant mode is generated, and then a wide upper band consisting of two resonant modes is achieved to cover the GSM1800/1900/UMTS2100/LTE2300/2500 band. By further embedding strip 2 of the shorting strip and the RF switch to form the proposed antenna, a wide lower band for the antenna can be generated covering the desired LTE700/GSM850/900. Detailed, the RF switch is placed between the shielding wall and the root of strip 2. The single-pole four-throw RF switch has four states and can control the ON and OFF states of four inductances. The inductance values of L1, L2, L3 and L4 are 2.7 nH, 4.7 nH, 8.2 nH and 12 nH. By selecting one of the inductances mentioned above, the proposed antenna can cover any frequency band between 698 MHz and 960 MHz, and the upper band of the antenna almost has no effect. To analyze the operating principle of the RF switch, the corresponding input impedance results of the lower band with different inductance on the Smith chart are shown in Fig. 5(c). Results indicate that the embedded inductance has strong effects on the impedance matching over the lower band. It can be seen from the Smith chart, the series inductor values are increasing from L1 to L4. As we know from matching principle on the Smith chart, when the series inductance becomes larger, the combination impedance of

Fig. 5. (a) Simulated return loss for the proposed antenna and reference antenna, (b) comparison of the simulated input impedance for the proposed antenna and reference antenna, (c) the corresponding input impedance results of the lower band with different inductance on the Smith chart.

the antenna and the series inductor will move up along the constant resistance circles, just as the change of the impedance curves shown in Fig. 5(c). Hence, different switch states can make different frequency bands of the lower band inside the 3:1 VSWR circle and then different bandwidth coverage can be achieved. So by properly selecting the values of the inductances, the whole operating band of the lower band can be achieved. Fig. 6 shows the simulated return loss as a function of the width and the length of the ground plane; where other dimensions are the same as given in Fig. 1. Considering the actual size of the tablet computer, the discussed sizes here are reasonable. Also, in order to express it more is chosen and the results of other clearly, only the state states are similar. It is clearly seen from the two figures that changing the width and length of the ground plane has little influence on the coverage of the antenna’s bandwidth. This behavior once again proved that the frequency reconfigurable technique is not serious reliant on the ground plane. In this study, the tablet computer with a 150 200

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Fig. 6. Simulated return loss as a function of (a) the width of the ground plane and (b) the length of the ground plane (other dimensions are the same as given in Fig. 1).

Fig. 7. Measured and simulated 2-D radiation patterns for the proposed antenna: at (a) 830 MHz, (b) 1750 MHz, and (c) 2450 , , MHz ( , and ).

display is considered, which is currently commercially available size for tablet computer. The measured and simulated radiation patterns at 830 MHz, 1750 MHz and 2450 MHz are plotted in Fig. 7. 830 MHz is the

Fig. 8. Measured and simulated antenna gain and measured radiation efficiency versus frequency: (a) The lower operating bands of LTE700/GSM850/900. (b) The upper operating bands of GSM1800/1900/ UMTS2100/LTE2300/2500.

