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Very-Low-Profile Monopole Antennas for Concurrent 2.4- and 5-GHz WLAN Access-Point Applications
 

Very-Low-Profile Monopole Antennas for Concurrent 2.4- and 5-GHz WLAN Access-Point Applications

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A very-low-profile, six-antenna MIMO system aimed at operating in the concurrent 2.4 and 5 GHz bands for WLAN access-point applications is proposed. The MIMO system consists mainly of an antenna ...

A very-low-profile, six-antenna MIMO system aimed at operating in the concurrent 2.4 and 5 GHz bands for WLAN access-point applications is proposed. The MIMO system consists mainly of an antenna ground plane and six short-circuited monopole antennas, among which the three antennas are designated for 2.4 and 5 GHz operation respectively. The antennas are set in a sequential, rotating arrangement on the ground plane, and the 2.4 and 5 GHz antennas are facing each other one by one. The results show that well port isolation can be obtained together with good radiation characteristics. With a low profile of 6 mm in height, the proposed design can easily fit into wireless access points or routers and allow the 2.4- and 5-GHz band signals to be simultaneously received and transmitted with no need of external diplexer.

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    Very-Low-Profile Monopole Antennas for Concurrent 2.4- and 5-GHz WLAN Access-Point Applications Very-Low-Profile Monopole Antennas for Concurrent 2.4- and 5-GHz WLAN Access-Point Applications Document Transcript

    • VERY-LOW-PROFILE MONOPOLE ANTENNAS FOR CONCURRENT 2.4- AND 5-GHz WLAN ACCESS-POINT APPLICATIONS Saou-Wen Su Network Access Strategic Business Unit, Lite-On Technology Corporation, Taipei County, Taiwan 23585, Republic of China; Corresponding author: susw@ms96.url.com.tw Received 22 February 2009 ABSTRACT: In this article, a very-low-profile, six-antenna multiple- input multiple-output (MIMO) system aimed at operating in the concur- rent 2.4 and 5 GHz bands for WLAN access-point applications is pro- posed. The MIMO system mainly consists of an antenna ground plane and six short-circuited monopole antennas, among which the three an- tennas are designated for 2.4 and 5 GHz operation, respectively. The antennas are set in a sequential, rotating arrangement on the ground plane, and the 2.4 and 5 GHz antennas are facing each other one by one. The results show that well port isolation can be obtained together with good radiation characteristics. With a low profile of 6 mm in height, the proposed design can easily fit into wireless access points or routers and allow the 2.4- and 5-GHz band signals to be simultaneously received and transmitted with no need of external diplexer. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 2614 –2617, 2009; Published online in Wiley InterScience (www.interscience.wiley. com). DOI 10.1002/mop.24700 Figure 5 Current distribution at (a) 0.5, (b) 1.0, (c) 1.5, and (d) 2 GHz Key words: monopole antennas; WLAN access-point antennas; concur- rent dual-band operation; MIMO antennas 1. INTRODUCTION reduction in size has been calculated, and the performance of Many “11n” or “pre-n” [1] wireless applications are readily ac- the simulated and measured result has been compared and cessible on the open market. Most of the 11n products have analyzed. In conclusion, the reductions in size of the fabricated adopted multiple-input multiple-output (MIMO) technology, in antenna have been reduced and depend on the degree of flare which multiple transmit and/or receive antennas [2–5] are used to angle. In this work, the size has been reduced by 7% for 30° of increase data throughput without additional spectrum and also flare angle and it can be reduced up to 25% for 60° of flare make use of multipath propagation to improve signal quality and angle. reliability. For access-point (AP) or router applications, external dipole and/or monopole antennas are commonly used; however, there is a great demand for internal AP antennas [4] simply from ACKNOWLEDGMENTS an esthetic point of view that external antennas are not very pleasing to the end user. In addition, to have