JOURNAL OF TELECOMMUNICATIONS, VOLUME 7, ISSUE 1, FEBRUARY 2011

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Metamaterial Embedded Electrically Small Planar Loop Antenna J.G. Joshi, Shyam S. Pattnaik, S. Devi and M.R. Lohokare Abstract—This paper presents electrically small planar square loop antenna loaded with embedding metamaterial multiple split ring resonator (MSRR). The unloaded planar loop antenna resonates at 10 GHz and after loading with MSRR it resonates at 5.58 GHz. Acccoring to Chu limit, size of the proposed antenna is ka = 0.948<1 which satisfies the condition for electrically small antenna that is ka <1. The bandwidth and gain of this antenna is 303 MHz (-10dB) and 4.80 dBi respectively in the small size with the directivity of 7 dBi. The size of this antenna is 0.214 λ × 0.214 λ. The estimated radiation Q factor (Qrad) is 18.43 which is much larger than the Q minimum that is Qchu = 2.22. Index Terms— Electrically small antenna (ESA), Chu limit, negative permeability, multiple split ring resonator (MSRR), metamaterial

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1 INTRODUCTION

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ITH the rapid escalation of wireless and mobile communication technology various applications like RF ID, PDAs, BANs, WLANs etc. have become part and parcel of everyday activities of human life [1]. The size of equipments mainly handheld equipments is reducing due to advancement in technology so as the case for antenna. But with size reduction, the demands to communicate voice, data and multimedia information at high data rates are also increasing. This is a great challenge for microwave researchers to design miniaturized antenna that can congregate gain, bandwidth and efficieny requirements of the antenna to support the particular application. Electrically small antennas (ESAs) are proved to be good candiadates for such applications [2-9]. These antennas can easily be integrated in a printed chip with rest of the circuitary of the equipment. Wheeler defined electrically small antennas (ESAs) as; whose maximum dimensions can fit inside a radiansphere that is an imaginary sphere of radius equal to λ/2π (where λ is the free space wavelength) [10]. It is experessed by the condition ka<1; where k = 2π / λ and a = the radius of sphere enclosing maximum dimensions of the antenna [10]. Further, Chu derived a theorotical relationship which implies the relation between size of the antenna (ka) and the minimum quality factor to be attained by the antenna [11]. It is also revalidated by Mclean [12]. ————————————————

• J.G. Joshi is with the Department of Electronics and Communication, National Institute of Technical Teachers’ Training and Research, Chandigarh, India. E-mail: [email protected] • Shyam S.Pattnaik is with the Educational Televison Centre, National Institute of Technical Teachers’ Training and Research, Chandigarh, India. E-mail: [email protected]. • S.Devi is with the Department of Electronics and Communication, National Institute of Technical Teachers’ Training and Research, Chandigarh, India. E-mail: [email protected]. • M.R. Lohokare is with the Department of Electronics and Communication National Institute of Technical Teachers’ Training and Research, Chandigarh, India. E-mail: [email protected].

The limitations of ESAs are narrow impedance bandwidth, low radiation efficiency and gain [6-8]. Various techniques like slot cutting, shorting pin, and meandering have been employed to overcome these limitations but the performance get affected by one or the other way. Metamaterials with elecromagnetic properties of negative permittivity (ε) and/or negative permeability (µ) [13] have attracted the attention of microwave researchers to improve the performance of microstrip patch antennas by loading them with metamaterials [8, 14-15]. Metamaterials are prominently used to reduce the size as well as to enhance the performance parameters of microstrip patch antennas like bandwidth, gain and efficiency. In recent times, antenna designers are using metamaterial based ESAs in different RF communication applications [2-9]. R.W. Ziolkowski et al. demonstrated different configurations of electrically small antennas using metamaterials [2, 3, 7]. ESA is also presented using SRR unit cell satisfying the condition ka = 0.904 [5]. Z. Duan exemplified the SRR based electrically small antenna of size ka = 0.6745 [6]. In 2010, J.G. Joshi et al. presented a planar metamaterial loaded electrically small rectangular microstrip patch antenna [8]. Recently, electrically small wired monopole antenna is presented using metamaterial MSRR and split ring resonator (SRR) [9]. Filiberto Bilotti et al. designed the metamaterial multiple split ring resonator (MSRR), spiral resonator (SR) and labyrinth resonator (LR) which exhibit the negative permeability in microwave and millimeter wave frequency range [16]. In line to their previous research work on metamaterial loaded electrically small rectangular microstrip patch antenna [8], in this paper the authors present a metamaterial MSRR embedded electrically small planar loop antenna for WLAN applications. It is useful for HYPERLAN/2 in the frequency band of 5.470 GHz to 5.725 GHz. Remaining part of the paper is organized into four sections. The detailed geometrical structure and design of the metamaterial MSRR embedded planar square loop antenna is proposed in section 2. The metamaterial characteristics of MSRR are presented in section 3. By using equivalent circuit theory resonant frequency of the MSRR

