A novel hybrid of a 2.4-GHz monopole antenna and a 5-GHz dipole antenna is presented to provide concurrent 2.4 and 5 GHz band operation for access- point applications. The two antennas are arranged in a collinear structure and printed on a compact dielectric substrate with dimensions 12 mm × 60 mm. The monopole antenna has a meandered radiating strip and is short-circuited to a small ground plane through a shorting strip. The dipole antenna includes two sub-dipoles at the opposite side of a narrow ground plane and fed by a simple T-junction microstrip-line network. The two antennas are closely set with a distance of 1 mm only, yet good port isolation (S21) well below –20 dB can be obtained. With a low profile, the proposed design can easily fit into the casing of some standard access points and allow the 2.4 and 5 GHz band signals to be simultaneously received or transmitted with no external diplexer required. Good omnidirectional radiation has been observed too.

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© 2009 Wiley Periodicals, Inc.
HYBRID OF MONOPOLE AND DIPOLE
ANTENNAS FOR CONCURRENT 2.4-
AND 5-GHz WLAN ACCESS POINT
Jui-Hung Chou and Saou-Wen Su
Network Access Strategic Business Unit, Lite-On Technology (a)
Corporation, Taipei County 23585, Taiwan; Corresponding author:
susw@ms96.url.com.tw
Received 4 September 2008
ABSTRACT: A novel hybrid of a 2.4-GHz monopole antenna and a
5-GHz dipole antenna is presented to provide concurrent 2.4 and 5 GHz
band operation for access-point applications. The two antennas are ar-
ranged in a collinear structure and printed on a compact dielectric sub-
strate with dimensions 12 mm 60 mm. The monopole antenna has a
meandered radiating strip and is short-circuited to a small ground plane
through a shorting strip. The dipole antenna includes two sub-dipoles at
the opposite side of a narrow ground plane and fed by a simple T-junc-
tion microstrip-line network. The two antennas are closely set with a
distance of 1 mm only, yet good port isolation (S21) well below 20 dB
can be obtained. With a low profile, the proposed design can easily fit
into the casing of some standard access points and allow the 2.4 and
5 GHz band signals to be simultaneously received or transmitted
with no external diplexer required. Good omnidirectional radiation
has been observed too. © 2009 Wiley Periodicals, Inc. Microwave
Opt Technol Lett 51: 1206 –1209, 2009; Published online in Wiley
InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24281
Key words: antennas; monopole antennas; dipole antennas; hybrid an-
tennas; WLAN antennas; concurrent operation
1. INTRODUCTION
With a great success in developing WLAN technology over the
past few years, many Wi-Fi-enabled consumer-electronic devices
are ubiquitous in the market, along with pervasive wireless access-
point (AP) infrastructure. The laptops nowadays are almost (b)
equipped with 802.11a/b/g wireless functionality as a basic, re-
quired specification. For indoor AP applications, several printed Figure 1 (a) Configuration of the proposed hybrid of the 2.4-GHz
antennas have been reported to cover single- or dual-band WLAN monopole and 5-GHz dipole antennas for a concurrent WLAN access
operation in the 2.4 GHz (2400 –2484 MHz) band and/or 5 GHz point. (b) Detailed dimensions of the two printed WLAN antennas. [Color
[5.2 GHz (5150 –5350)/5.8 GHz (5725–5825 MHz)] band [1– 6]. figure can be viewed in the online issue, which is available at www.inter-
Among these designs, the dual-band antenna usually has a single science.wiley.com]
RF feed port only. This suggests that an extra external diplexer
between the conventional single-feed antenna and two separate 2.4
and 5 GHz modules is needed when concurrent 2.4 and 5 GHz
1206 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 5, May 2009 DOI 10.1002/mop

2. shorting strip has been meandered such that better impedance
matching can be realized. As for the dipole antenna in the lower
part, the antenna consists of two sub-dipoles at the opposite side of
a narrow ground of width 4 mm. This back-to-back dipole con-
figuration [2, 4] can result in good omni-directional radiation
characteristics. The ground plane is set on the same layer where the
printed monopole antenna is located. The dipole arms are printed
on both sides of the substrate. To excite both the sub-dipoles with
equal power and in phase, a simple T-junction 50- microstrip-
line network is utilized in this study.
To feed the design prototype, two short, 50- mini-coaxial
cables with I-PEX connectors are used. The inner conductors of
the coaxial cables are connected to the feed point A and C, and the
outer braided shielding are connected to the ground point B and D.
