COMPACT COUPLED-FED WIDEBAND ANTENNA FOR INTERNAL EIGHT-BAND LTE/WWAN TABLET COMPUTER APPLICATIONS

Transcription

1 J. of Electromagn. Waves and Appl., Vol. 26, x y, 2012 COMPACT COUPLED-FED WIDEBAND ANTENNA FOR INTERNAL EIGHT-BAND LTE/WWAN TABLET COMPUTER APPLICATIONS Y.-L. Ban 1, *, S.-C. Sun 1, J. L.-W. Li 1, and W. Hu 2 1 Institute of Electromagnetics, University of Electronic Science and Technology of China, 2006 Xi-Yuan Avenue, Western High-Tech District, Chengdu, Sichuan , P. R. China 2 System Planning Division, Potevio Institute of Technology Co. Ltd., Beijing , P. R. China Abstract In this article, a compact coupled-fed antenna for eight-band LTE/WWAN tablet computer applications is proposed. The designed antenna consists of an L-shaped monopole, a long meandered strip, a long radiation strip, and a short shorted strip. The long meandered strip and long radiation strip can generate a dual-resonance characteristic for the antenna s lower band, making it capable of a wideband operation to cover the desired LTE700/GSM850/900 ( MHz) bands. Furthermore, the notch in the long radiation strip can also influence the bandwidth of the antenna s lower band. The long radiation strip can generate an additional higher-order resonant mode at the upper band, combining the two resonant modes contributed by the L-shaped monopole and the short shorted strip to form a wide upper band, covering GSM1800/1900/UMTS2100/LTE2300/2500 ( MHz) bands. With the proposed scheme, the antenna only occupies a small volume of mm 3 and can mainly be disposed on a 0.8-mm thick FR4 substrate. The antenna is mounted at the top edge of the display ground which supports a mm 2 display for the tablet computer. Reasonably good radiating efficiency and antenna gain are also achieved for the practical tablet computer. 1. INTRODUCTION Recently, with the rapid development of wireless communication technology, mobile terminals such as tablet computers or mobile phones Received 3 July 2012, Accepted 4 August 2012, Scheduled 10 September 2012 * Corresponding author: Yong-Ling Ban (byl@uestc.edu.cn).

2 2 Ban et al. have evolved into personalized smart devices. In addition, the 3G and 4G mobile communication systems will integrate different standards of wireless communication and achieve multiple standards on-demand access. It requires mobile terminals to have a wideband or multiband antenna with attractive features of compact size, simple structure, low profile, and ease of fabrication. The challenge is much huger for tablet computer antenna than mobile phone antenna [1 9], because the system ground plane of a tablet computer is very large and cannot assist in achieving a wide lower band for the embedded internal antenna. Many wireless USB dongle computer antennas [10 14] have been reported recently, but these antennas are more suitable for laptop computer than portable tablet computer. Fortunately, in recent years, extensive research activities have been dedicated toward the development of multiband antennas for tablet computer applications. In [15], the designed tablet computer antenna occupies a small size of 9 47 mm 2 on the mid top of the display ground, but the antenna can only cover partial upper operating bands of GPS (1.575 GHz), WLAN (2.4, 5.8 GHz), Bluetooth (2.45 GHz) and WiMAX (2.5, 3.5, 5.5 GHz) operation. In [16], although the antenna with a size of mm 3 can cover the LTE/WWAN operation, the antenna does not include the band from 698 to 704 MHz, and a chip inductor is loaded in the antenna which adds some complexity in practical production. Moreover, most of the existing designs [17 22] usually cover a few operating bands and cannot cover a broad bandwidth for future communication requirements, especially for LTE700/GSM850/900/1800/1900/UMTS2100/LTE2300/2500 bands. In this article, based on the reported designs [16, 23 26], we present a LTE/WWAN multi-network operation tablet computer antenna without any external matching circuit or setting additional resonators. The proposed design has a simple structure, comprising an L-shaped monopole, a long meandered strip, a long radiation strip, and a short shorted strip. The long meandered strip is connected to the edge of the L-shaped monopole and its end section gap-couples to the long radiation strip. As the long meandered strip is very narrow, it can lead to distributed inductance effect over the desired lower band which can adjust the impedance matching of the proposed antenna. The long radiation strip is capacitively excited by the L-shaped monopole and can excite a quarter-wavelength resonant mode at about 710 MHz and a higher-order resonant mode at about 2800 MHz. Hence, a dualresonance mode is achieved to provide a wide lower-band bandwidth for the antenna. Moreover, the notch in the long radiation strip can also improve the impedance matching of the antenna s lower band.

