Design of A Reconfigurable Miniaturized Microstrip ...

This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/LAWP.2015.2476363, IEEE Antennas and Wireless Propagation Letters

Borhani et al.: Design of A Reconfigurable Miniaturized Microstrip Antenna for Switchable Multiband Systems

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Design of A Reconfigurable Miniaturized Microstrip Antenna for Switchable Multiband Systems M. Borhani, P. Rezaei, and A. Valizade, Member, IEEE  Abstract—In this letter, a novel printed frequency reconfigurable microstrip square slot antenna for switchable Bluetooth, WiMAX and WLAN applications is presented. The proposed antenna has a small size of 20×20 mm2 in which in order to be able to cover lower frequencies; as for Bluetooth applications, miniaturization techniques such as modification of the ground plane and inserting an adjustable backplane crossshaped sleeve have been employed. Moreover, by implementation of PIN diodes within the antenna structure switchable frequency responses are achieved. The presented antenna has a small size while providing suitable switchable radiations at 2.3-2.51 GHz (BW=8.7%) bluetooth, 3.35-3.75 GHz (BW=11.2%) WiMAX, and 4.95-5.53 GHz (BW=11%) WLAN.

matching and the radiation pattern at other resonance frequencies especially when they are introduced unsymmetrically to the antenna structure [6].

Index Terms—Microstrip Slot Antenna, Multiband Systems, Reconfigurable Structure

I. INTRODUCTION

T

HE Interest in wireless compact handheld devices which can operate at multiple frequency standards or wideband tunable spectrum has been dramatically raised in recent years and as a result the effort for designing compact reconfigurable wideband or switchable multiband antennas [1-5] has become a challenging task. In order to create frequency reconfigurable antennas, external parameters, typically a DC voltage is implemented through embedding a variable element into the antenna structure such as PIN diodes [3-4] or varactor diodes [6]. Nowadays, Microstrip antennas have an important role in realization of the handheld devices due to their unique merits such as light weight, compact size and ease of fabrication and integration [7]. In order to create multiband antennas, several techniques such as cutting slots on the metal parts of the antenna or using multiple radiating sections has been proposed recently [8-9]. Using slots or multiple radiating parts to create additional resonance usually causes inevitable degradation of the input A. Valizade is with Young Researchers and Elites Club, Qaemshahr Branch Islamic Azad University, Qaemshahr, Iran (e-mail: [email protected]). M. Borhani and P. Rezaei are with with the Department of Electrical Engineering, Semnan University, Semnan, Iran (e-mail: borhani.mehri@ semnan.ac.ir, [email protected]).

Fig. 1. The geometry of the proposed reconfigurable antenna: (a) top view, (b) bottom view

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In this paper, a new microstrip monopole antenna with triple band coverage capability is proposed. The presented antenna utilizes a simple uniform square shaped radiating stub which has a single band radiation characteristic. In order to change the resonance frequency band, the needed modifications have been done on the ground plane and the feed-line and the square radiating stub are left intact in order to minimize the degradation of input matching and radiation. Finally, inserting PIN diodes on the ground plane of the antenna has led to three different desirable switchable bands of operation. The resonance frequency can be effectively controlled by changing the bias conditions of the PIN diodes. The following sections deal with the antenna geometry, design theory, simulation and experimental results.

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miniaturization techniques have been employed and by cutting the ground plane and adding a T-shaped sleeve to it (Fig. 2(b)) the antenna is able to cover frequencies near 2.4 GHz, as shown in Fig. 3. Reconnecting the two sides of the T-shaped sleeve to the ground plane and forming two symmetrical slots on the ground plane (Fig. 2(c)), brings back the resonance frequency to 3.5 GHz, as can be seen in Fig. 3. Adding a cross-shaped sleeve to the ground plane and forming the antenna structure in Fig. 2(d), shifts the resonance frequency toward higher frequencies, 5.5 GHz here. In order to modify the input matching characteristic of the proposed structure at the highest resonance frequency band and tune the frequency response and shift it toward the desired range (5-5.5 GHz), the feed-line is etched symmetrically as shown in Fig. 2(e) and as can be seen in Fig. 3; the frequency response can be effectively tuned and improved according to [8-9].

II. ANTENNA DESIGN AND CONFIGURATION The proposed reconfigurable microstrip slot antenna configuration with its design parameters is shown in Fig. 1, which is printed on a FR4 substrate with thickness of 0.8 mm, permittivity of 4.4, and loss tangent of 0.018. As it is observed in Fig. 1, the antenna structure consists of a simple squareshaped radiating stub, a modified ground plane with a crossshaped sleeve arm, and a modified feed-line. A SMA connector is connected to the feed-line for signal transmission. In order to explain the switchable performances of the proposed antenna, the various antenna structures which were investigated during the simulation studies are presented in Fig. 2 and their frequency responses are compared in Fig. 3.

Fig. 3. The frequency responses of various antenna structures shown in Fig. 2.

Moreover in order to illustrate the phenomenon behind the multiband performance of the proposed antenna, the surface current distribution is depicted for various structures in Fig. 2. From this figure, it can be seen that for different modes, different part of the structure has a corresponding resonating path, and therefore the multiband performance is achieved.

Fig.2. Different simulated antenna structures: (a) ordinary microstrip slot antenna, (b) antenna with a modified ground plane having a T-shaped sleeve, (c) antenna with a modified ground plane having symmetrical slots, (d) antenna with a modified ground plane having symmetrical slots and a crossshaped sleeve, (e) antenna with a modified ground plane having symmetrical slots and a cross-shaped sleeve, and a modified etched feed-line.

