Dual-Band, Switched-Beam, Reconfigurable Antenna ... - IEEE Xplore

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IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 12, 2013

Dual-Band, Switched-Beam, Reconfigurable Antenna for WLAN Applications M. Z. M. Nor, S. K. A. Rahim, Member, IEEE, M. I. Sabran, P. J. Soh, Student Member, IEEE, and G. A. E. Vandenbosch, Fellow, IEEE

Abstract—The development of WLAN technology requires antennas with additional capabilities using simple and low-cost mechanisms. This letter presents a low-cost microstrip antenna integrated with simple p-i-n diodes as RF switches. The proposed antenna is capable of steering the beam in a dual-band mode: , and 16 , and 61 within the 2.45-GHz band, and 6 within the 5.8-GHz band, which meets the IEEE 802.11a, b/g requirements. The antenna consists of three different dual-band elements, connected using an H-shaped transmission line at the radiating side. The novelty of this structure lies in the ground plane design. The H-shaped transmission line is used to integrate three different ground-plane structures. Index Terms—Beam-reconfigurable antenna, dual-band antenna, half-ground plane, microstrip antenna, p-i-n diode, parabolic ground plane, quarter-wavelength transformer.

I. INTRODUCTION

T

HE RECENT growth of wireless technology requires antennas with multiple capabilities and functions. Antenna reconfigurability is one of the popular antenna features that enables selectiveness in operating frequencies, polarizations, radiation patterns, and gains [1]. With the integration of RF switches in an antenna, a reconfigurable antenna enables use in a wide variety of wireless applications. For example, in [2], RF MEMS switches are implemented in the proposed designs. However, the major problem with RF MEMS switches is their complexity when integrated with the antenna. Other popular RF switches used in reconfigurable antennas include the photo-conducting switch, found in [3], which enables frequency and beam reconfigurability. However, a complex switching circuit is needed to trigger this type of switch. The outcome is that p-i-n diodes are the most efficient choice to implement this feature, as seen in [4] and [5]. Manuscript received March 18, 2013; revised July 08, 2013; accepted October 20, 2013. Date of publication November 07, 2013; date of current version November 20, 2013. This work was supported in part by the Ministry of Science, Technology and Innovation (MOSTI), Universiti Teknologi Malaysia (UTM), and the Wireless Communication Center (WCC). M. Z. M. Nor, S. K. A. Rahim, and M. I. Sabran are with the Wireless Communications Center (WCC), Universiti Teknologi Malaysia (UTM), 81310 UTM Skudai, Malaysia (e-mail: [email protected]; [email protected]; [email protected]). P. J. Soh is with the ESAT-TELEMIC Research Division, KU Leuven, 3001 Leuven, Belgium, on leave from the School of Computer and Communication Engineering, Universiti Malaysia Perlis (UniMAP), 02000 Kuala Perlis, Malaysia (e-mail: [email protected]). G. A. E. Vandenbosch are with the ESAT-TELEMIC Research Division, KU Leuven, 3001 Leuven, Belgium (e-mail: [email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LAWP.2013.2289919

In switched beam antennas, most researchers utilize a multiport antenna approach. In [6], a four-port switched beam antenna is designed using four slotted antennas that are simultaneously activated using four ports. Other studies in [7] utilize a beamforming technique such as a butler matrix, which does not require RF switches. However, the need for 3-dB couplers, phase shifters, and crossovers further complicates the antenna structure. In this letter, a simple and novel reconfigurable antenna structure is proposed. The antenna has been improved from previous work [8], which featured two directive antennas with defected ground planes (DGS). An enhanced directive antenna is integrated with the antenna of [8] in an H-shaped array configuration. The main goal is to create a directive antenna that is able to switch its main beam between right and left directions. RF p-i-n diodes are integrated in the transmission lines to enable this reconfigurability feature. Designed and fabricated on ), the low-cost FR4 material (with dielectric constant, this improved antenna is capable of operating simultaneously at 2.45- and 5.8-GHz WLAN frequencies with satisfactory reflection and radiation performances, as will be detailed in the following sections. II. ANTENNA DESIGN The four-element array is formed using two types of antenna topologies, two elements from the topology in [8], and another two elements are modified based on this topology. A comparison of the two topologies and their dimensions is shown in Fig. 1. Fig. 1(b) clearly shows the modifications in the proposed antenna with respect to the existing one, given in Fig. 1(a). The differences are the size of the inner patch ( and ) and the gaps between the antenna arm and the inner patch . In this proposed antenna, mm relative to [8], while the dimensions of mm and mm are maintained. In its design process, the use of 45 chamfers for each slot in antenna radiating aperture is utilized to improve current flow. In addition, a circular slot is centered on the inner patch to effectively tune the higher resonant frequency while keeping the lower resonance constant. This is shown in Fig. 2. However, this additional center slot also introduces a change of the current path within the inner patch; consequently, its patch size is reduced in order to compensate for this. The DGS with a parabolic-shaped edge is used to manipulate the current distribution on the ground plane for directivity improvements. The current is concentrated along the parabolic edge, facilitating beam-shaping toward the right. The shape of the parabolic slot is determined by the basic equation of a parabolic curve where the origin of the parabolic slot is shown in Fig. 3. The letter in Fig. 3(a) denotes the distance between the parabolic curve and the origin.