more efficient The authors thank the Ministry of Higher Education, Research spectrum usage, concurrent dual-WLAN-band operation [5–7] is Management Centre (RMC) and Department of Radio Engineering becoming a current trend in wireless-AP specification requirement, (RaCED), Universiti Teknologi Malaysia, for supporting this re- which is very different from the conventional APs utilizing single- search work. fed, dual-band antennas [8] and switch circuits or diplexers. In this article, we present a very-low-profile AP antennas for concurrent WLAN operation in the 2.4 GHz (2400 –2484 MHz) and 5 GHz REFERENCES (5150 –5825 MHz) bands as internal MIMO antennas. The pro- posed design mainly consists of six monopole antennas, all bent 1. L.K. Kim, J.G. Yook, and H.K. Park, Fractal shape small size microstrip two times to obtain a low profile and short-circuited for ease of patch antenna, Microwave Opt Technol Lett 34 (2002). mounting, and these six monopoles are evenly distributed on a 2. C.A. Balanis, Antenna theory: Analysis and design, 3rd ed., Wiley, New hexagonal antenna ground plane. Details of a constructed proto- York, 2005. type of the proposed, six-antenna MIMO system are described, and 3. R. Garg, P. Bhartia, I. Bahl, and A. Ittipiboon, Microstrip antenna design handbook, Artech House, Norwood, MA, 2001, Chapter 9, the results thereof are discussed. The radiation characteristics, in 533 p. the 3D and 2D forms, are also elaborated in the article. 4. P.E. Mayes, Frequency independent antennas and broadband deriva- tives thereof, Proc IEEE, 80 (1992). 2. ANTENNA CONFIGURATION AND DESIGN CONSIDERATIONS 5. A.A. Gheethan and D.E. Anagnatou, The design and optimization of planar LPDAs, Proc Electromagn Res 4 (2008). Figure 1(a) shows the configuration of a six-antenna MIMO sys- tem for concurrent 2.4- and 5-GHz WLAN band operation. The © 2009 Wiley Periodicals, Inc. design includes a hexagonal ground plane of side length 60 mm (the center to vertex is also 60 mm long), three 2.4 GHz antennas 2614 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 11, November 2009 DOI 10.1002/mop
    • and at the same time, short-circuited to the ground for ease of mounting [together with foam rubber as shown in Fig. 1(c)]. The near optimal dimensions for the 2.4 and 5 GHz antennas are detailed in Figure 2. The antenna height in this study is 6 and 5 mm for the 2.4 and 5 GHz monopoles, respectively, making it possible for the design to be integrated into a wireless AP or router as internal MIMO antennas. Notice that the thickness of the proposed design is merely about 4.8% free-space wavelength at 2442 MHz, the center frequency of the 2.4-GHz WLAN band. Moreover, one can easily alter the center operating frequency (fc) of the 2.4 GHz antenna by fine-tuning parameter d. With an increase in d, the fc increases too. As for the 5 GHz antennas, the center frequency fc can be adjusted by fine-tuning parameter g with all the other dimensions untouched and in general, goes up from lower to higher frequencies as g decreases. These parameters are substan- tially useful, especially when the antennas are installed inside the housing of a wireless AP, because the operating frequencies of both 2.4- and 5-GHz antennas are affected by dielectric loading (device housing) [9] and usually shifted to lower frequency band. 3. RESULTS AND DISCUSSION On the basis of design dimensions given in Figure 2, the proposed, MIMO AP antennas was constructed, studied, and tested. Figures 3(a) and 3(b) show the measured reflection coefficients and isola- tion between antennas. The reflection coefficients are plotted by the curves of S11, S22, S33 for the 2.4 GHz antennas and of S44, S55, S66 for the 5 GHz antennas. The isolation between any two of the six antennas is only presented by the curves of S21, S31, S41, S51, S61, S54, S64 due to symmetrical structure of the proposed antenna system. It can be first seen that all measured impedance bandwidth of the 2.4 and 5 GHz antennas satisfy the required bandwidth Figure 1 (a) Configuration of the proposed, low-profile monopole an- tennas for concurrent, dual-WLAN-band operation for MIMO access-point applications. (b) Top view of the proposed six-antenna MIMO system. (c) Photograph of a design prototype. