Hybrid of Monopole and Dipole Antennas for Concurrent 2.4- and 5-GHz WLAN Access Point

Saou-Wen Su1, Jui-Hung Chou2 Network Access Strategic Business Unit

Lite-On Technology Corp., No. 9, Chien I Road, Chungho, Taipei County 23585, Taiwan 1stephen.su@liteon.com 2alex.chou@liteon.com

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 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.

I. 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 equipped with 802.11a/b/g wireless functionality as a basic, required specification. For indoor access-point applications, several printed antennas have been reported to cover single- or dual-band WLAN operation in the 2.4 GHz (2400-2484 MHz) band and/or 5 GHz [5.2 GHz (5150-5350)/5.8 GHz (5725-5825 MHz)] band [1-6]. Among these designs, the dual-band antenna usually has a single 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 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 individual 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 employing a common shorting portion [7] or sharing a common antenna ground plane [7, 8] with port isolation below –15 dB.

In this paper, we propose a novel design of a hybrid of printed monopole and dipole antennas for concurrent, WLAN access-point applications. Each of the two antennas has its

own radiating element 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.

II. ANTENNA DESIGN Fig. 1(a) shows the proposed hybrid of the 2.4 GHz

monopole and the 5 GHz dipole antennas for access-point 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, in which the monopole is located at the top of the dipole, and there is only 1 mm isolation gap therein between. Further detailed dimensions of each antenna are shown in Fig. 1(b). The monopole antenna in the upper part has a small ground plane of size 12 mm × 15.5 mm and an S-shaped radiating strip, which is further short-circuited to the ground through a thin shorting strip. The 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 configuration [2, 4] can result in good omnidirectional radiation characteristics. Notice that 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-Ω micro-strip-line network is utilized in this study.

To feed the design prototype, two short, 50-Ω mini-coaxial cables with I-PEX connectors are used. The inner conductors of the coaxial cables are connected to feed point A and C, and the outer braided shielding are connected to ground point B and D. Because the 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 this 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 manufacture sample in Fig. 2(a)]. In this

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(a)

(b)

Fig. 1 (a) Configuration of the proposed hybrid of the 2.4-GHz monopole and 5-GHz dipole antennas for a concurrent WLAN access point. (b) Detailed dimensions of the two printed WLAN antennas.

case, both the antennas can be fed at or below the end of the two-antenna system, making it possible for very practical applications in some swivel-type access point, as seen in an example photo in Fig. 2(b). Also notice that the two coaxial cables in the two-antenna system can affect the mutual coupling between the antennas. The results of measured isolation (S21) may be inaccurate if no special consideration is given for cable routing.

(a) (b)

Fig. 2 (a) Photo of the proposed antennas printed on a double-layered FR4 substrate and fed by two 50-Ω mini-coaxial cables. (b) Photo of a swivel-type access point.

III. EXPERIMENTAL RESULTS AND DISCUSSION Fig. 3(a) and 3(b) show the measured and simulated

reflection coefficients (S11 for the 2.4 GHz antenna, S22 for the 5 GHz antenna) and isolation (S21) of a design prototype. The experimental data in general compare well with the simulation results, which are based on the finite element method (FEM). Some discrepancies are also found due to manufacture tolerance and effect of mini-coaxial cables. The measured impedance bandwidth, defined by 10 dB return loss, can easily meet the bandwidth specification for 2.4 and 5 GHz WLAN operation and the isolation between the antennas is well below –20 dB. The isolation is even better than –30 dB in the 5 GHz band. Notice that when there is no distance [that’s gap equal to 0 in Fig.1(a)] between the two antennas, the isolation behavior in both resonant modes is seen from the simulation results (not shown here for brevity) to rapidly deteriorate by about 10 dB.

