JOURNAL OF TELECOMMUNICATIONS, VOLUME 10, ISSUE 2, SEPTEMBER 2011 10

A Compact Wideband Antenna with CPW-fed Monopole for WLAN/WiMAX Operation M. Koohestani, M. Khaghani and H. Asadi Abstract—This paper presents a new compact planar wideband antenna with CPW–fed monopole, suitable for WLAN and WiMAX operations, providing a tunable bandwidth. By simply tuning the lengths of the CPW ground plane which results in an asymmetric ground plane structure, the range of bandwidth can be obtained from 2.3 to 7.6 GHz which can operate in WLAN (2.4–2.484 GHz, 4.9–5.1 GHz, 5.15–5.35 GHz, 5.7–5.9 GHz) and WiMAX (2.5–2.7 GHz, 3.4–3.6 GHz, 5.7–5.9 GHz). The antenna is as small as 23 mm × 20 mm × 1 mm. The performance of the antenna have been simulated and measured, showing good impedance bandwidth and omnidirectional radiation patterns across its entire tunable frequency range. Index Terms—Wideband antennas, Compact size, CPW-fed, WLAN/ WiMAX application.

—————————— u ——————————

1 INTRODUCTION

S

INCE wireless communication has been popular and used more, especially during the recent decade, wireless equipment has been developed more than ever. In this way different application bands are available for wireless usage. Wireless local area network (WLAN) is one of the most important applications of the wireless communication technology. The WLAN standards have been developed by the institute of electrical and electronics Engineers (IEEE) and the 802.11 standard is a family of specifications for WLAN technology. The frequency ranges based on this standard for WLAN are: 2.4 GHz (2.4 – 2.484 GHz), 5.2 GHZ (5.15 – 5.35 GHz) and 5.8 GHz (5.725 – 5.825 GHz). WLAN system is used not only in the office but also in the home recently by users of the mobile terminal devices like laptop computers or PDAs (Personal Digital Assistant). Many other wireless and mobile communication systems, such as WiMAX has also developed rapidly. IEEE 802.16 standard named WiMAX provides maximum of 10 Mbps wireless transmission of data using variety of transmission modes from point to multipoint links to portable and fully mobile internet access devices. WiMAX operating bands are 2.5 GHz (2.5 – 2.69 GHz), 3.5 GHz (3.4 – 3.69 GHz) and 5.5 GHz (5.25 – 5.85 GHz). WiMAX can be used for a number of applications, including “last mile” broadband connections, hotspots and cellular backhaul, and high speed enterprise connectivity for business applications. Many systems nowadays operate in multiple frequency bands, requiring dual– or triple–band operation of fundamentally narrowband antennas. Advances in software– ————————————————

• Mohsen Koohestani is with the Faculty of Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran. • Mahyar Khaghani is with the Faculty of Engineering, Shahre Rey Branch, Islamic Azad University, Tehran, Iran. • Hadi Asadi is with the Faculty of Engineering, Shahre Rey Branch, Islamic Azad University, Tehran, Iran.

defined and reconfigurable radio networks necessitate their operation over a wide range of frequencies or operation in a multi–band manner. Hence, to cover more wireless communication services, antennas which operating at a wideband range are highly demanded. One popular design for antennas using for the mentioned applications is the coaxial–fed monopole antenna because of providing the various radiation features for dual–band or multi–band beside the wide bandwidth and low profile communication systems. But as it needs a large ground plane at the opposite side of radiation plane on substrate, it is necessary to make a via-hole connection for feeding the signal which make it hard to produce and with higher costs. Therefore, microstrip–fed and coplanar waveguide (CPW)–fed monopole antennas are more popular in dual– band or multi–band and wideband communication systems. However, the CPW feed line can offer wide bandwidth, simple configuration, low cost, low radiation loss, simplicity in construction and easy integration with monolithic microwave integrated circuits (MMIC). For the available designs, the printed monopole antennas reported in [1–10] have wideband characteristics. However, most of them were addressed to the needs of WLAN applications [1, 3–5, 7, 9–10] and WiMAX applications [1, 6–10]. Very limited compact antenna designs have the ability to tune the bandwidth or have included both the bands simultaneously. Therefore, in this paper we propose a new compact CPW–fed planar monopole antenna that covers the operating bands of both WLAN in IEEE 802.11 b/a/g at 2.4/5.2/5.8 GHz and WiMAX based on IEEE 802.16 at 2.5/3.5/5.5 GHz, while it enables frequency tuning by adjusting the ground dimensions of CPW. The proposed antenna, with fairly small size was designed to operate over the frequency bands between 2.3– 7.6 GHz. It should be noted that the size of the proposed antenna is smaller than that of achieved wideband frequency range.