1536-1225 © 2013 IEEE

NOR et al.: DUAL-BAND, SWITCHED-BEAM, RECONFIGURABLE ANTENNA FOR WLAN APPLICATIONS

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Fig. 3. (a) Parabolic slot axis. (b) Effect of rotation angle of parabolic shaped slot on the steering angle of proposed antenna.

Fig. 1. (a) Original antenna topology from [8]. (b) Improved dual-band antenna with circular slot and DGS. (c) Improved antenna’s dimensions in millimeters: .

Fig. 2.

with the center slot’s diameter

varied.

In this case, mm. The coefficient in the equation is 0.022. The angle of rotation of the parabola affects the steering of the enhanced antenna. Fig. 3(b) shows the effect of rotation angle on the steering. In this figure, Beam 1 represents the normalized beam pattern when the origin of the parabolic-shaped slot is rotated 42 . Beam 2 corresponds to 40 of rotation angle, and Beam 3 represents 38 of rotation angle. As illustrated in this figure, the rotation angle of the parabolic-shaped slot can modify the steering angle of the proposed antenna. Furthermore, 40 of rotation angle is used since this gives an optimum overall performance. Note that the main effect of the rotated parabola is that the horizontal symmetry of the ground is broken. This unconventionally shaped ground conductor then exhibits a changed current flow, contributing to the steering effect. It is clear that other shapes than a parabola could be used, yielding a very similar steering effect. Although a detailed study of all possible shapes is beyond the scope of this letter, it can be mentioned that the steering effect is not that sensitive to the exact shape.

Fig. 4. Proposed antenna array with locations of p-i-n diodes: (a) front view; (b) back view with ground-plane topology.

The simulated 10-dB bandwidths of the enhanced antenna are 258.5 MHz at 2.45 GHz and 517 MHz at 5.8 GHz. In the proposed array, as shown in Fig. 4, two antenna elements from [8] have been integrated with two elements of the proposed enhanced antenna with a separation of at the reference frequency of 3.75 GHz. Note that the parabolic edges of two enhanced antennas form a mirror image toward each other with symmetrical mirror axis along the feed point of proposed antenna. This symmetry enables steering at three directions. Also note the innovating layout of the ground plane structure; the ground planes of the elements are connected through an H-shaped strip. This is novel, as most planar arrays utilize full ground-plane structures. Together with an identical strip on the upper surface, this strip structure forms a dual-strip transmission-line topology, containing a quarter-wavelength (at 3.75 GHz) impedance transformer and containing the eight p-i-n diodes, four each on the radiating surface and ground plane with coaxial SMA at the structure’s center. The overall size of the antenna is 160 170 mm . Note that the placement of the diodes is such that it enables the beam-steering feature by ensuring that only the wanted patches are excited. Further integration of a microcontroller into this system will enable a fully automated feedback control in sensing the strongest direction of arrival (DoA) and consequently beam-steering. In determining the best p-i-n diode locations, the reflection coefficient was studied while varying the p-i-n diode locations. An example of this investigation is illustrated in Fig. 5 for p-i-n diodes 1 and 5. The array’s optimal is obtained when both diodes are placed very near to the coaxial feed. To achieve reconfigurability, three p-i-n diode configurations are defined. In the first configuration, p-i-n diodes 1 and 5 are switched ON, and the others are OFF. This enables forward beamsteering. In the second configuration, p-i-n diodes 2, 3, 6, and

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IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 12, 2013

Fig. 5. Effect of location variation for p-i-n diode 1 and 5 on the (a) lower frequency response; (b) higher frequency response.

’s:

Fig. 6. Fabricated antenna: (a) front view; (b) back view.

P -I -N

Fig. 7. Simulated and measured

for Configuration 1.





