A new design of the printed omnidirectional antenna for applications in 2.4/5-GHz dual-WLAN-band access points is proposed. The antenna consists of a conventional collinear antenna for 2.4 GHz operation and two U stubs for 5 GHz operation. The two U stubs are located near the points where the maximum currents at about 5.5 GHz occurring on the strips of the collinear antenna, and arranged back to back in the same phase for achieving better antenna gain. Detailed analyses of the U stub on the impedance matching over the 5 GH band is presented. A prototype with good omnidirectional radiation across the 2.4/5-GHz WLAN bands is demonstrated.
3. (a)
(b)
Figure 3 Measured radiation patterns for the antenna studied in Fig. 2 with U stubs 1 and 2: (a) at 2442 MHz; (b) at 5490 MHz. [Color figure can be viewed
in the online issue, which is available at www.interscience.wiley.com]
demonstrated in Figure 1(b). The antenna mainly consists of a ductivity 0.02 S/m). That is, all the data in the next section
conventional 2.4-GHz collinear antenna [2], and two U stubs were obtained under the conditions, in which the antenna was
arranged back-to-back. Notice that a hollow metal cylinder (diam- installed within the housing.
eter 4 mm) soldered to the ground portion is used for manufac- The 2.4-GHz collinear antenna (the proposed antenna without
ture purposes. The hollow structure also allows a 50- minicoaxial U stubs) can be considered as two half-wavelength elements
cable (the inner conductor connected to the feed point A; the outer bridged by such a half-wavelength phase reversal that both of the
braided shielding connected to the ground point B) to get through. elements have in-phase currents, which result in constructive ra-
Also, notice that the plastic housing [see Fig. 1(b) but not drawn in diation [2]. For upper frequencies in the 5 GHz band, the element
Fig. 1(a) for brevity] is taken into account in this study for both 1 (comprising the section 1, ground and metal cylinder) and
measurement and simulation (relative permittivity r 3.5, con- element 2 (the section 2) become one-wave -length resonant struc-
DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 9, September 2008 2405
4. Figure 4 Measured peak antenna gain and measured radiation efficiency
for the antenna studied in Figure 2 with U stubs 1 and 2. [Color figure can
be viewed in the online issue, which is available at www.interscience. Figure 6 Simulated return loss as a function of g for the antenna with a
wiley.com] U stub 1. [Color figure can be viewed in the online issue, which is available
at www.interscience.wiley.com]
tures and have out-of-phase currents with two current nulls in each
element. It has been found that by adding a U stub of proper size MHz. When adding first one U stub to the antenna section 1, much
to the antenna around the feed point A, where the maximum wider bandwidth with better impedance matching can be achieved.
current occurs, much wider achievable bandwidth for 5 GHz As for the proposed design [denoted as with stubs 1 and 2 in Fig.
operation can be obtained. Furthermore, the second U stub (the 2(b)], the lower-edge frequency and the return loss is about the
same size as the first stub) is added to the section 2 around the same as that for the antenna with a single U stub 1 only. That is,
portion where the maximum current is seen and arranged back-to- adding one more stub to the initial antenna with a single U stub 1
back for in-phase currents for better antenna gain. Notice that the is not considered in this case to have a major impact on the desired
maximum-current occurrence is very close the center of the section impedance bandwidth of the proposed antenna.
2; thus, the U stub 2 is connected to the antenna in about the Figures 3(a) and (b) plot the measured radiation patterns for the
middle of the section 2. Because the 5-GHz impedance bandwidth antenna with the U stubs 1 and 2 at 2442 and 5490 MHz. Dipole-
can be much improved by adding a single U stub 1 to the antenna, like radiation patterns are obtained, and good omnidirectional
the analyses here only focus on the effects of the U stub 1 for radiation in the horizontal (x–y) plane with gain variation less than
brevity. Once the near optimal size of the U stub 1 is decided, the 1.5 dBi is seen. In the elevation plane are also found side lobes in
U stub 2 will be as the same. The results will be elaborated more the radiation at 5490 MHz, which is due largely to the overall
fully in the next section. resonant path of the antenna (excluding the path for meandered
currents in the phase reversal) longer than two wavelengths at
3. RESULTS AND DISCUSSION about 5.5 GHz and also the null current distribution on the sections
Figure 2(a) shows the measured and simulated return loss. For the 1 and 2 and the metal cylinder. The radiation patterns for the
2.4/5 GHz bands, the impedance matching is all within VSWR of conventional 2.4-GHz collinear antenna [shown as w/o stubs 1 and
two. Notice that when there are no stubs at all [denoted as w/o 2 in Fig. 2(b)] at 2442 and 5490 MHz were measured too. No
stubs 1 and 2 in Fig. 2(b)], one resonant mode with relatively much difference in the radiation patterns (normalized in respect of
narrow bandwidth in the 5 GHz band is excited at about 5420 the largest gain in each plane) between the proposed and conven-
tional antennas was observed. This behavior is expected mainly
because the stubs have major effects on the impedance matching
and are also beneficial for the 5-GHz-band antenna gain. The
measured antenna gain and radiation efficiency are shown in
Figure 4. The peak gain in the 2.4 GHz band has a gain level of
about 4.1 dBi, and the radiation efficiency exceeds about 80%. As
for the 5 GHz band, the peak gain level reaches about 3.4 dBi, with
radiation efficiency in a range of 79 – 85%.