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unit cell is found to be in good aggrement with the simulated results. The results of unloded and loaded conditions of the planar square loop antenna are presented and discussed in this section. In particular, the quality factor, bandwidth and efficiency of the loaded planar loop antenna are calculated and analyzed. Finally, the paper is concluded in section 4.

2 ANTENNA DESIGN Fig.1 illustrates the sketch and geometrical structure of metamaterial MSRR embedded electrically small planar square loop antenna.

Planar square loop antenna WL

MSRR

GL

In unloaded situation, the planar loop antenna resonates at 10 GHz. Further, to reduce the resonant frequency of loop antenna keeping the dimensions same, it is loaded with embedding the metamaterial MSRR inside it. In loaded condition the wavelength is calculated to be 53.76 mm at its resonant frequency is 5.58 GHz. Therefore, by Chu limit ka = 0.948<1; which satisfies the condition that in loaded configuration the square loop antenna is electricaly small antenna. The complete dimensions of proposed loop antenna are fitted inside a radian sphere of radius ‘a’. In loading condition the resonant frequency of loop antenna is reduced to 5.58 GHz. This makes an entire structure as an electically small antenna. Initially, metametrial chracteristics of the MSRR are verified. In loaded condition, the MSRR get excited because it is magnetically coupled with the loop antenna. This makes the MSRR to exhibit metamaterial characteristics.

g

b Ground plane

a

c Lm h y

εr

e

Substrate z

x

Fig.1. Sketch and geometrical structure of metamaterial MSRR embedded electrically small planar square loop antenna.

This antenna is an integrated structure of planar square loop antenna and metamaterial MSRR unit cell. In this antenna, the MSRR unit cell is embedded inside the square loop antenna. The dimensions of the planar loop antenna are; width WL = 0.625 mm, length a=b=c=11.50 mm respectively and length e=4.25 mm. The gap between loop antenna and the MSRR is GL = 0.13 mm. The electrical dimension of the proposed antenna is 0.214 λ × 0.214 λ.The finite ground plane of size 11.50 mm × 11.50 mm is used for this antenna. Fig. 2 depicts the geometrical structure of MSRR unit cell. The number of rings inthe MSRR are N = 12. The dimensions of MSRR are; length of the outermost split ring Lm = 10 mm, width of the ring (w), gap at the splits (g) and separation between the two adjacent rings (s) are in the order of w= g = s = 0.2 mm respectively. The dimensions of the MSRR are 0.055 λ (λ is the free space wavelength at lowest resonant frequency 1.70 GHz of the MSRR). The antenna structure is designed on RT Duriod 5880 substrate of thickness h = 3.175 mm, dielectric constant (εr) = 2.20 and simulated using method of moment based IE3D electromagnetic simulator. The loop antenna is co-axially fed at x = 2.4 mm and y = -5.4 mm whereas the other end of the loop is grounded.

w

s

Fig.2. Geometrical structure of multiple split-ring resonator (Number of rings N = 12).

