Dual-Band Dual-Polarized Compact Log-Periodic Dipole Array for ...

IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 14, 2015

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Dual-Band Dual-Polarized Compact Log-Periodic Dipole Array for MIMO WLAN Applications Jia-Jun Liang, Jing-Song Hong, Member, IEEE, Jia-Bei Zhao, and Wei Wu

Abstract—A compact dual-band dual-polarized log-periodic mm for dipole array with a dimension of multiple-input–multiple-output (MIMO) WLAN applications is proposed. In this antenna, there are 12 antennas: six for horizontal polarization and six for vertical polarization. In order to achieve dual linear polarizations and beam switching, six horizontal antennas are placed in sequential, rotating arrangement on a horizontal substrate panel with an equal inclination angle of to form a symmetrical structure, while the other six antennas for vertical polarization are inserted through slots made on the horizontal substrate panel. Furthermore, six pairs of double T-type slits are introduced to mainly reduce the mutual coupling between the horizontal antennas. The proposed array is manufactured and exhibits the characteristics of high isolation, good front-to-back ratio, and average gains of 5 and 6 dBi over the 2.4- and 5-GHz band, respectively. The MIMO performance of the array is analyzed and evaluated by mutual coupling and the envelope correlation coefficient. Index Terms—Antenna array, dual-band, dual-polarization, logperiodic, multiple-input–multiple-output (MIMO).

I. INTRODUCTION

M

ULTIPLE-INPUT–MULTIPLE-OUTPUT (MIMO) wireless systems, characterized by multiple antennas at the transmitter and receiver, have demonstrated the potential for increasing capacity in rich multipath environments. MIMO antenna systems have attracted considerable interest as an effective way of improving the date and increasing the channel capacity. Like other wireless communication systems, MIMO WLAN systems suffer from multipath fading. It is well known that MIMO technology can be used to provide multiplexing gain and diversity gain to improve the capacity and link quality. The basic concept of the MIMO diversity is to use multiple antenna elements to transmit or receive signals with different fading characteristics. Switched beam arrays with directional antennas have the advantage of simplicity since several fixed beams could be chosen to reduce the interference by controlling the state of a number

Manuscript received August 07, 2014; revised October 23, 2014; accepted November 29, 2014. Date of publication December 08, 2014; date of current version March 02, 2015. This work was supported by the National Natural Science Foundation of China under Grants No. 61172115 and No. 60872029, the High-Tech Research and Development Program of China under Grant No. 2008AA01Z206, the Aeronautics Foundation of China under Grant No. 20100180003, Project 9140A07030513DZ02098, and the Fundamental Research Funds for the Central Universities under Grant No. ZYGX2009J037. The authors are with the School of Physical Electronics, University of Electronic Science and Technology of China, Chengdu 610054, China (e-mail: [email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LAWP.2014.2378772

of switches. Compared to the adaptive beamforming arrays, the simplicity of the switched beam arrays makes it suitable for low-cost applications in MIMO systems. This letter presents a dual-band dual-polarized compact logperiodic dipoles array for MIMO WLAN, which supports beam switching. In this array, there are 12 antennas: six for horizontal polarization and six for vertical polarization. Six horizontal antennas are placed in sequential, rotating arrangement on a horito zontal substrate panel with an equal inclination angle of form a symmetrical structure, while the other six antennas for vertical polarization are inserted through slots made on the horizontal substrate panel. Furthermore, six pairs of T-type slits are introduced to mainly reduce the mutual coupling between the horizontal antennas. In this design, the log-periodic dipole array comprises two dipole elements. Some of the research works on log-periodic antenna were reported in [1] and [2]. Compared to the existing MIMO WLAN antenna arrays [3]–[7], the proposed array has the advantage of a compact structure, dual linear polarizations, and high isolation. In this array, the high isolation between any two antennas is achieved in combination with two kinds of decoupling strategies, which are T-type slits on the ground and dual-polarization arrangement. A prototype of the array with a dimension of mm is manufactured to support three dates streams with beam switching over the 2.4-GHz band and the 5-GHz band. In Section II, the antenna geometry and design consideration of the array are described. Advantages of the array are demonstrated by the measured and simulated results, including -parameters, envelope correlation coefficients (ECCs), and radiation patterns. Finally, a conclusion is presented in Section III. II. ANTENNA DESIGN Log-periodic dipole array is a well-known ultrawide bandwidth prototype. A conventional printed log-periodic dipole array antenna consists of linear dipole arrays whose properties vary periodically with the logarithm of the frequency. The complete and practical design procedure of a log-periodic dipole array antenna is given in [1]. Three important steps are needed for the designing procedure. In the first step, the scale factor , spacing factor , and the number of the dipole elements should be determined, where , , and are chosen in this study. Second, the length of the longest dipole , which responds to the lowest resonance frequency , should be computed by