© 2011 JOT http://sites.google.com/site/journaloftelecommunications/

11

3 RESULTS AND DISCUSSIONS

Fig. 1. Antenna geometry and its design parameters (left), Photograph of the fabricated antenna (right).

The analysis and design of this antenna were carried out using a well–known simulator, namely, Ansoft High Frequency Structure Simulator (HFSSTM) [11]. Furthermore, the proposed antenna was fabricated and measured to verify its characteristics. The empirical results validate its wide impedance bandwidth and omnidirectional radiation pattern.

2 ANTENNA STRUCTURE AND DESIGN

At the beginning of this section to obtain the optimum performance from the antenna, design procedure of the proposed monopole antenna with simulated reflection coefficient curves is studied. Note that the proposed structure was simulated using Ansoft HFSS solver. As it will be shown below the performance of the proposed antenna is mainly affected by its geometrical parameters, i.e. radiator shape and CPW ground size. First, the effect of varying the radiated patch on the impedance bandwidth is studied. The characteristics features of the proposed patch and proposed patch without notch are compared to understand the function of the notch in the antenna patch radiator. In order to examine the effect of the patch radiator notch shape on the antenna impedance matching performance, the notch in Fig. 1 was removed while keeping all other antenna dimensions constant. Fig. 2 illustrates the results of the simulation. It can be seen that for the antenna without a notch is only able to achieve an impedance bandwidth with a lower and upper edge frequency of 2.35 GHz and 5.02 GHz, respectively. Therefore, resonance at the upper frequency is caused by placing a notch on the patch radiator. In other words, the notch creates another band to fully cover the required bandwidth of the IEEE 802.11a (5.15–5.35 GHz), and IEEE 802.16d (5.7–5.9 GHz). The upper resonance frequency has dependence on the radius of semicircle notch while the lower frequency still almost keeps the resonance point. Based on the simulation results, when the radius of semicircle notch becomes larger or smaller, the antenna impedance bandwidth decreases. Therefore, the best performance is obtained when the notch radius is 6 mm.

The new antenna configuration with its optimal parameters is shown in Fig. 1 (a). The antenna is located in x–y plane and the normal direction is parallel to the z–axis. The radiation patch which radiates the incident energy across a wide frequency range comprises an inverted semicircle with a rectangular section. An inverted semicircle notch with smaller radius ‘r1’ is also etched under the rectangular section of the radiating element to perturb the resonant response for widening the antenna impedance bandwidth. The input of the antenna is via a CPW feed line. The CPW line has line width and slot width of 2 and 0.5 mm, respectively. To enhance the antenna 10 dB impedance bandwidth, the ground planes of CPW are designed to have unequal lengths that the resonance frequencies can be finely tuned according to the variation of the ground dimensions of CPW. In this study, parameter ‘k’ represents the feed gap (distance between radiating element and larger CPW ground plane), as defined in Fig. 1 (a). Parameter ‘r2’ is the radius of the larger semicircle. The width and length of patch rectangular section are 2בr2’ and ‘e’, respectively. The Fig. 2. Effect of the radiating element notch on the antenna impedsmaller and larger CPW ground planes have length of ‘a’ ance bandwidth and ‘b’, respectively. The proposed antenna is fabricated on one side of a standard FR4 microwave substrate with a thickness of 1 mm and relative permittivity of 4.4. The other side of the substrate is devoid of any metallization. The area of the antenna is 23 mm × 20 mm. The photograph of the fabricated antenna is shown in Fig. 1 (b). To minimize the antenna dimensions, the antenna parameters have been optimized. The optimal dimensions of the monopole antenna are set as follows: m = 23 mm, n = 20 mm, a = 2 mm, b = 8 mm, w = 2 mm, i = 0.5 mm, k = 1 mm, r1 = 6 mm, r2 = 7 mm, e = 7 mm. Fig. 3. Effect of the CPW ground length on the antenna impedance bandwidth.