3 RESULTS & DISCUSSION Fig. 3 depicts the reflection coefficient (S11) and transmission coefficient (S21) characteristics of the MSRR. It resonates at 1.70 GHz, 2.68 GHz and 3.44 GHz respectively. The MSRR unit cell is placed between the transmitting and receiving planar monopole antennas as suggested in [16-17] to get the S-parameters. The distance between these two antennas is kept fixed. The magnetic field produced in the near zone by the antenna excites the MSRR. In this work, IE3D simulator is used to obtain the Sparameters. The obtained S-parameters and subsequently using the mathematical equations (1) and (2) the metamaterial characteristics of MSRR have been verified. The effective medium theory is used to extract the permeability (µ) and permittivity (ε) from the reflection and transmission coefficient parameters (S-parameters) using Nicolson-Ross-Weir (NRW) approach [8, 18-19]. The expressions of equations (1) and (2) are used to determine the effective parameters.

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µr = εr =

2 1 − V2 jk 0 d 1 + V2

(1)

2 1 − V1 jk 0 d 1 + V1

(2)

where k0 is wave number and d is the thickness of substrate, V1 and V2 are the composite terms to represent the addition and subtraction of S- parameters. In MSRR the factor k0d = 0.113 which is <<1 [8, 18-19] The values of V1 and V2 are estimated using equations (3) and (4).

V1 = S 21 + S11

(3)

V2 = S 21 − S11

(4)

Fig. 4 signifies the relative permeability (µr) characteristics

of the MSRR. From this figure strong matching is observed near the respective resonant frequencies of the MSRR. In the same frequency bands the magnetic permeability (Fig.4) is negative therefore; it exhibits the negative refractive index. Thus, the MSRR structure which is embedded inside the planar loop antenna reveals the metamaterial behavior. Using the principle of equivalent circuit theory, the MSRR is modelled as a LC resonant circuit. The values of equivalent inductance (L) and capacitance (C) are calculated by using mathematical equations [16, 20]. Thus, the estimated values of equivalent circuit elements are L = 28.4 nH and C = 0.320 pF. Theoretically, by using the values of L and C in equation (5) the resonant frequency (fr) of MSRR is calculated as 1.68 GHz.

fr = 1

2π LC

(5)

The simulated resonant frequency of an isolated MSRR structure is fr = 1.70 GHz (Fig. 3) which is in good agreement with the theoretical results.

Fig.3.Reflection coefficient (S11) and transmission coefficient (S21) characteristics of MSRR.

Fig.5. Return loss (S11) characteristics of unloaded planar square loop antenna.

Fig.6. Return loss (S11) characteristics of metamaterial MSRR embedded electrically small planar square loop antenna.

Fig. 4. Relative permeability (µr) characteristics of MSRR as a function of frequency.

Fig.5 shows the return loss (S11) characteristics of unloaded planar square loop antenna that resonates at 10 GHz. Hence, to lower down the resonant frequnecy of the proposed loop antenna in same dimensions it is loaded with the MSRR unit cell.

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Fig. 6 depicts the return loss (S11) characteristics of the metamaterial MSRR loaded electrically small planar loop antenna. In this configuration, the loop antenna resonates at 5.58 GHz with the bandwidth and gain of 303 MHz (-10 dB) and 4.80 dBi respectively. The performance of ESA is analyzed by its radiation quality factor. Chu therotically derived the mathematical expression of fundamental limit of quality factor Qchu [1112]. It is expressed by equation (6).

1   1 Qchu =  3 3 +  ka  k a

(6)

The estimated minimum quality factor using equation (6) is Qchu =2.22. The fractional bandwidth of the proposed antenna is 0.054. The radiation quality factor Qrad of the antenna is expressed by equation (7) and it is to be greater than 10 (Qrad>10). Mathematically, it is derived from bandwidth of the antenna and expressed by equation (7).

