The document proposes a dual-dipole-antenna system for concurrent dual-radio operation at 2.4 GHz and 5 GHz bands. It first studies the mutual coupling between two dipole antennas and finds good isolation when the antennas are orthogonal in polarization. Based on this, the design places a 2.4 GHz dipole perpendicular to a 5 GHz dipole on a two-layer substrate. Simulations show the antennas, stacked just 0.8 mm apart, achieve over 15 dB isolation across bands. Measurements of a prototype confirm bandwidth and radiation pattern specifications for both WiFi standards are met while maintaining low coupling between closely packed antennas.
2. (a)
(a)
(b)
(b)
Figure 2 Simulated isolation against distance D between the two dipole
antennas in Figure 1: (a) for the parallel dipole antennas; (b) for the
perpendicular dipole antennas. [Color figure can be viewed in the online (c)
issue, which is available at www.interscience.wiley.com]
(5150 –5350)/5.8 GHz (5725–5825 MHz)] band operation [7]. The
2.4 and 5 GHz dipoles are etched on a square, two-layered dielec-
tric substrate. The 2.4 GHz dipole is set to be perpendicular to the
5 GHz dipole. Both dipoles are fed by mini-coaxial cables, which
provide the design much flexibility in the placement inside a
WLAN device. Detailed description of the proposed dual-dipole-
antenna system and design consideration thereof are given, and the
simulation and experimental results of a realized prototype are
presented and discussed.
2. MUTUAL COUPLING OF DIPOLE ANTENNAS
Figure 1 shows two simple dipole antennas placed D mm apart and
configured to be of two parallel dipoles in Figure 1(a) and one
dipole antenna perpendicular to the other in Figure 1(b). The two
dipoles are identical in dimensions and designed to operate in the (d)
2.4 GHz band. Each dipole consists of two radiating strips (26.5
mm in length and 1.5 mm in width), which is separated by a feed Figure 3 (a) Proposed dual-dipole-antenna system with a 2.4 GHz
gap (2 mm in length). With near optimal antenna size selected, the dipole on the front layer and a 5 GHz dipole on the bottom layer. (b)
10-dB return-loss bandwidth of the antenna reaches about 250 Detailed dimensions of the 2.4 GHz dipole antenna. (c) Detailed dimen-
MHz (2340 –2600 MHz). The design and optimization of the strip sions of the 5 GHz dipole antenna. (d) Photo of a design prototype of a
dipole antennas are realized with the aid of Ansoft HFSS [8], dual-dipole-antenna system. [Color figure can be viewed in the online
issue, which is available at www.interscience.wiley.com]
which uses the finite element method, in this study.
1726 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 7, July 2009 DOI 10.1002/mop
3. arrangement of the two dipoles in Figure 1(b) results in radiation
or waves of orthogonal polarization, which do not interact a lot,
and thus antennas transmitting orthogonal waves are well isolated
despite the fact that antennas are set closely. Following this char-
acteristic, we expect that good decoupling can still exist between
two dipoles set in the shaped of a cross when the antennas are
operating at different frequencies, which motivates us to study and
propose a very compact dual-dipole antenna system in this article.
3. PROPOSED DUAL-DIPOLE-ANTENNA DESIGN
Figure 3(a) shows the proposed dual-dipole-antenna system, which
includes a 2.4 GHz dipole etched on the front layer and a 5 GHz
dipole etched on the bottom layer of a 0.8-mm-thick FR4 substrate
with dimensions 30 mm 30 mm. More details of the two dipoles
are given in Figures 3(b) and 3(c). To achieve polarization diver-
sity, the 2.4 GHz dipole is first arranged to be perpendicular to the
Figure 4 Reflection coefficients (S11 for the 2.4 GHz dipole, S22 for the 5 GHz dipole, similar to the shape of a cross. This orthogonal
5 GHz dipole) and isolation (S21) between the two antennas. [Color figure configuration of two dipoles allows the two antenna ports to be
can be viewed in the online issue, which is available at www. easily decoupled, as has been explored in the previous section. The
interscience.wiley.com] dipole arms of the 2.4 GHz dipole are further bent to have a
compact structure. Each dipole arm is also of a constant width and
The mutual coupling or the port isolation between the two as general rule of thumb, the wider the width is chosen, the broader
dipoles against the separation distance D is presented in Figure 2. the bandwidth becomes. That is the reason why the dipole-arm
In the case of two parallel dipoles, port isolation 15 dB occurs width for the 5 GHz dipole is wider when compared with that for
when the separation distance D is larger than 80 mm, as seen in the 2.4 GHz dipole. In addition to the width of the dipole arms, the
Figure 2(b). This distance corresponds to about 0.65-wavelength feed gap of each dipole also plays an important part in determining
( c) at the center operating frequency, 2442 MHz in the 2.4 GHz input matching of the antenna. The near optimal value of the feed
band, of the antenna. In another word, to obtain isolation well gap in between the dipole arms is found to be 2 mm for the 2.4
below 15 dB between the two parallel dipoles that have good GHz dipole and 1 mm for the 5 GHz dipole. Finally, to feed the
input matching of 10 dB return loss, a minimal space of 0.65 c is design prototype [see Fig. 3(d)], two short 50- mini-coaxial
required between the two dipole antennas. Notice that with reflec- cables with I-PEX connectors are used. The inner conductors of
tion coefficients 10 dB and isolation 15 dB in this case, the coaxial cables are connected to the feed point A and C, and the
the envelop correlation can be smaller than 0.1 [9]. As for the case outer braided shielding are connected to the ground point B and D.