12

tenna normalized radiation pattern at 2.4, 3.5, and 5.2 GHz. It is observed that the radiation patterns of the antenna in the H–plane is nearly omnidirectional and almost bidirectional in the E–plane that these results are desirable. It is obvious from these results that the omnidirectional radiation patterns are acceptable across the operating band.

4

CONCLUSION

A new compact wideband planar monopole antenna, CPW–fed, is presented for WLAN/WiMAX applications. The antenna occupying a small area of 23 × 20 mm2. The measured reflection coefficient extends from 2.3 to 7.6 GHz and confirms the antenna wideband characteristics. By adjusting ground dimensions of CPW, the range of bandwidth can be obtained across its operating band. The measured radiation patterns indicate that the antenna radiates approximately omnidirectional in the H–plane and bidirectional in the E– plane through its operating band. The proposed antenna has advantages of low cost, compact size, bandwidth tunability and omnidirectional radiation patterns. Accordingly, the proposed antenna can be apFig. 5. Measured radiation pattern of the antenna at: (a) H–plane, and plied to modern wireless communications. Fig. 4. Measured and simulated reflection coefficient of the antenna.

(b) E–plane.

REFERENCES In next stage, to examine the effect of the CPW ground lengths on the antenna impedance matching performance, various CPW ground lengths are studied. Comparison has been done for different CPW ground sizes where ‘a’ and ‘b’ have been varied. The reflection coefficient response of the antenna concerned with the length of CPW ground is shown in Fig. 3. It can be seen that 10 dB bandwidth is reduced when the length of the ground is either wide or narrow. It is found that designed frequencies and operational bandwidth are sensitive to change in these parameters. It should be noted that, the resonance frequencies can be finely tuned according to the variation of the CPW ground size. For example, when ‘a’ = 2 mm and ‘b’ = 7 mm the antenna has a dual–band behavior which covers the require band for WLAN 2.4/5.2/5.8 GHz. When ‘a’ = 2 mm and ‘b’ = 8 mm the antenna can additionally cover WLAN 4.9–5.1 GHz and WiMAX 3.4–3.6 GHz. To validate the proposed design, the antenna was fabricated based on optimal dimensions and measured using an Agilent E8363B Network Analyzer (10 MHz–40 GHz). Fig. 4 depicts the simulated and measured reflection coefficient of the antenna. This graph indicates that the 10 dB bandwidth of the measured reflection coefficient reaches 5.3 GHz (2.3–7.6 GHz) which meets the bandwidth requirement for WLAN 2.4 GHz (2.4–2.484 GHz), 5.2 GHZ (5.15–5.35 GHz), 5.8 GHz (5.725–5.825 GHz) and WiMAX 2.5 GHz (2.5–2.69 GHz), 3.5 GHz (3.4–3.69 GHz), 5.5 GHz (5.25–5.85 GHz) applications. Reasonable agreement between simulated and a measured result is observed and the difference between them may be from SMA connector effects and fabrication imperfections. Two principle planes are selected to show the radiation pattern of the antenna. These planes are referred to as the y–z plane (E– plane) and the x–z plane (H–plane). Fig. 5 show the an-

[1]