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com] (denoted as antenna 1, 2, 3), and three 5 GHz antennas (denoted as antenna 4, 5, 6). Each antenna is situated next to another which operates in different frequency band so as to mitigate mutual coupling therein between. The six antennas are set 15 mm away from the vertex and mounted on the ground with an equal incli- nation angle (formed by two adjacent vertices and the center) of 60°. In this case, the proposed antennas are in a sequential, rotating arrangement and of a symmetrical structure. Figure 1(b) gives a comprehensible drawing of the design described earlier. A photo- graph of the design prototype is shown in Figure 1(c), too, for better understanding. To feed each antenna, six 50- mini-coaxial cables with I-PEX connectors are utilized [see Fig. 3(c)]. The inner conductors of the coaxial cables are connected to the feed points, and the outer braided shielding are connected to the hexagonal ground plane. Figure 2 Dimensions of the 2.4- and 5-GHz monopole antennas in In order to realize the proposed AP antennas that have a very detail. [Color figure can be viewed in the online issue, which is available low profile, the designed monopole antennas are bent two times at www.interscience.wiley.com] DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 11, November 2009 2615
    • (a) Figure 4 Measured 2D radiation patterns at 2442 MHz for antenna 1 studied in Figure 3(a). [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com] conical radiation patterns in the y-z plane and omnidirectional radiation patterns in the x-y plane. For the 5 GHz antennas, similar conical-pattern (in the y-z plane) and omnidirectional (in the x-y plane) radiation has been also observed but with less backward radiation (below the x-y plane) obtained. Figure 6 gives the 3D radiation patterns at 2442 and 5490 MHz for antennas 1 and 6. Other frequencies in the bands of interest were also measured, and no appreciable difference in radiation patterns was found. It can be seen that both 2.4 and 5 GHz antennas have maximum field strength in the lateral directions instead of horizontal directions in elevation planes. The proposed design is favorable to ceiling- mount AP applications in this case. (b) Figure 7 presents the measured peak antenna gain and radiation efficiency against frequency. The peak gain over the 2.4 GHz band Figure 3 Measured S-parameters for the antennas of a constructed is seen at a constant level of about 2.1 dBi; the radiation efficiency prototype; d 1.7 mm, g 1 mm: (a) reflection coefficients (S11, S22, S33 exceeds about 75%. For the 5 GHz band, the peak gain varies from for the 2.4 GHz antennas, S44, S55, S66 for the 5 GHz antennas); (b) 3.7 to 4.6 dBi with radiation efficiency larger than 79%. Notice isolation (S21, S31, S41, S51, S61, S54, S64) between any two of the six antennas. [Color figure can be viewed in the online issue, which is avail- that the radiation efficiency was obtained by calculating the total able at www.interscience.wiley.com] radiated power of the antenna under test (AUT) over the 3D spherical radiation first and then dividing the total amount by the input power (default value is 0 dBm) given to the AUT. Finally, specification for 2.4 and 5 GHz WLAN operation with reflection the studies on substituting a circular ground of diameter 120 mm coefficient below 9.6 dB (or VSWR of 2). Second, the isolation for the hexagonal ground were also conducted. The simulation between any two antennas is found to be below 15 and 20 dB over the 2.4 and 5 GHz bands, respectively. In general, poor isolation occurs between the two antennas that operate in the same frequency band, as can be observed in Figure 3(b). The variation between S21 and S31 (or between S54 and S64) is very small largely due to the symmetrical multiantenna structure. Notice that the decoupling between antennas 4 and 1 is better than that between antennas 5 and 1 despite the fact that the two antennas (4, 1 and 5, 1) are spaced the same distance apart. This behavior is probably because the shorting portion of antenna 1 faces antenna 4. In this case, the