Qrad =

1 BW

(7)

tance of MSRR unit cell form the LC resonator circuit. Due to magnetic coupling the resonant frequency of loaded planar loop antenna becomes lower than the resonanace frequency of single uncoupled resonator that is square loop antenna and controlled by the capacitance associated with the MSRR [8, 21]. Thus, the planar loop antenna and MSRR strongly resonates at the shifted lower resonant frequency maintaining the same dimensions. Fig. 7 shows the input impedance characteristics of metamaterial MSRR embedded electrically small planar loop antenna. In this antenna, the selected co-axial feed point is matched to 50 Ω without incorporating any impedance matching network. The efficiency curve of the metamaterial MSRR embedded electrically small planar loop antenna is shown in Fig.8. The radiation and antenna efficiency at the resonant frequency 5.58 GHz is 60% which is considerably good in such a compact size antenna. The directivity of this antenna is 7 dBi. Fig. 9 (a) and Fig.9 (b) respectively shows the azimuth and elevation radiation patterns of the proposed metamaterial MSRR embedded planar square loop antenna.

From equation (7) the calculated radition quality factor is Qrad = 18.43 which is much larger than the mimimum limit i.e. Qchu. Thus, a practically realizable bandwidth is achieved in the proposed electrially small loop antenna. The loop antenna has a large inductance. The loop antenna is magnetically coupled to the MSRR hence large voltage get induced across the gap capacitance at the splits and distributed capacitance between the rings of MSRR. The inductance of loop antenna and the capaci-

Imaginary part

Fig.8. Efficiency curve of metamaterial MSRR embedded electrically small planar square loop antenna.

Real part Fig.7. Input impedance (Zin) characteristics of metamaterial MSRR embedded electrically small planar square loop antenna.

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4 CONCLUSION This paper presents a planar electrically small square loop antenna. Metamaterial embedding is an advantegeous approach to miniaturization as well as it leads to achieve better bandwidth, efficiency, reasonable gain and directivity. Though the proposed antenna is electrically small there is appropriate balance between the radiation quality factor and bandwidth which is realizable. Because of these advantages the proposed antenna may find its application in WLAN systems. For further studies, by varying the number of MSRR rings the antenna can be used for different applications.

ACKNOWLEDGMENT The support of Director, National Institute of Technical Teachers’ Training and Research (NITTTR), Chandigarh, India is thankfully acknowledged. J. G. Joshi is highly indebted to Director, Directorate of Technical Education (M.S.), India and Principal, Government Polytechnnic; Pune (M.S.), India for sponsoring him to pursue full time Ph.D. under AICTE sponsored QIP (POLY) scheme.

REFERENCES (a)

(b) Fig. 9. Radiation patterns of metamaterial MSRR embedded electrically small planar square loop antenna (a) azimuth (b) elevation.

[1]