3 Antenna for LTE/WWAN tablet computer applications 3 Besides, the L-shaped monopole and short shorted strip can also resonate at the antenna s upper band, and then a wide upper band for the GSM1800/1900/UMTS2100/LTE2300/2500 operation is achieved, which means that this proposed wideband antenna can provide a whole eight-band LTE700/GSM850/900/GSM1800/1900/UMTS2100/LTE2300/2500 operation with a small compact structure size of mm 3, which is very suitable to be applied in the modern handy tablet computer. The details of the antenna design and experimental results are presented and discussed in the following sections. (a) (b) Figure 1. Proposed antenna configuration. (a) Geometry of the compact coupled-fed antenna for eight-band LTE/WWAN operation in the tablet computer. (b) Detailed dimensions of the metal pattern in the antenna area (unit: mm).

4 4 Ban et al. 2. PROPOSED ANTENNA DESIGN AND PARAMETRIC STUDY Figure 1(a) shows the geometry of the proposed eight-band LTE/WWAN antenna connected to the top shielding metal wall of the display ground for tablet computer application, and detailed dimensions of the unfolded printed metal pattern of the presented antenna are given in Figure 1(b). The antenna is mainly printed on a 0.8 mm thick FR4 substrate (relative permittivity 4.4 and loss tangent 0.02) of size mm 2, and is mounted along the edge of the top shielding metal wall (5 mm in width) of the display ground ( mm 2 ). The display ground is for accommodating a 9.7- inch display for the tablet computer, which is commercially available on the market. The top shielding metal wall can reduce coupling between the internal antenna and the circuitry on the back side of the tablet display ground. However, the shielding metal wall usually has a negative influence on the impedance matching of the internal antenna, because the system ground plane of the tablet computer is very large and cannot assist in achieving a wide lower band for the embedded internal antenna. In this antenna design, the proposed antenna with a small volume of mm 3 can still generate two wide operating bands to respectively cover the desired eight-band LTE/WWAN operation. Note that the antenna is placed close to one corner of the shielding wall with a distance of 10 mm, which can allow more possible internal antennas to be mounted along the shielding wall in practical applications where there are usually a variety of internal antennas required to be embedded in the tablet computer. The antenna comprises an L-shaped monopole, a long meandered strip, a long radiation strip, and a short shorted strip. The antenna ground with a narrow width of 0.5 mm and a length of 44 mm is disposed at the bottom edge of the antenna. For a practical tablet computer application, the antenna is grounded to the shielding wall at points C and D, and is fed by a 50 Ω mini coaxial line. The central conductor and outer grounding sheath of the 50 Ω mini coaxial line are connected to points A and B, respectively, as shown in Figure 1. In the proposed design, the short shorted strip and the long radiation strip are both capacitively excited by the L- shaped monopole. The L-shaped monopole with a length about 45 mm (0.37 wavelength at 2350 MHz) can excite a resonant mode at about 2350 MHz. The short shorted strip having a length about 37 mm (0.24 wavelength at 1950 MHz) contributes a quarter-wavelength resonant mode at about 1950 MHz to enhance the bandwidth of the antenna s upper band. Besides, the long radiation strip having a

5 Antenna for LTE/WWAN tablet computer applications 5 length about 74 mm can generate a quarter-wavelength resonant mode in the desired lower band and a higher-order resonant mode in the desired upper band of the antenna. Combining the three resonant modes, a wide upper band larger than 1 GHz for the antenna to cover the GSM1800/1900/UMTS2100/LTE2300/2500( MHz) operation is obtained. The operating bandwidth of the antenna s lower band provided by the long radiation strip is far from covering the desired lower band. Then by using the long meandered strip, the antenna can realize a dual-resonance mode in the lower band, which makes the bandwidth of the lower band greatly enhanced to cover the LTE700/GSM850/900 ( MHz) operation. The long meandered strip is connected to the edge of the L-shaped monopole, and its end section is coupled to the long radiation strip through a coupling gap of 0.5 mm. As the long meandered strip is very narrow, it can provide distributed inductance, and the coupling between the long meandered strip and long radiation strip provides an equivalent capacitance. The equivalent inductance and capacitance lead to generating an additional resonance (zero reactance) at about 920 MHz, resulting in a dual-resonance mode excited in the antenna s lower band. Furthermore, the notch in the long (a) (b) Figure 2. (a) Photo of the manufactured antenna for eight-band LTE/WWAN operation in the tablet computer. (b) Overall photo of the fabricated antenna attached to a tablet computer.