For better understanding, the function of the U stub in terms of
matching the antenna in the 5 GHz band, the simulation studies on
the analyses of the parameters d, g, and L of the U stub (see each
inset in Figs. 5–7) were made. Because adding the U stub 2 (in
addition to the U stub 1) to the antenna does not affect the
antenna’s upper frequencies to a great degree, and the analyses
here only focus on the parametric effect of the U stub 1 for brevity.
Figure 5 shows the return-loss results for the small distance d
varying from 3 to 5 mm. For 5 GHz operation, the lower-edge
frequency decreases as the value d increases, which is due to an
Figure 5 Simulated return loss as a function of d for the antenna with a increase in the overall resonant path (stating from the feeding point
U stub 1. [Color figure can be viewed in the online issue, which is available A) for upper frequencies. Notice that the lower frequencies, the
at www.interscience.wiley.com] 2.4-GHz band, are almost unaffected in this case. Effects of the
2406 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 9, September 2008 DOI 10.1002/mop
![demonstrated. © 2008 Wiley Periodicals, Inc. Microwave Opt
Technol Lett 50: 2403–2407, 2008; Published online in Wiley Inter-
Science (www.interscience.wiley.com). DOI 10.1002/mop.23629
Key words: antennas; printed antennas; omnidirectional antennas;
WLAN antennas
1. INTRODUCTION
Printed collinear antennas with omnidirectional radiation are at-
tractive to applications of access points, routers, and gateways in
WLAN [1-4]. Except for the multisection microstrip antenna re-
ported in [4], the printed collinear antenna, in general, comprises
(a)
(b)
Figure 2 (a) Measured and simulated return loss for the antenna with U
stubs 1 and 2. (b) Measured return loss with U stub 1, U stubs 1 and 2, and
without U stubs 1 and 2. [Color figure can be viewed in the online issue,
which is available at www.interscience.wiley.com]
a ground, quarter-wavelength and half-wavelength sections, and a
phase reversal. One can easily manipulate the peak gain of this
kind of antenna by cascading further more half-wavelength sec-
tions together with phase reversals. However, for these referenced
antennas, the antennas are merely operating in the single band and
cannot provide dual-band 2.4/5-GHz (2400-2484/5150-5825
MHz) WLAN operation. This motivates us to find a simple solu-
tion for obtaining a dual-band omnidirectional antenna, based on
the conventional collinear antenna reported in [1-3]. In this letter,
we demonstrate a new design of a printed omnidirectional antenna
capable of dual-band operation in the 2.4/5 GHz bands. To some
degree, the proposed antenna is derived from the 2.4 GHz collinear
antenna reported in [2] for 2.4 GHz operation, and furthermore,
two U stubs of proper size are carefully added to the antenna for
achieving the 5 GHz operation. Details of the design consideration
of the proposed antenna are described, and the results are presented
and discussed.
Figure 1 (a) Detailed dimensions of the printed omnidirectional antenna
for a 2.4/5-GHz WLAN access point. (b) Photo of a mass-production 2. ANTENNA DESIGN AND CONFIGURATION
sample for the antenna with a plastic housing. [Color figure can be viewed Figure 1(a) shows the geometry of the proposed access-point
in the online issue, which is available at www.interscience.wiley.com] antenna, and a photo of a working sample for mass production is
2404 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 9, September 2008 DOI 10.1002/mop](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)