center frequency of the lower band, which is representative. Smooth over all of the angles are variations in the vertical polarization seen in the azimuthal plane ( -plane) at the three frequency points. and components are observed in the In addition, comparable radiation patterns, which is advantageous since the position of the tablet computer is usually complex for practical applications. It can be seen that the experimental data exhibit an excellent agreement with the simulation results. Some discrepancies were found too, largely due to the manufacture tolerance and the effects of coaxial cables as well as the switch and the accuracy of the software. Fig. 8 shows the measured and simulated antenna gain and measured radiation efficiency of the designed antenna. As the simulated efficiency calculated by the HFSS V12.0 is not exact, so only the simulated gain is added in Fig. 8. For the lower band shown in Fig. 8(a), by combing the curves of four modes, the improvements of efficiency and gain are clearly observed. For the LTE700 band, the efficiency is greater than 50% and the gain is greater than 1.4 dBi; for the GSM850 band, the efficiency is greater than 58% and the gain is greater than 1.4 dBi; for the GSM900 band, the efficiency is greater than 57% and the gain is greater than 1.4 dBi. The gain and efficiency of the lower bands when connecting RF1, RF2, RF3 and RF4 have been marked with dotted lines respectively in Fig. 8(a). In Fig. 8(b), results for the upper band are presented. As the gain and efficiency of the upper band in the four states are almost the same, so only the results under one state are given. Larger variations of the antenna gain in the range of about 2.3–4.4 dBi are seen for the desired GSM1800/1900/UMTS2100/LTE2300/2500 operation, and the measured radiation efficiency varies from about 50% to 77%. The results are all acceptable for the tablet computer applications. IV. CONCLUSIONS In this communication, a compact frequency reconfigurable coupled-fed antenna for tablet computer applications is proposed. By combining four states of the single-pole four-throw RF switch, eight-band including LTE700/GSM850 /900 and GSM1800/1900/UMTS2100/LTE2300/2500 can be covered with


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an antenna volume of 40 12 4 , which is much smaller than previous internal antenna designs. A prototype of the proposed antenna is being designed and tested. Good results of efficiency and gain illustrate the potential use of the proposed antenna for tablet computer applications.

Theoretical Limitations for TM Surface Wave Attenuation by Lossy Coatings on Conducting Surfaces

REFERENCES

Abstract—In this work, we theoretically analyze the limitations for TM surface wave attenuation on lossy coated conducting surfaces containing electric and/or magnetic loss. We use both an analytical approach as well as numerical simulations, and find excellent agreement between them. We also find that the loss can be described by a simple approximate expression for a wide range of material properties. Furthermore, we analyze lossy slabs with simple equivalent circuit boundaries on top, such as may be provided by frequency selective surfaces or other patterned structures. We find that such composite lossy coating can exceed the attenuation of a simple lossy slab, but with limited bandwidth. We also find that only by increasing permeability, and not permittivity, can the peak absorption frequency be lowered for a given thickness without reducing the relative absorption bandwidth.

[1] Y. L. Ban, S. C. Sun, J. L. W. Li, and W. Hu, “Compact coupled-fed wideband antenna for internal eight-band LTE/WWAN tablet computer applications,” J. Electromagn. Waves Applicat., vol. 26, pp. 2222–2233, 2012. [2] M. Zheng, H. Y. Wang, and Y. Hao, “Internal hexa-band folded monopole/dipole/loop antenna with four resonances for mobile device,” IEEE Trans. Antennas Propag., vol. 60, no. 6, pp. 2880–2885, Jun. 2012. [3] K. L. Wong and P. J. Ma, “Small-size WWAN monopole slot antenna with dual-band band-stop matching circuit for tablet computer application,” Microw. Opt. Technol. Lett., vol. 54, pp. 875–879, 2012. [4] Z. Chen, Y. L. Ban, S. C. Sun, and L. W. Li Joshua, “Printed antenna for penta-band WWAN tablet computer application using embedded parallel resonant structure,” Progr. Electromagn. Res., vol. 136, pp. 725–737, 2013. [5] C. L. Hu, D. L. Huang, H. L. Kuo, C. F. Yang, C. L. Liao, and S. T. Lin, “Compact multibranch inverted-F antenna to be embedded in a laptop computer for LTE/WWAN/IMT-E applications,” IEEE Antenna Wireless Propag. Lett., vol. 9, pp. 838–841, 2010. [6] L. Pazin and Y. Leviatan, “Inverted-F laptop antenna with enhanced bandwidth for Wi-Fi/WiMAX applications,” IEEE Trans. Antennas Propag., vol. 59, no. 3, pp. 1065–1068, Mar. 2011. [7] K. L. Wong, Y. W. Chang, and S. C. Chen, “Bandwidth enhancement of small-size planar tablet computer antenna using a parallel-resonant spiral slit,” IEEE Trans. Antennas Propag., vol. 60, no. 4, pp. 1705–1711, Apr. 2012. [8] K. L. Wong, T. J. Wu, and P. W. Lin, “Small-size uniplanar WWAN tablet computer antenna using a parallel-resonant strip for bandwidth enhancement,” IEEE Trans. Antennas Propag., vol. 61, no. 1, pp. 492–496, Jan. 2013. [9] Y. F. Yu, J. Xiong, H. Li, and S. L. He, “An electrically small frequency reconfigurable antenna with a wide tuning range,” IEEE Antenna Wireless Propag. Lett., vol. 10, pp. 103–106, 2011. [10] H. Li, J. Xiong, Y. F. Yu, and S. He, “A simple compact reconfigurable slot antenna with a very wide tuning range,” IEEE Trans. Antennas Propag., vol. 58, no. 11, pp. 3725–3728, Nov. 2010. [11] Y. Li, Z. J. Zhang, J. F. Zheng, Z. H. Feng, and M. F. Iskander, “A compact hepta-band loop-inverted F reconfigurable antenna for mobile phone,” IEEE Trans. Antennas Propag., vol. 60, no. 1, pp. 389–392, Jan. 2012. [12] Y. Li et al., “Compact heptaband reconfigurable loop antenna for mobile handset,” IEEE Antenna Wireless Propag. Lett., vol. 10, pp. 1162–1165, 2011. [13] Y. L. Ban, J. H. Chen, L. J. Ying, L. W. Li Joshua, and Y. J. Wu, “Ultrawideband antenna for LTE/GSM/UMTS wireless USB dongle applications,” IEEE Antennas Wirel. Propag. Lett., vol. 11, pp. 403–406, 2012.

