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.
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