Fig. 4 and 5 give the far-field, 2-D radiation patterns in Eθ and Eϕ fields at 2442 and 5490 MHz, the center operating frequencies of the 2.4 and 5 GHz bands. Other frequencies in the bands of interest were also measured, and no appreciable

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difference in radiation patterns was obtained. It is easy to see that good omnidirectional radiation patterns in the horizontal plane (that’s the x-y plane here) are obtained from the test results. Notice that though the monopole antenna is utilized for 2.4 GHz operation, the antenna system (radiating strip and ground plane thereof) can radiate a dipole-like radiation pattern. This is because both the radiating strip and the ground plane are of quarter-wavelength resonant structure with no null surface currents occurring in both portions at the same operating frequency.

Fig. 6 plots the measured peak antenna gain and radiation efficiency. The peak-gain level in the 2.4 GHz band is about 2.1 dBi; the radiation efficiency exceeds about 83%. As for the 5.2 GHz band, the peak gain is in the range of 2.6-3.1 dBi with radiation efficiency larger than 79%. Notice that the radiation efficiency was obtained in the 3-D test system by calculating the total radiated power of an antenna under test (AUT) over the 3-D spherical radiation first and then dividing the total amount by the input power (default value is 0 dBm) given to the AUT.

(a)

(b)

Fig. 3 Reflection coefficients (S11 for the 2.4 GHz antenna, S22 for the 5 GHz antenna) and isolation (S21) between the two antennas: (a) measured results; (b) simulated results.

Fig. 4 Measured 2-D radiation patterns at 2442 MHz for the antenna studied in Fig 3(a).

Fig. 5 Measured 2-D radiation patterns at 5490 MHz for the antenna studied in Fig 3(a).

Fig. 6 Measured peak antenna gain and radiation efficiency for the two antennas studied in Fig. 3(a).

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IV. CONCLUSIONS A two-antenna system formed by arranging a monopole

antenna and a dipole antenna both printed on a dielectric substrate for concurrent, WLAN access-point applications has been demonstrated, studied, and tested. The results show that though the distance between the two WLAN antennas is 1 mm, good isolation of less than –20 dB over the 2.4 and 5 GHz bands is still obtained. In addition, dipole-like radiation patterns with good omnidirectional radiation in the horizontal plane have been observed. Peak antenna gain is about 2.1 and 2.9 dBi for the 2.4 and 5 GHz antennas respectively. The proposed design is well suited for concurrent 2.4 and 5 GHz band operation in an access point, which does not lose extra gain compared with the case of a single-feed, dual-band access point using an external diplexer for concurrent operation.

REFERENCES

[1] S. W. Su, Y. T. Chen, and K. L. Wong, “Printed dual-band U-slotted monopole antenna for WLAN access point,” Microwave Opt. Technol. Lett., vol. 38, pp. 436–439, Sep. 2003.

[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., vol. 39, pp. 33–36, Oct. 2003.

[3] K. M. Luk and S. H. Wong, “A printed high-gain monopole antenna for indoor wireless LANs,” Microwave Opt. Technol. Lett., vol. 41, pp. 177–180, May 2004.

[4] F. R. Hsiao and K. L. Wong, “Omnidirectional planar folded dipole antenna,” IEEE Trans. Antennas Propagat., vol. 52, pp. 1898–1902, Jul. 2004.

[5] R. Bancroft, “Design parameters of an omnidirectional planar microstrip antenna,” Microwave Opt. Technol. Lett., vol. 47, pp. 414–418, Dec. 2005.

[6] S. W. Su and J. H. Chou, “Printed omnidirectional access-point antenna for 2.4/5-GHz WLAN operation,” Microwave Opt. Technol. Lett., vol. 50, pp. 2403–2407, Sep. 2008.

[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., vol. 47, pp. 263–265, Nov. 2005.

[8] S. W. Su, J. H. Chou, and Y. T. Liu, “Printed coplanar two-antenna element for 2.4/5 GHz WLAN operation in a MIMO system,” Microwave Opt. Technol. Lett., vol. 50, pp. 1635–1638, Jun. 2008.

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