30thNATIONAL RADIO SCIENCE CONFERENCE - IEEE Xplore

30thNATIONAL RADIO SCIENCE CONFERENCE (NRSC 2013) April 16‐18, 2013, National Telecommunication Institute, Egypt

B10. Design of A Compact Tri-band Antenna for RFID Handheld Applications using Optimization Techniques. A. M. Montaser1, K. R. Mahmoud2, Adel B. Abdel-Rahman3, H. A. Elmikati4 Senior Member IEEE 1 Sohag University, Sohag, Egypt, [email protected] 2 Helwan University, Helwan, Egypt, [email protected] 3 South Valley University, Qena, Egypt, [email protected] 3 Department of Electronics and Communications Engineering, E-Just-Alexandria, Egypt [email protected] 4 Mansoura University, Mansoura 35516, Egypt, [email protected]

ABSTRACT:In this article, the hybrid approach involving Central Force Optimization and Nelder-Mead (CFO-NM) algorithm is considered to optimize a compact dual bow-tie slot antenna for 915/2450/5800 MHz triple band RFID applications. The size of the proposed antenna is determined by the upper resonant frequency, and thus it is compact in nature. The middle and lower operating frequencies are obtained by inserting metal strip pairs near the ends of the slotted dual bow-tie without increasing the overall antenna area. The CFO-NM algorithm program was implemented using MATLAB-software which linked to the CST Microwave studio software to simulate the antenna. In addition the optimized antenna is assessed using the Finite Difference Time Domain (FDTD) program written with MATLAB to validate the results. Finally, the performance of the designed antenna in the presence of the human-hand model is investigated in addition to evaluating the spatial peak specific absorption rate (SAR).

Key Words: Slotted bow-tie antenna, RFID, Hybrid CFO-NM algorithm, FDTD method. I. INTRODUCTION In recent years, Radio Frequency Identification (RFID) technology has been widely used in service industries as an automatic identification tool [1]. A basic RFID system comprises a radio-scanner unit, called reader, and a set of remote transponders, denoted as tags, which include an antenna and a microchip transmitter with internal read/write memory. The tag's antenna receives signals from an RFID reader and then sends back the signals, usually including some additional data such as a unique serial number or customized information. There are four common frequency bands have been assigned for this technology, low frequency (LF, 125-134 kHz), high frequency (HF, 13.56 MHz), ultra-high frequency (UHF, 902-928 MHz), and the so-called microwave (MW, 2.4-2.483) GHz, in addition to (5.725-5.875) GHz. A low frequency operation, such as in the LF band, would suffer very slow reading in a heavily tag populated environment. Also, LF requires large antenna components and hence is difficult to implement and is susceptible to electrical noise, which HF can handle [2]. Many RFID antenna designs for achieving operation at one or more of the LF, HF, UHF, and MW bands have been reported in the open literature [3-9].The bow-tie antenna is one of the most suitable antennas for various RFID applications. It has many advantages such as low profile, high radiation efficiency, ease of manufacturing and low fabrication cost. In [3], a hybrid approach involving Bacterial Swarm Optimization (BSO) and NelderMead (NM) algorithm is proposed to design a bow-tie antenna for 2.45 GHz RFID readers. In [4], the paper emphasized a novel bow-tie slot antenna with CPW-fed structure suitable for 5.8 GHz RFID applications. The proposed antenna adopts dual bow-tie structure which could decrease the antenna's dimension and broaden the bandwidth. In [5], a dual-band antenna that allows operation in the 13.56MHz band as well as in the 868MHz band is presented. A novel design for the modified bow-tie slot antenna with a rectangular tuning stub for 2.4/5.2/5.8 GHz triple band applications is presented in [6]. Each of ultra-high frequency and microwave RFID frequency bands has their own advantages. Therefore, the urge of multiband RFID readers and consequently multiband reader antennas are also becoming vivid. There are few papers embraced both advantages contributed by UHF and microwave band in one patch. In [7], a triple-band printed dipole tag antenna is proposed for RFID. The triple-band printed dipole antenna is designed to operate at 0.92 GHz, 2.45 GHz and 5.8 GHz. However, the antenna size is found to be relatively large. A novel compact microstrip antenna is presented for tri-band RFID applications in [8]. The prototyped antenna showed broad impedance bandwidths (10 dB ≤ return loss) of 17.5, 32.7, and 25.5% accordingly in UHF (902–928MHz), 2.45GHz and 5.8GHz RFID bands with stable radiation characteristics. In this paper, the hybrid approach consisting of Central Force Optimization and Nelder-Mead (CFO-NM) algorithm is considered to optimize the antenna dimensions. The CFO had been introduced as a new deterministic 978-1-4673-6222-1/13/$31.00 ©2013 IEEE