Because unwanted leakage currents on the surface of the coaxial
cable usually occur, the cable routing of the monopole antenna
thus needs more concern. For minimizing the cable effect, the
coaxial cable is arranged to first go through the monopole ground
and then the center of the dipole ground [see photo of a manufac-
ture sample in Fig. 2(a)]. In this case, both the antennas can be fed
at or below the end of the two-antenna system, making it possible
for practical applications in some swivel-type access point, as seen
in an example photo in Figure 2(b). Also notice that the two
Figure 2 (a) Photo of the proposed antennas printed on a double-layered
FR4 substrate and fed by two 50- mini-coaxial cables. (b) Photo of a
swivel-type access point. [Color figure can be viewed in the online issue,
which is available at www.interscience.wiley.com]
operation is demanded for the purpose of having more efficient
spectrum usage. However, even a good diplexer can still yield
1-dB insertion loss over the 2.4 and 5 GHz bands, which is really
unsatisfying and unwanted. Recently, the integration of two indi-
vidual antennas with two separate feeds has been introduced as a
good solution to concurrent operation [7, 8]. The 2.4 GHz antenna
and the 5 GHz antenna can be integrated into a compact structure
by using a common shorting portion [7] or sharing a common
antenna ground plane [7, 8] with port isolation below 15 dB.
In this article, we propose a novel design of a hybrid of printed
monopole and dipole antennas for concurrent, WLAN AP appli-
cations. Each of the two antennas has its own radiating element (a)
and ground plane, different from the antenna configuration shown
in [7, 8], in which the two antennas share the same ground plane.
The two antennas in this study are arranged in a collinear structure,
the monopole at the top of the dipole, to achieve an upright but
narrow profile to fit into the casing of an access point. Though
there is only 1 mm small gap between the two antennas, low
mutual coupling with good port isolation (S21 20 dB) can still
be obtained. Details of the design consideration of the proposed
antenna are described in this article, and the experimental results of
a realized prototype are presented and discussed.
2. ANTENNA DESIGN
Figure 1(a) shows the proposed hybrid of the 2.4 GHz monopole
and the 5 GHz dipole antennas for AP applications. The two-
antenna system is formed on a 0.8-mm thick FR4 substrate with
dimensions 12 mm 60 mm. The two antennas are arranged in a
collinear structure, the monopole at the top of the dipole, and there (b)
is only 1 mm isolation gap therein between. Further detailed
dimensions of each antenna are shown in Figure 1(b). The mono- Figure 3 Reflection coefficients (S11 for the 2.4 GHz antenna, S22 for the
pole antenna in the upper part has a small ground plane of size 12 5 GHz antenna) and isolation (S21) between the two antennas: (a) measured
mm 15.5 mm and an S-shaped radiating strip, which is further results; (b) simulated results. [Color figure can be viewed in the online
short-circuited to the ground through a thin shorting strip. The issue, which is available at www.interscience.wiley.com]
DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 5, May 2009 1207

3. Figure 4 Measured 2-D radiation patterns at 2442 MHz for the antenna studied in Figure 3(a). [Color figure can be viewed in the online issue, which is
available at www.interscience.wiley.com]
coaxial cables in the two-antenna system can affect the mutual tal data in general compare well with the simulation results, which
coupling between the antennas. The results of measured isolation are based on the finite element method. Some discrepancies are
(S21) may be inaccurate if no special consideration is given for also found due to manufacture tolerance and effect of coaxial
cable routing. cable. The measured impedance bandwidth, defined by 10 dB
return loss, can easily meet the bandwidth specification for 2.4 and
3. EXPERIMENTAL RESULTS AND DISCUSSION 5 GHz WLAN operation and the isolation between the antennas is
Figures 3(a) and 3(b) show the measured and simulated reflection well below 20 dB. The isolation is even better than 30 dB in
coefficients (S11 for the 2.4 GHz antenna, S22 for the 5 GHz the 5 GHz band. Notice that when there is no distance [that’s gap
antenna) and isolation (S21) of a design prototype. The experimen- equal to 0 in Fig. 1(a)] between the two antennas, the isolation
Figure 5 Measured 2-D radiation patterns at 5490 MHz for the antenna studied in Figure 3(a). [Color figure can be viewed in the online issue, which is
available at www.interscience.wiley.com]
1208 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 5, May 2009 DOI 10.1002/mop

4. 2. K.L. Wong, J.W. Lai, and F.R. Hsiao, Omnidirectional planar dipole-
array antenna for 2.4/5.2-GHz WLAN access points, Microwave Opt
Technol Lett 39 (2003), 33–36.
3. K.M. Luk and S.H. Wong, A printed high-gain monopole antenna for
indoor wireless LANs, Microwave Opt Technol Lett 41 (2004), 177–
180.
4. F.R. Hsiao and K.L. Wong, Omnidirectional planar folded dipole an-
tenna, IEEE Trans Antennas Propag 52 (2004), 1898 –1902.
5. R. Bancroft, Design parameters of an omnidirectional planar microstrip
antenna, Microwave Opt Technol Lett 47 (2005), 414 – 418.
6. S.W. Su and J.H. Chou, Printed omnidirectional access-point antenna
for 2.4/5-GHz WLAN operation, Microwave Opt Technol Lett 50
(2008), 2403–2407.
7. 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.