R. Zhu, X. Wang, and G. Yang, “A Wideband Monopole Antenna Using Parasitic Elements,” Applied Mechanics and materials, Trans. Tech. Publications, Switzerland, Vol. 52–54, pp. 1515-1519, 2011. [2] C. R. Medeiros, E. B. Lima, J. R. Costa, and C. A. Fernandes, “Wideband Slot Antenna for WLAN Access Points,” IEEE Antennas and wirel. Propag. Lett., vol. 9, pp. 79-82, 2010. [3] H. I. Hraga, C. H. See, R. A. Abd-Alhameed, D. Zhou, S. Adnan, I.T.E. Elfergani, and P. S. Excell, “Small wideband antenna for GSM and WLAN applications,” IEEE Proceedings of the Fourth European Conference on Antennas and Propagation (EuCAP), pp. 1-4, 2010. [4] G. Augustin, P. C. Bybi, , V. P. Sarin, P. Mohanan, C. K. Aanandan, and K. Vasudevan, “A Compact Dual-Band Planar Antenna for DCS1900/PCS/PHS, WCDMA/IMT-2000, and WLAN Applications,” IEEE Antennas and wirel. Propag. Lett., vol. 7, pp. 108-111, 2008. [5] M. H. Al Sharkawy, “Miniaturized wideband slotted monopole antenna for WLAN applications,” IEEE APS, Middle East Conference on Antennas and Propagation (MECAP), Cairo, Egypt, 2010. [6] B. Purahong, P. Jearapradikul, T. Archevapanich, N. Anantrasirichai, O. Sangaroon, “CPW-Fed Slot Antenna with Inset U-Strip Tuning Stub for Wideband,” International Conference on Control, Automation and Systems, COEX, Seoul, pp. 1781-1784, 2008. [7] Y. K. Choukiker, S. K. Behera, “Compact ACS-Fed Koch Fractal Shape Antenna for Wideband Application,” International Conference on Electronic Systems (ICES-2011), NIT Rourkela, India, pp. 247-251, 2011. [8] M. S. Kumar, M. D. Mujumdar, “CPW-Fed Antenna with Two Rectangle Slots for RFID/Wideband applications,” IEEE ACS, International Conference on Advances in Computer Engineering, pp. 259-261, 2010. [9] Z. Katbay, S. Sadek, “Wideband Antenna for WLAN Applications,” IEEE ACTEA, Zouk Mosbeh, Lebanon, pp. 197-199, 2009. [10] N. Arsusiri, O. Sangaroon, S. Puntheeranurak, N. Anantrasirichai, “Trapezoid-Stub Fed Rectangular Slot Antenna for WLAN and WiMAX Applications,” International Conference on Control, Automation and Systems, KINTEX, Korea, pp. 2206-2209, 2010.

13

[11] Ansoft HFSS User’s manual, Ansoft Corporation, Beta Release 11.0, Apr 2007.

Mohsen Koohestani was born on 28 August, 1985 in Iran. He received his B.Sc. and M.Sc. degrees in Electrical EngineeringTelecommunication from Islamic Azad University, Tehran, Iran in 2007 and 2010, respectively. His main field of research includes antenna theory and design, particularly in small and broadband antennas, diversity antennas, antennas for on-body communications, microwave circuit and filters, wireless communication and Metamaterial cloaking. Mahyar Khaghani was born on 5 December, 1983 in Iran. He received his B.Sc. degrees in Electrical EngineeringTelecommunication from Islamic Azad University, Tehran, Iran in 2008.He is currently working toward the M.SC degree in communication Engineering. In 2010, he joined the ICS co as one of major companies in telecom systems in Iran. His research interests are antenna design and theory in wideband antennas, microwaves and antenna wave propagation, wireless communication and optical networks. Hadi Asadi was born on 22 June, 1985 in Iran. He received his B.Sc. degrees in Electrical Engineering-Telecommunication from Islamic Azad University, Tehran, Iran in 2009. He is currently working toward the M.SC degree in communication Engineering. His research interests are antenna theory and design in multiband and ultra wideband antennas, wireless communication.