As can be seen in Fig. 3, the ordinary microstrip antenna (Fig. 2(a)) has a resonance at frequencies near 3.5 GHz. In order to move the resonance frequency toward lower frequencies without increasing the size of the antenna,

Fig. 4. The frequency responses of the proposed reconfigurable antenna for three different biasing condition combinations of the implemented PIN diodes.

In summary, it can be concluded that the proposed antenna has three different switchable frequency responses which are suitably separated from each other. Inserting three PIN diodes to the proposed structure as depicted in Fig. 1, creates a reconfigurable antenna which is able to cover Bluetooth (2.4




GHz), WiMAX (3.5 GHz), and WLAN (5.5 GHz) systems. The simulated frequency response of the proposed reconfigurable antenna for different bias conditions of the PIN diodes in Fig. 1 is plotted and compared in Fig. 4. III. RESULTS AND DISCUSSION A prototype of the proposed reconfigurable microstrip slot antenna with its final modified parameters is designed, fabricated and tested and in this section the numerical and experimental results of its reflection coefficient and radiation characteristics are presented and discussed. The simulated results are obtained using Ansoft High Frequency Structure Simulator (HFSS) [10]. To obtain the final design values, parametric studies have been done and the final values are listed in TABLE I. An example is shown in Fig. 5 which reveals the procedure of determining the resonating path for the WLAN band by altering the design parameters LC2. Same thing can be done to the other two resonances by changing the corresponding parameters. TABLE I

The final design parameters values of the proposed antenna Param. WSub W Wf Wfs Ws Wc Wc1

mm 20 10 1.5 0.5 17 10 1

Param. W1 W2 W3 LSub Lf Ls Lc

mm 14 1.5 8 20 3 11 5.5

Param. Lc1 Lc2 L1 L2 L3 L4

mm 1 1.5 1.5 2.5 1.5 2

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diodes in order to avoid DC short circuiting. The DC supply is applied to the PIN diodes by means of wires. These wires may affect the antenna performance specially the radiation pattern however in a practical implementation of the designed antenna in an integrated circuit, the wires can be replaced by microstrip lines and the needed consideration can be performed in order to avoid any degradation of the radiation characteristic.

Fig. 6. The Picture of realized reconfigurable antenna

According to Fig. 7, good agreement between the measured results and simulation predictions is achieved; however, there exist some discrepancies which can be due to some reasons such as the implementation of the PIN diodes and their biasing circuits [4]. Also, the measured radiation pattern for the fabricated antenna at its three bands of operation is compared with the simulated results in Fig. 8. As can be seen in this Figure, the fabricated antenna has suitable radiation at all its switchable frequency bands of operation with omnidirectional characteristics at H-plane which is desired for most related applications [6]. However there exists some deviation in the measured radiation patterns (especially at 3.5 GHz) which can be mainly due to the effect of the biasing wires.

Fig. 5. The effect of variation in the design parameter LC2 on the frequency response for the condition in which D1: on, D2: on, and D3: on.

Moreover, a prototype of the designed antenna was manufactured and its radiation characteristics were measured. The Photograph of the realized antenna is presented in Fig. 6 and its measured reflection coefficient characteristic for various bias conditions of the PIN diodes are plotted and compared to simulation results in Fig. 7. BAR64-3W PIN diodes which have low capacitance at zero volt reverse bias (typically 0.17 pF at frequencies above 1 GHz) and low forward resistance (typically 2.1 Ω at 10 mA) were used as switches. As can be observed in both Fig. 1 and Fig.6 a DCblocking capacitor is used in the biasing circuit of the PIN

Fig. 7. Comparison between simulated and measured reflection coefficient of the presented reconfigurable antenna for various bias conditions of the embedded PIN diodes

Moreover, the measured maximum gain for various switchable performances of the fabricated antenna is presented in Fig. 9. As it is observed in this figure the presented antenna has suitable gain characteristics in its different switchable frequency bands of operation. A comparison between the proposed antenna with previously published works is provided in Table II.




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Fig. 9. Measured maximum gain for various biasing condition of the embedded PIN diodes

Ant. [1] [4] This work

TABLE II Comparison with previously published works No. of switches No. of bands Size (mm2) 4 4 3

3 3 3

40×40 30×30 20×20

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[2]

[3]

[4] Fig. 8. Normalized measured radiation patterns: (a) for D1: off, D2: off and D3: off at 2.4 GHz, (b) D1: on, D2: on and D3: off at 3.5 GHz, D1: on, D2: on and D3: on at 5.2 GHz.

[5]

IV. CONCLUSION A novel design of reconfigurable microstrip antenna with switchable multiband performance was presented in this paper. The main merit of the proposed structure is its compact size while being able to cover three widely separated frequency bands of 2.3-2.51 GHz Bluetooth, 3.35-3.75 GHz WiMAX, and 4.95-5.53 GHz WLAN with desired omnidirectional radiation patterns at H-plane. In the presented antenna three PIN diodes were employed into the ground plane and changing on and off statuses of the PIN diodes leads to a frequency reconfigurable property. The experimental measurements were in good correlation with the simulation results showing that the proposed antenna can be a good candidate for modern cognitive multiband systems.

[6]

[7]

[8]

[9]

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[10] Ansoft High Frequency Structure Simulator (HFSS), Ver. 13,

Ansoft Corporation, 2010.

ACKNOWLEDGMENT The authors are thankful to Dynamic Microwave Electronics (DMWE) Company staff for their professional help and support (www.dmw-electronics.com).
