Ultra-Compact Three-Port MIMO Antenna with High Isolation And
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IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 13, 2014 1545 Ultra-Compact Three-Port MIMO Antenna With High Isolation and Directional Radiation Patterns Han Wang, Longsheng Liu, Zhijun Zhang, Senior Member, IEEE,YueLi,Member,IEEE,and Zhenghe Feng, Fellow, IEEE Abstract—In this letter, a disk-shaped ultra-compact three-port similar radiation patterns. Undoubtedly, achieving such a design planar multiple-input–multiple-output (MIMO) antenna is pro- is very challenging because of the tradeoffs existing between posed and fabricated. The antenna is designed for the 2.4-GHz these features. Many attempts have been taken in this area, such WLAN band, the radius of which is only 16 mm ( ). Three microstrip-fed T-shaped open-ended slots are used as radiating as those reported in [3]–[9]. elements, whose edge-to-edge separation is only .They To fit the MIMO antenna into a limited space, the element size are rotationally symmetric and generate directional patterns that should be as small as possible. Many miniaturization methods, aim at 0 , 120 , and 240 , respectively. The peak gain of these such as folded or meander-line technique, have been adopted patterns is 2.3 dBi, and the pattern correlation coefficient is below in [3] and [4], which can reduce the element size effectively. 0.03, which provides good pattern diversity for an MIMO system. However,withtheplanarinverted-Fantenna(PIFA)ormonopole Meanwhile, the 3-dB beamwidth of these patterns is 105 ,which can realize nearly full azimuth coverage with its three beams. By as the radiating element, the metal ground in these designs also introducing a newly designed neutralization network, the isola- participates in the radiation. It cannot be separated from the tion between elements reaches 24 dB at the resonant frequency. radiating elements, which are still too large for size-sensitive Moreover, the proposed antenna is manufactured on the FR-4 applications. substrate, which is cost-effective and easy to be implemented in a Apartfromtheelementdesign,loweringtheintensifiedmutual size-limited MIMO application. coupling between highly integrated elements is another chal- Index Terms—Antenna radiation patterns, multiple-input–mul- lenge. Orthogonally placing the elements in an MIMO antenna is tiple-output (MIMO), mutual coupling, slot antennas. one solution [5]. This will be very effective if the elements have directional patterns. However, the elements are independent, which cannot push the size into the limit. In similar fashion, the I. INTRODUCTION mutual coupling can be controlled by combining the elements ULTIPLE-INPUT–MULTIPLE-OUTPUT (MIMO), as with orthogonal modes/polarizations [6]; nevertheless, the var- M a technique that can offer significant performance im- ious element types provide different radiation patterns and gains, provement without using additional bandwidth or transmission which cannot achieve consistent performance in all angle ranges. power, has received growing interest ever since it was intro- In addition, the modifying ground structure [7], electromagnetic duced in the 1990s [1]. By using multiple antennas in both the band-gap (EBG) material [8], and neutralization line techniques transmitter and receiver, the MIMO technique can detect mul- [9], [10] can also reduce the mutual coupling effectively. How- tiple independent channels in free space, which can achieve a ever, these structures are wavelength-related, which makes them higher bit rate compared to the classic single-antenna design. difficult to apply in an ultra-compact MIMO antenna design. Attracted by this unique feature, newly developed wireless In this letter, an ultra-compact three-port planar MIMO an- communication devices, such as the 802.11ac receiver, have tenna designed for the 2.4-GHz WLAN band is proposed and adopted this technique [2]. The miniaturization trends in these fabricated. The antenna has a round contour with a radius of devices impose more stringent requirements on the MIMO an- 16 mm ( ), which is the most compact planar multielement tenna design. A state-of-art design should not only possess good ( ) MIMO antenna design in the existing literature. By impedance matching and low mutual coupling, but should also introducing three rotationally symmetric T-shaped open-ended have small size, low price, and full-range coverage along with slotsastheradiating elements, the proposed antenna provides three directional patterns with an interval of 120 . The peak gain Manuscript received June 02, 2014; revised July 11, 2014; accepted July 25, reaches 2.3 dBi, and the 3-dB beamwidth covers 105 ,which 2014. Date of publication July 30, 2014; date of current version August 14, can provide good pattern diversity and nearly full azimuth cov- 2014. This work was supported by the National Basic Research Program of erage in free space. Moreover, a newly designed neutralization China under Contract 2013CB329002, and in part by the National High Tech- nology