6 6 Ban et al. radiation strip can change the surface current distribution on the long radiation strip and hence can also improve the impedance matching of the antenna s lower band. A photograph of the manufactured internal tablet computer antenna is displayed in Figure 2. To analyze the excited resonant modes of the antenna, Figure 3 shows the simulated return losses and input impedances of the proposed antenna, Ref1 (Ref1 denotes the L-shaped monopole antenna), Ref2 (Ref2 denotes the corresponding antenna without the long meandered strip). Figure 3(a) presents the return losses of the proposed antenna, Ref1, and Ref2. Results show that there is almost no resonance with return loss better than 6 db by using the L-shaped monopole antenna (Ref1). After adding the long radiation strip and short shorted strip to form Ref2, the two shorted strips are capacitively excited by the L-shaped monopole. Then the L-shaped monopole can provide a resonant path at about 2500 MHz, and the short shorted strip can add a resonant mode at about 1950 MHz. Also, the long radiation strip is excited to generate a fundamental resonant mode at (a) (b) (c) Figure 3. (a) Simulated return loss of the proposed antenna and the reference antennas, (b) simulated input impedance of the proposed antenna and Ref1, and (c) simulated input impedance of the proposed antenna and Ref2 (other dimensions are the same as given in Figure 1).

7 Antenna for LTE/WWAN tablet computer applications 7 about 760 MHz and a higher-order resonant mode at about 2800 MHz. To clearly illustrate the impedance match of the proposed antenna and reference antennas, the simulated input impedance is plotted in Figures 3(b) and (c). In Figure 3(b), the input resistance of Ref1 only has one peak, and over other desired operating bands the input resistance is too small to generate radiation. So the desired band of and MHz cannot be obtained for the case using a monopole feed strip only. Also, from Figure 3(c), it can be seen that the impedance matching of Ref2 is poor at the frequency near 2500 MHz and especially at the lower band. In detail, the input resistance in the entire lower band of Ref2 is less than 50 Ω, and the input reactance has no zero point. From Figure 3(c), it can be seen that by using the long meandered strip, the input resistance of the proposed antenna is promoted, and two additional zero input reactance points occur at the lower band. Hence, a double-resonance mode at the lower band can be realized to cover the desired LTE700/GSM850/900 operation. As the long meandered strip can be coupled to the long radiation strip, it can also influence the impedance matching of the antenna s upper band. Several main parameters, in this design, are also studied. The simulated return loss results for the length t of the long meandered strip varied from 12 to 24 mm are plotted in Figure 4. The impedance matching of the antenna s lower band is affected strongly, and the upper band has a small influence, because the lower band resonances are mainly controlled by the lengths of long meandered strip and long radiation strip. With varying the length t of the long meandered strip, the coupling strength between the long radiation strip and long meandered strip is changed. Then the effective length of the long Figure 4. Simulated return loss as a function of the length t of the long meandered strip (other dimensions are the same as given in Figure 1). Figure 5. Simulated return loss as a function of the length a of the short shorted strip (other dimensions are the same as given in Figure 1).

8 8 Ban et al. radiation strip and the capacitance provided by the coupling between the long meandered strip and long radiation strip are also changed. These reasons lead to the variation of impedance matching of the antenna s lower band in common. Figure 5 depicts simulated return loss results while a parameter is varied from 16 to 10 mm. The results show that the resonant mode contributed by the short shorted strip shifts to higher frequencies with the length a decreasing from 16 to 10 mm and other dimensions fixed. As the variations in length a leads to the variations in the resonant length of the short shorted strip, the resonant mode (the third resonant mode at about 2000 MHz) contributed by the short shorted strip is greatly affected. Whereas the effects on the upper band resonant modes related to the L-shaped monopole and long radiation strip are comparatively smaller. Figure 6 shows the simulated return loss as a function of length b of the long radiation strip (other parameters are the same as those in Figure 1). Results for length b varied as b = 4, 7 and 10 mm are presented. Big effects are seen in the resonant modes (the resonant modes at 710 and 2800 MHz in the proposed antenna) related to the long radiation strip, and comparatively smaller effects in the resonant modes at 1950 and 2350 MHz which are related to the short shorted strip and L-shaped strip. This also confirms the results analyzed in Figure 3, hence, by properly selecting length b of the long radiation strip, wide lower and upper operating bands can be obtained for LTE/WWAN operation. Figure 7 depicts simulated return loss results while g parameter is Figure 6. Simulated return loss as a function of the length b of the long radiation strip (other dimensions are the same as given in Figure 1). Figure 7. Simulated return loss as a function of the width g between the long meandered strip and the long radiation strip (other dimensions are the same as given in Figure 1).