Sanghoon Kim and Daniel F. Sievenpiper

Index Terms—Attenuation measurement, conducting materials, dielectric materials, microwave propagation, surface waves.

I. INTRODUCTION It is well-known that a grounded dielectric slab can support bound surface waves [1], [2]. The bound waves are related to the ordinary surface currents that occur in any metal surface in the limit where the dielectric thickness approaches zero. The surface currents contribute significantly to interference between nearby antennas or electronics [3], [4], and suppressing the currents such as by using a lossy coating can be an important tool for interference reduction [5], [6]. An analytical solution to the attenuation by a dielectric slab has been provided by Attwood [7], where he calculated loss terms for both the conductor loss in the metal surface and dielectric loss due to the slab. However, there has been no attempt to analyze general trends or theoretical limits of the attenuation by lossy coatings, particularly for surface waves. The closest general analysis is for plane waves at normal incidence [8]. Furthermore, Attwood’s original analysis cannot be applied to many modern lossy coatings which may contain magnetic materials, as well as patterned composite materials such as frequency selective surfaces [9] or metamaterials [10]. In this work, we extend Attwood’s analysis to include both of these effects and we verify our analytical solution with numerical simulations. We refer to lossy slabs which may contain impedance sheets, FSS, or other such structures collectively as “coatings”. Our analytical solution allows us to sweep a wide range of material properties to derive general trends and establish theoretical limits. In particular, we find that the attenuation by a lossy slab can be approximated by a simple formula. Adding an impedance sheet on top of the slab that can be described by an equivalent circuit can provide greater attenuation than the slab alone, but over limited bandwidth. Thus, lossy coatings combined with frequency selective surfaces, metamaterials or other such patterned surfaces can be effective narrowband absorbers. Manuscript received October 11, 2012; revised May 21, 2013; accepted September 26, 2013. Date of publication November 01, 2013; date of current version December 31, 2013. This work was supported in part by ONR contract N00014-11-1-0460. S. Kim is with the Applied Electromagnetic Group, Electrical and Computer Engineering Department, University of California at San Diego, La Jolla, CA 92093 USA (e-mail: [email protected]). D. F. Sievenpiper is with the University of California at San Diego, La Jolla, CA 92093 USA. Color versions of one or more of the figures in this communication are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TAP.2013.2288091

0018-926X © 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.