shorting portion acts as a shield [4, 10, 11] against nearby fringing field from antenna 4 and also suppressing coupling effect on antenna 4. Figures 4 and 5 plot the far-field, 2D radiation patterns at 2442 and 5490 MHz, the center operating frequencies of the 2.4 and 5 GHz bands, in E and E fields. Because of the sequential, rotating arrangement of the six antennas, it is only needed to analyze the radiation of one 2.4 GHz antenna and one 5 GHz antenna. Thus, Figure 5 Measured 2D radiation patterns at 5490 MHz for antenna 6 antennas 1 and 6 are chosen to suit the convenience of defining the studied in Figure 3(a). [Color figure can be viewed in the online issue, antenna coordinates. For the 2.4 GHz antennas, the antenna yields which is available at www.interscience.wiley.com] 2616 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 11, November 2009 DOI 10.1002/mop
    • results (not shown for brevity) indicate that both reflection coef- ficients and isolation are about the same. However, the peak antenna gain is slightly increased by 1 and 2 dBi for 2.4 and 5 GHz operation, respectively. 4. CONCLUSION A very-low-profile six-antenna system able to provide concurrent WLAN operation in the 2.4 and 5 GHz bands has been studied, measured, and demonstrated. To attain the proposed design, six monopole antennas have been used and mounted on a hexagonal ground with an equal inclination angle between each antenna. The Figure 7 Measured peak antenna gain and radiation efficiency for an- tennas 1 and 6 studied in Figure 6. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com] antenna system has a height of 6 mm only and shows low mutual coupling with port isolation of less than 15 dB in the bands of interest. Conical radiation patterns and maximum field strength of fine antenna gain have been found in elevation planes. The pro- posed multiple antennas are well suited for internal MIMO anten- nas embedded in wireless APs or routers for concurrent 2.4- and 5-GHz WLAN operation and does not lose extra gain compared with the case of a single-feed, dual-band AP using external di- plexer for concurrent operation. REFERENCES 1. 11n, Wikipedia, the free encyclopedia, available at: http://en.wikipedia. org/wiki/11n. 2. C.C. Chiau, X. Chen, and C.G. Parini, A compact four-element diver- sity-antenna array for PDA terminals in a MIMO system, Microwave f = 2442 MHz Opt Technol Lett 44 (2005), 408 – 412. (a) 3. M. Manteghi and Y. Rahmat-Samii, A novel miniaturized triband PIFA for MIMO applications, Microwave Opt Technol Lett 49 (2007), 724 –731. 4. J.H. Chou and S.W. Su, Internal wideband monopole antenna for MIMO access-point applications in the WLAN/WiMAX bands, Mi- crowave Opt Technol Lett 50 (2008), 1146 –1148. 5. S.W. Su, J.H. Chou, and Y.T. Liu, Printed coplanar two-antenna element for 2.4/5 GHz WLAN operation in a MIMO system, Micro- wave Opt Technol Lett 50 (2008), 1635–1638. 6. J.H. Chou and S.W. Su, Hybrid of monopole and dipole antennas for concurrent 2.4- and 5-GHz WLAN access point, Microwave Opt Technol Lett 51 (2009), 1206 –1209. 7. S.W. Su, J.H. Chou, and Y.T. Liu, Realization of dual-dipole-antenna system for concurrent dual-radio operation using polarization diver- sity, Microwave Opt Technol Lett 51 (2009), 1725–1729. 8. F.S. Chang, K.C. Chao, C.H. Lu, and S.W. Su, Compact vertical patch antenna for dual-band WLAN operation, Electron Lett 44 (2008), 612– 613. 9. Antenna platform-level integration, Ansoft technical library, An- soft, LLC, 2008, available at: http://www.ansoft.com/ie/Track1/ Antenna%20Platform-Level%20 Integration.pdf. 10. K.L. Wong and J.H. Chou, Integrated 2.4- and 5-GHz WLAN antennas with two isolated feeds for dual-module applications, Microwave Opt Technol Lett 47 (2005), 263–265. f = 5490 MHz 11. S.W. Su, J.H. Chou, and T.Y. Wu, Internal broadband diversity dipole antenna, Microwave Opt Technol Lett 49 (2007), 810 – 812. (b) © 2009 Wiley Periodicals, Inc. Figure 6 Measured 3D radiation patterns of the constructed proto- type: (a) at 2442 MHz for antenna 1; (b) at 5490 MHz for antenna 6. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com] DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 11, November 2009 2617