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[12] J.S. Mclean, “A Re-Examination of the Fundametal Limits on the Radiation Q of Electricallly Small Antennas,” IEEE Trans. on Antennas and Propagation, vol. 44, no.5, pp.672-676, 1996. [13] V.G.Veselago, “The Electrodynamics of Substances with Simultaneously negative ε and µ,” Sov. Physics USPEKHI, vol.10, pp. 509-514, 1968. [14] F. Bilotti, A.Alu, and L. Vegni, “Design of Miniaturized Metamaterial Patch Antenna with µ- negative Loading,” IEEE Trans. on Antennas and Propagtion, vol. 56, no. 6, pp. 1640-1647, June 2008. [15] P.Y.Chen and A. Alu, “Dual-Band Miniaturized Elliptical Patch Antenna with µ-Negative Metamaterials,” IEEE Antennas and Propagtaion Letters, vol.9, pp.351-354, 2010. [16] F. Bilotti, A. Toscano, L. Vegni, K. Aydin, K. B. Alici and E. Ozbay, “Equivalent-Circuit Models for the Design of Metamaterials Based on Artificial Magnetic Inclusions,” IEEE Trans. on Microwave Theory and Techniques, vol. 55, no.12, pp.2865-2873, 2007. [17] K. Aydin, I. Bulu, K. Guven, M. Kafesaki, C. M. Soukoulis and E. Ozbay, “Investigation of Magnetic Resonances for Different Split-Ring Resonator Parameters and Designs,” New J. of Physics, vol.7, pp.1-15, 2005. [18] D.R. Smith, D.C. Vier, Th. Koschny and C.M. Soukoulis, “Electromagnetic Parameter Retrieval from Inhomogeneous Metamaterials,” Physical Review, E 71, pp.036617-1-036617-10, 2005. [19] R.W. Ziolkowski, “Design, Fabricaton, and Testing of Double Negative Metamaterials,” IEEE Trans.on Antennas and Propagation, vol.51, no.7, pp. 1516-1529, July 2003. [20] J.G.Joshi, Shyam S. Pattnaik, S.Devi and M.R.Lohokare, “Extended Rectangular Split Ring (ERSR) Planar Metamaterial Antenna,” Proc. of 12th International Symposium on Microwave and Optical Technology (ISMOT-2009), New Delhi, India, pp.261-264, December 2009. [21] J. Garcia-Garcia, J. Bonache, I. Gil, F. Martin, M. del Castillo Velazquez-Ahumada and J. Martel,”Miniaturized Microstrip and CPW Filters using Coupled Metamaterial Resonators,” IEEE Trans. on Microwave Theory and Techniques, vol. 54, no.6 pp. 2628-2635, June 2006. J.G. Joshi received B.E.degree in Electronics & Telecommunication Engineering from Amravati University, Amravati, M. S. (Electronics & Control) from Birla Institute of Technology and Science, Pilani (Raj.), India in 1994 and 1996 respectively. Since 2008 he is pursuing Ph.D. under AICTE- MHRD-Govt. of India sponsored QIP (Poly) scheme at NITTTR, Chandigarh, India under the guidance of Prof. (Dr.) S.S. Pattnaik and Dr.Swapna Devi. His research interests include microstrip patch and planar metamaterial antennas. Dr. Shyam S. Pattnaik received Ph.D. degree in Engineering from Sambalpur University, India in 1992. Joined as a faculty member in the Dept. of Electronics and Communication Engineering at NERIST, India in the year 1991. He worked in the Department of Electrical Engineering, University of Utah, USA under Prof. Om. P. Gandhi. Since 2004, he is working as Professor and Head of Educational Television Center of National Institute of Technical Teachers’ Training and Research, Chandigarh, India. He is a recipient of National Scholarship, BOYSCAST Fellowship, SERC visiting Fellowship, INSA visiting Fellowship, UGC Visiting Fellowship, and Best Paper award etcs. He has been a member of many important committees at national and international level. He is a fellow of IETE, Senior Member of IEEE, life member of ISTE and has been listed in the Who’s Who in the world. He has 177 technical research papers to his credit. He has conducted number of conferences and seminars. His areas of interest are soft computing, information fusion, and their application to virtual learning, antenna design, metamaterial antennas and video processing. He has produced thirty-three M. Tech.

thesis and four PhDs. Eight Ph.D. students and four M.E. students are presently pursuing their thesis under the guidance of Prof. (Dr.) S.S. Pattnaik. Dr. Swapna Devi received Ph.D in Engineering from Tezpur University in the year 2008. M.E. degree from Regional Engineering College (Presently NIT), Raurkela, Orissa, in 1997. B. Tech degree in Electronics and Communication from NERIST, Arunachal Pradesh, in 1994. In 1997, she joined the Deptt. of Radiology, University of Utah as Research Assistant. On returns to India, she joined NERIST, India as a Lecturer in the Deptt. of Electronics and Communication and subsequently become a faculty in the Deptt. of Computer Science and Engineering in same Institute in 1999. She is an Associate Professor in the Deptt. of Electronics and Communication Engineering at National Institute of Technical Teachers’ Training and Research, Chandigarh, India. Her interests include, Medical Image Processing and Soft computing Techniques. She has contributed 130 technical papers in various journals and conferences. Dr. Swapna Devi is a life member of ISTE, member of IEEE and IETE. Since 2004, she has completed 03 sponsored projects. M.R. Lohokare received Bachelor degree in Electronics Engineering from Shivaji University, India in 1993 and M Tech. in Digital Communication, from Barkatullah University, Bhopal, India in 1999. Since 2008 he is pursuing Ph.D. under AICTE- MHRD-Govt. of India sponsored QIP (Poly) scheme, at NITTTR, Chandigarh, India under the guidance of Prof. (Dr.) S.S. Pattnaik and Dr. Swpana Devi. His research interests include Soft computing, Microstrip antenna, and video processing.