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(1)

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TABLE I OPTIMIZED PARAMETERS OF THE ANTENNA

where is the longest operating wavelength. Its value can be determined by (2) where is the speed of the light in vacuum and is effective dielectric constant. There is no closed-from or analytical formula to determine . Therefore, the formula of determining for the microstrip-line case is used to approximate for the dipole case. The formula is

Fig. 1. Geometry of the log-periodic dipole antenna. (a) Top view. (b) Back view.

(3) where is the relative permittivity of the substrate, is the substrate thickness, and is the width of the longest dipole. The relationship between the spacing and the length of the dipole elements is (4) where . Finally, once the dimensions of the longest dipole element are determined, the dimensions of the other dipoles can be determined in terms of the following relationship:

Fig. 2. Antenna array geometry.

(5) The final parameters of the proposed single antenna are given in Table I. The single antenna is a two-elements log-periodic dipole array seen in Fig. 1. The antenna consists of a microstrip line to feed the log-periodic dipole array, a ground plane, and two kinds of equal-sized arms named arm1, arm1 and arm2, arm2, respectively. Each kind of arm operates at a specific working frequency. Moreover, the log-periodic dipole antenna presents a good front-to-back ratio over the 2.4-GHz band and 5-GHz band. The array comprises six antennas for horizontal polarization and six antennas for vertical polarization as shown in Fig. 2. Six antennas (H1–H6) are placed in a sequential, rotating arrangement on the horizontal substrate panel with an equal inclination angle of 60 to form a symmetrical structure. The other six antennas (V1–V6) are inserted through slots made on the horizontal substrate panel. This configuration not only keeps the full broadside coverage on the azimuth plane for the WLAN bands, but also offers dual-linear polarizations that decrease the correlation between different streams in MIMO. Fig. 3 shows the physical parameters of the double T-type slits. Fig. 4 shows the equivalent feeding network of the proposed antenna array,

Fig. 3. Double T-type slits on ground plane.

and the symbols represent the six horizontal antennas ( ), represent the six vertical antennas ( ), respectively. The simplicity of the feeding network makes it easy to assemble the array. The -parameters of the proposed array are measured by a microwave vector network analyzer, employing a coaxial cable at the desired antenna port and connecting the others to 50loads, and the radiation patterns are measured by anechoic chamber. The photograph of the antenna array mounted on the measurement system is shown in Fig. 5. The measured results will be analyzed in the following. Fig. 6 indicates that the six horizontal antennas operate frequencies from about 2.33 to 2.57 GHz in the lower band, and from about 4.74 to 6.57 GHz in the upper band. Also, the bandwidths of the

LIANG et al.: DUAL-BAND DUAL-POLARIZED COMPACT LOG-PERIODIC DIPOLE ARRAY FOR MIMO WLAN APPLICATIONS

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Fig. 8. Coupling between the horizontal antennas.

Fig. 4. Equivalent feeding network of the proposed antenna array.

Fig. 9. Coupling between the vertical antennas. Fig. 5. Photographs of the single antenna and antenna array mounted on measurement system.

Fig. 10. Coupling between the horizontal and vertical antennas. Fig. 6. Reflection coefficients of the horizontal antennas.

Isolation between antennas is an important factor for the anti-interference MIMO WLAN. Due to the symmetry of the array, the isolation between any two of the 12 antennas is sufficiently indicated by

Fig. 7. Reflection coefficients of the vertical antennas.

six vertical antennas are from 2.36 to 2.65 GHz and from about 4.94 to 6.34 GHz, as shown in Fig. 7. The bandwidth of the array ensures that the system could cover the 2.4-GHz (2400–2484 MHz) and 5-GHz (5150–5850 MHz) bands for WLAN applications.