9 Antenna for LTE/WWAN tablet computer applications 9 varied from 0.5 to 1.5 mm. Big effects on the impedance matching of the resonant mode at about 920 MHz are clearly seen, because varying width g of the gap will lead to the change of the capacitance between the long meandered strip and long radiation strip. The capacitance and inductance generated by the long meandered strip will be not equivalent, so the impedance matching of the resonant mode contributed by the long meandered will become poor. The simulated surface current distributions of the proposed eightband antenna are also shown in Figure 8 at 710, 920, 1950, 2350 and 2800 MHz. At 710 and 920 MHz, strong surface current distributions on the long radiation strip and long meandered strip, respectively, are obviously observed from Figures 8(a) and (b), which confirm that the proposed antenna s lower band is mainly contributed by the two (a) (b) (c) (d) (e) (f) Figure 8. Simulated current distributions on the radiators and system ground of the tablet computer at (a) 710 MHz, (b) 920 MHz, (c) 1950 MHz, (d) 2350 MHz, and (e) 2800 MHz.

10 10 Ban et al. strips. From Figures 8(c) and (d), it can be seen that there are comparatively stronger current distributions on the short shorted strip and L-shaped monopole, which suggests that the resonant modes at 1950 and 2350 MHz are separately provided by the short shorted strip and L-shaped monopole. Moreover, in Figure 8(e), it is seen that strong current distributions flow along with the long radiation strip again, which provides a new resonant path for the resonant mode at around 2800 MHz. All of the simulated surface current distributions in the resonant modes comply with the parametrics studied above very well. 3. RESULTS AND DISCUSSION The proposed antenna was then fabricated and tested. Figure 9 shows the measured and simulated return losses of the prototype. It is noticed that the experimental results obtained on an Agilent N5247A vector network analyzer agree with the simulation results using Ansoft HFSS. Some discrepancies were found largely due to system circuit board manufacture tolerance and the effects of coaxial cables as well as the hand soldering performed. As can be seen from Figure 9, two wide operating bands are obtained to cover the eight-band LTE/WWAN operation, meaning that the lower band about 310 MHz ranging from 680 to 990 MHz and the upper band ranging about 1425 MHz from 1575 to 3000 MHz are realized with a return loss better than 6 db or 3 : 1 VSWR. Notice that the 3 : 1 VSWR bandwidth definition is widely used in the internal mobile device antenna for LTE/WWAN operation [16 19]. Radiation characteristics of the proposed antenna are also studied. Figure 10 plots the measured 2-D radiation patterns at 830, 1950, 2350 and 2660 MHz. In Figure 10(a) and Figure 10(b), smooth variations in the E θ over all of the ϕ angles are seen in the azimuthal Figure 9. antenna. Measured and simulated return loss of the proposed

11 Antenna for LTE/WWAN tablet computer applications 11 (a) (b) (c) (d) Figure 10. Measured 2-D radiation patterns at (a) 830 MHz, (b) 1950 MHz, (c) 2350 MHz and (d) 2660 MHz of the proposed antenna. plane (xy-plane) at 830 MHz and 1950 MHz, which can provide good coverage for LTE700/GSM850/900 and GSM1800/1900 operations, respectively. And there are generally no nulls seen in the radiation patterns of all the three frequency points, allowing no communication

12 12 Ban et al. (a) (b) Figure 11. Measured antenna gain and radiation efficiency of the proposed antenna. (a) The lower operating bands (LTE700/GSM850/900). (b) The upper operating bands (GSM1800/1900/UMTS2100/ LTE2300/2500). nulls expected in the practical applications. In addition, comparable E θ and E ϕ components are observed in the radiation patterns, which is advantageous since the position of the tablet computer is usually complex for practical applications. Figure 11 shows the measured antenna gain and radiation efficiency of the proposed antenna design. At the lower band as shown in Figure 11(a), the measured antenna gain varies from about 1.2 to 2.2 dbi, and the measured radiation efficiency ranges from about 58% to 69%. At the upper band as shown in Figure 11(b), the antenna gain ranges from about 1.9 to 4.5 dbi, and the radiation efficiency is varied from about 50% to 77%. As the internal antenna efficiency larger than 50% is sufficient for practical application, the measured results indicate that the proposed antenna is acceptable for the applications in tablet computer for LTE/WWAN operation. 4. CONCLUSION A WWAN/LTE tablet computer antenna with a small size of mm 3 has been reported. The compact coupled-fed antenna comprises an L-shaped monopole, a long meandered strip, a long radiation strip, and a short shorted strip. By using a long meandered strip, a dual-resonance characteristic is obtained for the antenna s lower band without increasing the antenna size. A detailed description and discussion of the operating principle of the proposed antenna in exciting these resonant modes have been presented and studied in depth. The antenna is fabricated. The radiation characteristics of the proposed antenna are also measured and discussed, and good

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