Figure 6 Measured peak antenna gain and radiation efficiency for the 8. S.W. Su, J.H. Chou, and Y.-T. Liu, Printed coplanar two-antenna
two antennas studied in Figure 3(a). [Color figure can be viewed in the element for 2.4/5 GHz WLAN operation in a MIMO system, Micro-
online issue, which is available at www.interscience.wiley.com] wave Opt Technol Lett 50 (2008), 1635–1638.
© 2009 Wiley Periodicals, Inc.
behavior in both resonant modes is seen from the simulation
results (not shown here for brevity) to rapidly deteriorate by about
10 dB.
Figures 4 and 5 give the far-field, 2-D radiation patterns in E RESONANCE TRANSMISSION
and E fields at 2442 and 5490 MHz, the center operating fre- THROUGH ELECTROMAGNETIC
quencies of the 2.4 and 5 GHz bands. Other frequencies in the
bands of interest were also measured, and no appreciable differ-
CRYSTALS CONSISTING OF METAL
ence in radiation patterns was obtained. It is easy to see that good
STRIPS
omni-directional radiation patterns in the horizontal plane (that’s Ruey Bing Hwang
the x-y plane here) are obtained from the test results. Notice that Department of Communication Engineering, National Chiao Tung
though the monopole antenna is utilized for 2.4 GHz operation, the University, Hsinchu, Taiwan, Republic of China; Corresponding
antenna system (radiating strip and ground plane thereof) can author: raybeam@mail.nctu.edu.tw
radiate a dipole-like radiation pattern. This is because both the
radiating strip and the ground plane are of quarter-wavelength Received 4 September 2008
resonant structure with no null surface currents occurring in both
portions at the same operating frequency. ABSTRACT: In this letter, we present frequency-selective transmission
Figure 6 plots the measured peak antenna gain and radiation of a 2D (two-dimensionally) electromagnetic (EM) crystal. Such a 2D
efficiency. The peak-gain level in the 2.4 GHz band is about 2.1 EM crystal consists of a finite stack of one-dimensionally metal-strip
gratings. The rigorous mathematical formulation using the mode-match-
dBi; the radiation efficiency exceeds about 83%. As for the 5.2
ing method incorporating the Floquet solution was employed to calcu-
GHz band, the peak gain is in the range of 2.6 –3.1 dBi with late the scattering and guiding characteristics of the structure under
radiation efficiency larger than 79%. Notice that the radiation consideration. Additionally, the experimental studies were also carried
efficiency was obtained in the 3-D test system by calculating the out to verify the numerical results. Significantly, the correlation be-
total radiated power of an antenna under test (AUT) over the 3-D tween frequency-selective transmission and the Fabry-Perot reso-
spherical radiation first and then dividing the total amount by the nance was clarified. © 2009 Wiley Periodicals, Inc. Microwave Opt
input power (default value is 0 dBm) given to the AUT. Technol Lett 51: 1209 –1212, 2009; Published online in Wiley Inter-
Science (www.interscience.wiley.com). DOI 10.1002/mop.24280
4. CONCLUSION
A two-antenna system formed by arranging a monopole antenna Key words: two-dimensionally periodic structures; electromagnetic
crystals; electromagnetic band-gap structures
and a dipole antenna both printed on a dielectric substrate for
concurrent, WLAN AP applications has been demonstrated, stud-
ied, and tested. The results show that though the distance between 1. INTRODUCTION
the two WLAN antennas is 1 mm, good isolation of less than 20 The frequency selective surface (FSS) has been extensively stud-
dB over the 2.4 and 5 GHz bands is still obtained. In addition, ied for many years. A typical FSS is a one-dimensionally (1D) or
dipole-like radiation patterns with good omni-directional radiation two-dimensionally (2D) periodic structure with the unit cell made
in the horizontal plane have been observed. Peak antenna gain is up of thin conducting elements printed on a dielectric substrate for
about 2.1 and 2.9 dBi for the 2.4 and 5 GHz antennas, respectively. supporting [1– 6]. In addition to the single-layer FSS, a cascade of
The proposed design is well suited for concurrent 2.4 and 5 GHz metal-strip gratings has been developed [5, 6]. In [5], the research-
band operation in an access point, which does not lose extra gain ers considered each individual metal-strip grating as a FSS and
when compared with the case of a single-feed, dual-band access synthesize the desired final behavior as a cascade of FSSs. On the
point using an external diplexer for concurrent operation. other hand, an alternative approach based on a “guided” viewpoint
instead of “diffraction” was proposed [6].
REFERENCES Although the guided wave concept was proposed, the detail
1. S.W. Su, Y.T. Chen, and K.L. Wong, Printed dual-band U-slotted mathematical formulation and guided wave phenomena; such as
monopole antenna for WLAN access point, Microwave Opt Technol the dispersion relation of the multilayer FSS were not tackled. In
Lett 38 (2003), 436 – 439. this letter, we study the transmission characteristic of a 2D elec-
DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 5, May 2009 1209