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30thNATIONAL RADIO SCIENCE CONFERENCE (NRSC 2013) April 16‐18, 2013, National Telecommunication Institute, Egypt metaheuristic for multidimensional search and optimization based on the metaphor of gravitational kinematics [912]. In [13], the CFO algorithm with the acceleration clipping scheme is applied to the optimal design of two different wideband microstrip patch antennas. A hybrid approach involving Central Force Optimization (CFO) and Nelder-Mead (NM) algorithm is proposed in [14] for accurate determination of resonant frequency and feed point calculation of rectangular microstrip antenna elements with various dimensions and various substrate thicknesses. A compact triple band antenna design for handheld RFID reader using CFO-NM algorithm is presented. The CFO-NM algorithm is applied to optimize the dimensions of the slotted dual bow-tie antenna to be resonant at 915 MHz, 2.45 GHz, and 5.8 GHz for matched input impedance (Zin) of 50 Ω. The performance of the designed antenna is assessed using a full EM analysis based on the Finite Difference Time Domain (FDTD) program written with MATLAB to validate the results.

II. DUAL BOW-TIE SLOT ANTENNA WITH THREE METAL STRIP PAIRS In this section, a compact tri-band dual bow-tie slot antenna fed by a CPW for handheld RFID applications optimized by CFO-NM algorithm has been presented with satisfactory antenna characteristics. The geometry of the proposed antenna is depicted in Fig. 1. Firstly, the proposed antenna is designed over a single layer of FR4 dielectric substrate with an initial dimension of 80 × 42 mm. The three different lengths of metal strip pairs inserted near the ends of the slotted dual bow-tie as shown in Fig. 1. The size of the proposed antenna is determined by the upper resonant frequency (f = 5.8 GHz), and thus it is compact in nature. The middle (f = 2.45 GHz)and lower (f = 915 GHz) operating frequencies are obtained by inserting three metal strip pairs near the ends of the dual bow-tie slot without increasing the overall antenna area[15].A dual bow-tie is considered to broaden the antenna bandwidth [4] to be applicable for other multiband applications such as in wireless communication systems. The longest strip pair has a total length of L1= 22.27 mm, and the second strip pair has a total length of L 2= 18.64 mmwhile the shortest strip pair has a total length of L3=13.68 mm. The strips width is assumed to be 0.5mmand the spacing between each two strips is 0.5mm.

Fig. 1.The basic geometry of dual bow-tie slot antenna with three metal strip pairs. In the beginning, the antenna dimensions (length and width) a, and b of the proposed antenna are optimized by the CFO-NM algorithm to be resonant at a centre frequency of 5.8 GHz which is the upper resonant frequency to be compact in size. The dimensions are allowed to vary from 30 mm to 50 mm for length a and from 20 mm to 35 mm for width b. Considering the hybrid CFO-NM algorithm, the parameters a and b were found to be 39.26 and 24.94 mm, respectively to give a return loss value of -18.5 dB at 5.8 GHz. Then, a dual bow-tie slot antenna construction with h, and t is considered to broaden the bandwidth [4]. The other required bands will be obtained by inserting three metal strip pairs near the ends of the bow-tie slot without increasing the overall antenna area. Therefore, we have five antenna parameters (L4, L5, L6, t, and h) need to be optimized to minimize the return loss less than -10 dB at the required RFID frequency bands. According to these remarks the fitness function is calculated using the following Equation: Fitness = min(S11 )915MHz + min(S11 )2450 MHz + min(S11 )5800 MHz

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(1)