and . The measured -parameters in Figs. 8–10 indicate that all the isolations between any two of the 12 antennas are about 30 dB at the operating band center frequency 2.45 and 5.5 GHz, respectively. The surface current distribution on the ground plane of the antenna array with slits is shown in Fig. 11. It is noticed that the current that flows to other antennas is trapped around the slits when H1 is excited. Therefore, the isolation between two ports is enhanced. The same effect is obtained for vertical antennas seen from Fig. 11(c) and (d). The simulated and measured radiation patterns are given in Figs. 12 and 13. The radiation characteristics of the array are obtained by the anechoic chamber. In this section, only the results of one horizontal antenna (H1) and one vertical antenna (V1) are reported since the array has a

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symmetrical arrangement. The measured and simulated radiation patterns at the operating band center frequency 2.45 and 5.5 GHz are shown in Figs. 12 and 13. According to the measured results, the horizontal (H1) or vertical (V1) antenna presents a good front-to-back ratio, which is approximately 14 dB at 2.45 GHz and 16 dB at 5.5 GHz. The single horizontal (H1) or vertical (V1) antenna presents a fixed main beam in the same direction, and the E-plane beamwidth of H1 or V1 is about , while the H-plane is about 110 . The ECCs are introduced in [6]–[8]. The ECCs of the proposed antenna are under 1.8e-5 within the bands of interest, indicating a good MIMO performance. III. CONCLUSION

Fig. 11. Current distribution of the proposed array: (a) H1 excited at 2.45 GHz, (b) H1 excited at 5.5 GHz, (c) V1 excited at 2.45 GHz, and (d) V1 excited at 5.5 GHz.

A dual-band dual-polarized compact log-periodic dipole array for MIMO WLAN applications is proposed. In the array, there are 12 antennas: six for horizontal polarization and six for vertical polarization. Six horizontal antennas are placed in sequential, rotating arrangement on a horizontal substrate panel with an equal inclination angle of to form a symmetrical structure, while the other six antennas for vertical polarization are inserted through slots made on the horizontal substrate panel. Furthermore, six pairs of T-type slits are introduced to mainly reduce the mutual coupling between the horizontal antennas. The MIMO performance of the array is analyzed and evaluated by mutual coupling. The measured mutual coupling is at least 30 dB between any two antennas in the array. The proposed antenna array is a good candidate for MIMO WLAN applications. REFERENCES

Fig. 12. Measured and simulated radiation pattern at 2.45 and 5.5 GHz in the E-plane (H1 and V1).

Fig. 13. Measured and simulated radiation pattern at 2.45 and 5.5 GHz in the H-plane (H1 and V1).

[1] D. Isbell, “Log periodic dipole arrays,” IRE Trans. Antennas Propag., vol. AP-8, no. 3, pp. 260–267, May 1960. [2] H. Jardon-Aguilar, J. A. Tirado-Mendez, R. Flores, and R. Linares-Miranda, “Reduced log-periodic dipole antenna using a cylindrical-hat cover,” Microw., Antennas Propag., vol. 5, no. 14, pp. 1697–1702, 2011. [3] S. Su, “High-gain dual-loop-antenna system for dual-WLAN-band access points,” IEEE. Trans. Antennas Propag., vol. 59, no. 5, pp. 1652–1659, May 2011. [4] T. W. Kang and K. L. Wong, “Isolation improvement of 2.4/5. 2/5.8 GHz WLAN internal laptop computer antennas using dual-band strip resonator as a wavetrap,” Microw. Opt. Technol. Lett., vol. 52, no. 1, pp. 58–64, Jan. 2010. [5] X. Zhang, Z. Zhang, Y. Li, and Z. Feng, “A planar wideband dualpolarized array for active antenna system,” IEEE Antennas Wireless Propag. Lett., vol. 13, pp. 544–547, 2014. [6] W. Han et al., “A six-port MIMO antenna system with high isolation for 5 GHz WLAN access points,” IEEE Antennas Wireless Propag. Lett., vol. 13, pp. 880–883, 2014. [7] W. Zheng, L. Zhang, Q. Li, and Y. Leng, “Dual-band dual-polarized compact bowtie antenna array for anti-interference MIMO WALN,” IEEE. Trans. Antennas Propag., vol. 62, no. 1, pp. 237–246, Jan 2014. [8] H. Li, J. Xiong, and S. He, “A compact planar MIMO antenna system of four elements with similar radiation characteristics and isolation structure,” IEEE Antennas Wireless Propag. Lett., vol. 8, pp. 1107–1110, 2009.