Research and Development Program of China (863 Program) under Con- network is applied to reduce the mutual coupling. This network tract 2011AA010202, the National Natural Science Foundation of China under reuses the feeding structure to reduce the size, and 24 dB Contract 61271135, the China Postdoctoral Science Foundation under Project coupling level is achieved with an edge-to-edge separation 2013M530046, and the National Science and Technology Major Project of the of between adjacent elements. Section II shows the Ministry of Science and Technology of China under Grant 2013ZX03003008- 002. geometry of the proposed antenna and describes the size reduc- The authors are with the State Key Lab of Microwave and Communications, tion and decoupling techniques in detail. Section III shows the Tsinghua National Laboratory for Information Science and Technology, simulation and measurement results, which verify the antenna Tsinghua University, Beijing 100084, China (e-mail: [email protected]) Color versions of one or more of the figures in this letter are available online performance and feasibility in practice. Section IV discusses the at http://ieeexplore.ieee.org. MIMO performance and provides a comparison to other planar Digital Object Identifier 10.1109/LAWP.2014.2344104 multielement MIMO antennas. 1536-1225 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. 1546 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 13, 2014 Fig. 1. (a) Geometry, (b) dimensions in millimeters, and (c), (d) prototype of the proposed antenna. II. ANTENNA DESIGN III. SIMULATION AND MEASUREMENT RESULT Fig. 1 shows the geometry of the antenna, which is fabricated To validate the performance of the proposed antenna, a pro- on a double-sided FR-4 ( , ) substrate. The totype is fabricated and tested in this study. Fig. 1(c) and (d) antenna has a rounded contour with a radius of 16 mm ( ). shows the photographs of the antenna, in which three semi-rigid In the bottom layer, three open-ended slots are etched on the coaxial cables are connected to perform the measurements. In metal ground, which are rotationally symmetric with an interval the following, the -parameters, radiation pattern, gain, and ef- of 120 . The open end of the radiation slot acts as a magnetic ficiency are measured and compared to the results acquired in wall, and the base mode (TM10) is excited with a resonant the full-wave simulation software HFSS. length of .Tofurther minimize the size, a T-shaped slot is applied in this design, with which the edge-to-edge separation A. -Parameters between elements is only 0.8 mm ( ). Compared to the The -parameters are measured with the Agilent VNA straight slot, the T-shaped slot can be fitted into ,and E5071B. The results are shown in Fig. 3. Good impedance its bandwidth can cover the whole 2.4-GHz WLAN band. matching and high isolation are achieved in both the simulation In the top layer, three microstrip feeding lines are placed and the measurement. Since the radiating elements are rota- above the T-shaped slots; these lines feed the slots through elec- tionally symmetric, only the return loss in port one ( )and tromagnetic coupling. Three feeding ports and three shorting the mutual coupling between ports 1 and 2 ( ) are provided. points are placed at the two ends of the feeding lines; these pro- In simulation, the return loss ( ) is lower than 10 dB vide 50- interfaces and achieve the impedance matching at the from 2400 to 2480 MHz, and the coupling level ( )isbelow desired frequency. 15 dB throughout this band. In measurement, the return loss To deal with the mutual coupling, a newly designed three-port and mutual coupling traces show similar trends to the simulation neutralization network is introduced. This network is composed ones. Comparatively, the matching range shifts around 25 MHz of three rotationally symmetric neutralization lines, which con- toward the higher frequency, which may result from uncertainty nect the feeding lines near the shorting points together. Through in the substrate permittivity or size error in manufacturing. The this neutralization network, a portion of the signal propagates isolation is better than 15 dB in the whole matching band and from one port to the adjacent ports, which has the same am- reaches 24 dB at the central resonant frequency, which verifies plitude but inversed phase to the original coupling signal. As the effectiveness of the proposed neutralization network. a result, the original coupling signal is canceled out, and the total coupling level is reduced. Fig. 2 shows the current distri- B. Radiation Pattern bution and the -parameters comparisons with and without this Due to the existence of the metal ground, the wave propa- network. As indicated in this figure, the current density around gation in the inward direction is blocked. As a result, three di- the adjacent ports is greatly reduced by introducing this net- rectional patterns are generated in the radial direction; these are work. The improves from 8to 24 dB at the central res- toward 0 ,120 , and 240 asshowninFig.4.