30thNATIONAL RADIO SCIENCE CONFERENCE (NRSC 2013) April 16‐18, 2013, National Telecommunication Institute, Egypt Table 1 shows the decision space for each variable and the best obtained value to achieve our goals for this design. It is found that the best antenna parameters are (L4 =10.13mm, L5 = 7.56mm, L6 =3.93mm, t =9.25mm, and h =9.18mm) to obtain a return loss of -10.9, -24.1, and-27 dB at centre frequencies of 915 MHz, 2.45 GHz, and 5.8 GHz, respectively. As shown in Fig. 2, the simulated result according to S11 less than -10 dB, the antenna achieves wide bands of 50 MHz (0.87 – 0.92 GHz), 320 MHz (2.26–2.58 GHz) and 1.21 GHz (5.37–6.58 GHz) in the three vital resonating bands. The bands are accordingly equivalent to 5.6, 13, and 20.7% with respect to the centre frequencies of 0.891, 2.458, and 5.83 GHz. To validate the results, a FDTD program written with MATLAB is used to simulate the optimized antenna. A non-uniform mesh was used to improve the efficiency of the FDTD calculation and help focus a large number of cells in regions of interest. FDTD analysis by using non-uniform mesh is based on the finite-difference representation of Maxwell's curl equations using a central-difference formulation and the Yee-cell notation [16]. The shape computational space is divided to fine mesh region and coarse mesh region. Fine mesh is used to model the region including the object, and coarse mesh is used to divide computed space between object and absorbing boundary. When using FDTD method with non-uniform mesh, the major problem was always that changing the size of neighbouring lattices continuously and simultaneously in all three directions introduces a first order error term. It is very important to investigate how to deal with electric and magnetic field components near boundary surface between the fine and coarse meshes [17]. In this structure the fine meshes in y-direction equal to 0.1 mm, otherwise coarse meshes equal to 0.85 mm. Fig. 2 shows the comparison between simulated antenna with FDTD method and the CST Microwave Studio software for return loss in the range 0 - 7 GHz. A good agreement between the results of EM Simulation and that produced from the FDTD are achieved. Table 1. The antenna parameters decision space and optimized values obtained using CFO-NM algorithm. Variables

Decision Space start end

Optimized value

a (mm)

30

50

39.26

b (mm)

20

35

24.94

L4 (mm)

0

21.83

10.13

L5 (mm)

0

19.45

7.56

L6 (mm)

0

16.86

3.93

t (mm)

6

31.5

9.25

h (mm)

3.14

18.1

9.18

0

Return Loss S11 (dB)

-5 -10 -15 -20 -25 -30 -35

CST MWS FDTD 0

1

2

3 4 Frequency (GHz)

5

6

7

Fig. 2 Return loss comparison between the FDTD method and the CST Microwave Studio software for the triband optimized antenna by CFO-NM algorithm.

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Fig. 3. The reader device with a compact triple band slotted dual bow-tie antenna in the presence of human-hand. Now the optimized tri-band RFID antenna will be covered with a dielectric material (ε r = 2.1) of thickness 1 mm, having external dimensions 104.6337 × 45.084× 6.1mm. The hand that provided by CST MWS is used [18]; the tissue that it contained has a relative permittivity and conductivity of 35.114 and 3.717 S/m respectively. These tissue-equivalent dielectric parameters were chosen according to [19] for simulating hand tissue at 5.8 GHz which is the highest operating frequency band. Fig. 3 shows the covered compact triple band slotted bow-tie antenna in the presence of human-hand. The effects of the covered dielectric and the human-hand on the antenna characteristic are studied. Fig. 4 shows the return loss frequency response comparison for the optimized antenna without cover, with cover, and the covered antenna in the presence of the human-hand. It is found that, either the handheld cover or the human-hand has an effect on the return loss results. However, in both cases the design still works adequately without the need for re-optimization.

00

(dB) S11 (dB) ReturnSLoss 11

-5 -5 -10 -10 -15 -15 -20 -20 -25 -25 CST Package Antenna FDTD (non-uniform Antenna with cover mesh) FDTD (uniform mesh) Antenna with cover and hand

-30 -30 -35 -350 0

11

22

33 4 4 Frequency (GHz) Frquency (GHz)

5 5

6

6

7

7

Fig. 4.S11 comparison of the antenna only, covered antenna with a dielectric case, and the covered antenna in the presence of the human-hand.

