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.

1. 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

2. 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

3. (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

4. 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.
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© 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