TABLE I DIODE CONFIGURATIONS

TABLE II SIMULATED AND MEASURED STEERING ANGLES FOR EACH CONFIGURATION

8 are ON, steering the main beam to the right. In the third configuration, the p-i-n diodes 2, 4, 6, and 7 are ON, activating the bottom left patch. Hence, steering is directed to the array’s left. The three p-i-n diode configurations are summarized in Table I. III. RESULTS AND DISCUSSION The antenna array was fabricated, integrating the eight p-i-n diodes and the 50- coaxial SMA feed, and is shown in Fig. 6. To validate the beam-steering concept, the radiation pattern of the proposed array was simulated and measured for all three p-i-n diode configurations. Simulations were performed using CST Microwave Studio, whereas measurements were performed with care to avoid the dc feeding wires for the diodes from influencing the radiation pattern. Simulated and measured steering angles for all three configurations are summarized in Table II. It is seen that at the higher frequency, the steering angle is relatively small compared to the lower frequency. This is due to the behavior of the current distribution. At 5.8 GHz, less current is flowing at the parabolic edge of the ground plane structure. The reflection coefficient for Configuration 1 is given in Fig. 7. The simulated and measured results are in reasonable agreement. Part of the disagreement can be attributed to the error in the fabrication process (etching accuracy and soldering quality). The measured bandwidth (BW)

of the proposed antenna at 2.45 GHz is 60 MHz, and 450 MHz at 5.8 GHz. Configuration 2 is depicted in Fig. 8, showing a measured BW of 360 MHz at 2.45 GHz (from 2.44 to 2.80 GHz), and 390 MHz at 5.8 GHz. Fig. 9 depicts Configuration 3, showing a measured BW of 180 MHz in the lower band (from 2.41 to 2.59 GHz) and 420 MHz (between 5.56 to 5.98 GHz) in the higher band. As can be seen in Figs. 8 and 9, there exist some ripples between the two resonance frequencies in the measurements. This is due to the p-i-n diodes. The actual impedances of these p-i-n diodes in the ON and OFF states do not correspond to a perfect short circuit and an open circuit. On top, even in the case of an open circuit, there is coupling between the left and right element’s feeding circuit in configurations 2 and 3. This means that reflections may occur and give rise to small ripples in the curves [9]. This type of coupling effect is not present for configuration 1 since only diodes 1 and 5 are active to enable this beam/mode. We would also not consider these ripples as resonances since they barely meet the minimum 10-dB reflection coefficient requirement. There exists only a slight difference between simulations and measurements for all configurations, as can be seen in Figs. 7–9. This is due to the imperfect fabrication of the low-profile anfor configurations 2 and 3 tenna. Note that the simulated ideally have to be identical due to the physical symmetry of these configurations. The fact that this is not perfectly the case is due to the different meshing obtained in CST. Fig. 10 shows the radiation pattern for Configuration 1. The antenna generates a main beam at 2.45 GHz directed toward

NOR et al.: DUAL-BAND, SWITCHED-BEAM, RECONFIGURABLE ANTENNA FOR WLAN APPLICATIONS

Fig. 10. Measured far field for Configuration 1: (a) H-field for 2.45 GHz; (b) E-field for 2.45 GHz; (c) H-field for 5.8 GHz; (d) E-field for 5.8 GHz.

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that due to the mirror symmetry in the topology, the Configurations 2 and 3 generate mirror-image radiation patterns. IV. CONCLUSION A low-cost, dual-band, beam-reconfigurable array antenna using simple switching has been designed and evaluated. In this innovative design, each element is activated using a specific p-i-n diode configuration and is capable of directing beams in three distinct directions, both at the desired frequencies of 2.45 and 5.2 GHz. In the lower frequency band, a larger steering and 40 ), while steering angles of angle is achieved ( and 23 are obtained in the higher band. Reflection coefficients and bandwidths meet the IEEE 802.11a, b/g standards at both frequencies. REFERENCES


(simulated) and (measured), while the main beam (simulated) and (meaat 5.8 GHz is pointing toward sured). Fig. 11 shows the radiation pattern for Configuration 2. (simulated) The main beam at 2.45 GHz is steered toward and (measured). The steering angle at 5.8 GHz is much lower, namely (simulated) and (measured). This shows the proposed antenna steered its main beam at the left side where this is due to the activation of the left bottom patch as a p-i-n diode configured in Configuration 2. Fig. 12 shows the radiation pattern for Configuration 3, activating the bottom right patch. The main beam at 2.45 GHz is steered toward 61 (simulated) and 40 (measured). The steering angle at 5.8 GHz is much lower, namely 16 (simulated) and 23 (measured). Note

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