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Now more concern is given towards antenna efficiency and possible different matching requirements (≠ 50 Ω) that are paramount importance for RFID applications. Fig. 5 shows the efficiency of the antenna with and without cover in addition to the antenna efficiency in the presence of the human hand. For antenna only, it is found that the efficiency at the highest resonance frequency of 5.8 GHz is 93.4%, but for middle frequency at 2.45 GHz the efficiency is decreased to 91.5% compared to 78% for the lowest frequency band at 915 MHz. However, in the presence of the human-hand, the antenna efficiencies at 915 MHz, 2.45 GHz, and 5.8 GHz are decreased to 68.6%, 82.2%, and 82.3% respectively. The simulated radiation patterns in x-z and y-z planes of the optimized antenna at resonance frequencies of 915 MHz, 2.45 GHz, and 5.8 GHz are illustrated in Fig. 6. It is seen that the antenna gives nearly omnidirectional pattern at different operating frequencies in both planes. It is found that the peak antenna gains at the operating frequency bands are 4.203, 2.8, and 2.46 dB, respectively. As described above, the proposed antenna has been demonstrated to basically satisfy the needs for RFID readers. In this section the impact of the handset on specific absorption rate (SAR) is studied. The SAR quantifies the power absorbed per unit mass of tissue. This quantity is defined as: SAR=

σ 2ρ

Ei

2

(2)

Where Ei is the max value of the electric field strength in the tissue in V/m, σ is the conductivity of body tissue in S/m, and ρ is the density of body tissue in kg/m3 . The SAR limit specified in IEEE C95.1: 2005 has been updated to 2 W/kg over any 10-g of tissue [20]. This new SAR limit specified in IEEE C95.1: 2005 is comparable to the limit specified in the International Commission on Non-Ionizing Radiation Protection (ICNIRP) guidelines [21]. The tissue that it contained has a relative permittivity and conductivity as shown in the Table 2. These tissueequivalent dielectric parameters were chosen according to [21] for simulating hand tissue at operating frequency band. Fig. 7 (a, b, and c), shows the SAR in hand at different frequency bands of 915 MHz, 2.45 GHz, and 5.8 GHz respectively. It is found that the highest spatial-peak SAR over 10-g is 0.114 W/kg at the highest resonance frequency of 5.8 GHz, but for middle frequency band at 2.45 GHz the SAR is decreased to 0.0716 W/kg compared to 0.0543 W/kg for the lowest frequency band at 915 MHz. It is noted that, the resulting SAR values for all cases are under the limits set by IEEE C95.1: 2005 or ICNIRP standards [20, 21].

1 0.9 0.8

Efficiency ()

0.7 0.6 0.5 0.4 0.3 0.2

Antenna only Antenna with cover Antenna with cover and hand

0.1 0

0

1

2

3 4 Frequency (GHz)

5

6

7

Fig. 5. The antenna efficiency comparison of the antenna only, covered antenna with a dielectric case, and the covered antenna in the presence of the human-hand.

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x-z plane

y-z plane

(a) f = 915 MHz

(d) f = 915 MHz

(b) f = 2.45 GHz

(e) f = 2.45 GHz

(c) f = 5.8 GHz

(f) f = 5.8 GHz

Fig. 6. The radiation patterns of the designed antenna at different operating frequencies. Eθ antenna only E φ antenna only Eθ antenna with cover and hand E φ antenna with cover and hand

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Table 2. The tissue-equivalent dielectric parameters at different frequencies. operating frequency 915 MHz

Palm (Skin) Dry Permittivity Conductivity (S/m) 41.329 0.87169

spatial-peak SAR over 10-g (W/kg) 0.0543

2.45 GHz

38.007

1.464

0.0716

5.8 GHz

35.114

3.717

0.114

(a) f = 915 MHz

(b) f = 2.45 GHz

(c) f = 5.8 GHz

Fig. 7. SAR in hand at different frequency bands.

III.

CONCLUSIONS

A compact dual bow-tie slot antenna fed with a coplanar waveguide with three metal strip pairs is designed for tri-band RFID applications. The antenna dimensions are optimized using CFO-NM algorithm which has been implemented in MATLAB and linked to the CST Microwave Studio simulator to simulate the antenna. The proposed antenna presented adequate matching and quite stable omnidirectional patterns in the selected operating frequencies even in the presence of the human-hand with high efficiency and low SAR value. These characteristics are acceptable not only for RFID systems but also for several multiband applications. Good agreement between the results of EM Simulator CST Microwave Studio and that produced by FDTD program written with MATLAB has been achieved.

ACKNOWLEDGEMENT We would like to acknowledge the Electronics Research Institute (ERI), Microstrip Department for the support, encouragement, help and cooperation during simulation of this research.

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