of one dipole perpendicular to the other, the results in Figure 2(b)
shows that even the two dipoles are set at very close proximity, the 4. RESULTS AND DISCUSSION
port isolation is still very small and almost separation distance Measurements of the reflection coefficients (S11 for the 2.4 GHz
independent too. That is, the isolation remains below 60 dB as dipole, S22 for the 5 GHz dipole) and isolation (S21) of a con-
separation distance D varies. This behavior is expected because the structed prototype were taken, and the results are given in Figure
Figure 5 Radiation patterns at 2442 MHz for the 2.4 GHz dipole antenna studied in Figure 4. [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. 7, July 2009 1727
4. Figure 6 Radiation patterns at 5490 MHz for the 5 GHz dipole antenna studied in Figure 4. [Color figure can be viewed in the online issue, which is
available at www.interscience.wiley.com]
4. It is first seen that both measured impedance bandwidth of the 5. CONCLUSION
2.4 and 5 GHz dipoles easily satisfy the required bandwidth The mutual coupling of the two simple strip dipole antennas has
specification for 2.4 and 5 GHz WLAN operation with reflection been studied first; the results show that good decoupling is ob-
coefficient well below 10 dB (even below 14 dB). The isola- tained when one of the two dipoles is put perpendicular to another,
tion between the two dipoles is found to be below 20 dB over the and orthogonal polarization is realized accordingly. Using this
2.4 GHz band and below 15 dB over the 5 GHz band. Notice that concept, a novel dual-dipole-antenna system has been presented,
the decoupling in the 2.4 GHz band is better than in the 5 GHz constructed, and tested. It has been demonstrated that the two
band, as can be observed in the sloping-up curve of S21 from the individual 2.4 and 5 GHz dipole antennas can be integrated into a
simulation results (not shown here for brevity). This is probably very compact structure such that the two antennas are stacked up
because the upper resonant mode of the 2.4 GHz dipole causes with a distance of mere 0.8 mm. In addition, though the antennas
somewhat overlapping radiation patterns of similar polarization to are set closely, low mutual coupling with port isolation of less than
those of the 5 GHz dipole. 15 dB in the bands of interest can still be achieved. Further,
Figures 5 and 6 plot the far-field, 2-D radiation patterns at 2442 omnidirectional and bi-directional radiation patterns in the hori-
and 5490 MHz, the center operating frequencies of the 2.4 and 5 zontal plane have been observed for the 2.4 and 5 GHz dipoles,
GHz bands. For the 2.4 GHz dipole, the antenna yields dipole-like respectively. The proposed design is well suited for concurrent 2.4
radiation with omnidirectional radiation patterns in the x-y plane and 5 GHz band operation and does not lose extra gain when
and nulls around the z directions. Because the two dipole arms compared with a single-feed, dual-band, dual-antenna system us-
are bent and extended toward the x directions, the radiation ing an external diplexer for concurrent operation.
patterns are inclined at an angle of 20 degrees in the x-z plane.
For the 5 GHz dipole, due to the effect of bent dipole arms of the
2.4 GHz dipole, the radiation energy in the z directions is much
less than in the y directions. In this case, bi-directional radiation
patterns are created with maximum radiation (peak antenna gain)
in the y directions. Interestingly notice that along the y axes,
the intersection of two H-planes of the 2.4 and 5 GHz dipoles, the
maximum radiation in the E field of the 2.4 GHz dipole is at the
right angle to the maximum radiation in the E field of the 5 GHz
dipole. This shows that the two dipoles have orthogonal waves
indeed.
Figure 7 presents the peak antenna gain and radiation efficiency
against frequency. The peak gain over the 2.4 GHz band is seen to
be at a constant level of about 2 dBi, and the radiation efficiency
exceeds about 90% ( 0.5 dB). As for the 5.2 GHz band, the peak
gain varies from 3.0 to 2.1 dBi with radiation efficiency larger than
74% ( 1.3 dB). Though the radiation efficiency seems to be low
(usually larger than 80% for WLAN metal-plate antennas of com- Figure 7 Peak antenna gain and radiation efficiency for the two dipole
parable size [10 –12]), the antenna gain is still good enough due to antennas studied in Figure 4. [Color figure can be viewed in the online
high directivity of the 5 GHz dipole. issue, which is available at www.interscience.wiley.com]
1728 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 7, July 2009 DOI 10.1002/mop