|Year : 2010 | Volume
| Issue : 6 | Page : 373-379
Electrically Small Patch Antenna Loaded with Metamaterial
JG Joshi, Shyam S Pattnaik, Swapna Devi, MR Lohokare
Department of ETV and ECE, National Institute of Technical Teachers' Training and Research, Sector-26, Chandigarh - 160 019, India
|Date of Web Publication||27-Jan-2011|
J G Joshi
Department of ETV and ECE, National Institute of Technical Teachers' Training and Research, Sector-26, Chandigarh - 160 019
| Abstract|| |
In this paper, the authors present a new planar metamaterial loaded electrically small microstrip patch antenna. The rectangular microstrip patch antenna is loaded by metamaterial using planar square spilt ring resonators (SRRs). The unloaded rectangular microstrip patch antenna resonates at 23 GHz, whereas after loading it by metamaterial, the same antenna resonates at 9.51 GHz. In loaded condition, the resonant frequency of rectangular microstrip patch antenna reduces due to magnetic coupling. The dimensions of the antenna structure are 0.161λ × 0.192λ. Using the Chu limit, the size of the antenna comes to ka = 0.775, thus, satisfies the condition of an electrically small antenna (ka < 1). The antenna exhibits an impedance bandwidth of 512 MHz at a resonant frequency of 9.51 GHz. The calculated radiation Qrad is 18.86 which is larger than Qchu minimum, that is Qchu = 3.43. The gain and directivity of this antenna structure are 3.2 dBi and 7.8 dBi, respectively. The proposed antenna structure is simulated on a dielectric substrate with coaxial fed without incorporating any additional matching network.
Keywords: Chu limit, Electrically small antennas, Magnetic coupling, Negative permeability, Planar metamaterial, Q-factor
|How to cite this article:|
Joshi J G, Pattnaik SS, Devi S, Lohokare M R. Electrically Small Patch Antenna Loaded with Metamaterial. IETE J Res 2010;56:373-9
| 1. Introduction|| |
In the growing era of wireless and mobile communication technologies, there exists a great prospective to electrically small antennas (ESAs) with size considerably lesser than usual half wavelength for applications like mobile handsets, sensor network, biomedical, wearable and RFID systems. In 1947, Wheeler defined an ESA whose maximum dimensions can fit inside a radiansphere that is an imaginary sphere of radius equal to λ/2π (λ is free space wavelength)  . This indicates that the sphere must enclose the maximum dimensions of an antenna. This is more explicitly expressed by Equation (1) as
where , and a = radius of sphere enclosing maximum dimensions of an antenna.
The other important characteristic of an ESA is the quality factor Q. Chu derived a fundamental relationship between the size of antenna and quality factor, referred as Chu limit , . This limit implies the minimum quality factor Q to be attained for an antenna of size ka  . ESA suffers from low radiation efficiency due to small radiation resistance, optimistically large bandwidth due to small Qrad and need of matching network to match the impedance ,,, . Antenna engineers are striving to overcome these limitations by using external matching network consisting of passive lumped elements, optimization of antenna topology, etc. It is a challenging task to achieve a better trade off between gain, bandwidth, impedance matching and radiation efficiency of an ESA. Recently, metamaterial loading has emerged as an innovative approach to improve the performance of ESAs ,,,,,, .
In 1967, Veselago discussed an interesting material having negative permeability (μ) and permittivity (ε) termed as metamaterials  . Positive permeability and permittivity are the basic properties of conventional materials available in nature called as double positive (DPS) materials. But metamaterial possesses negative permeability (μ) and/or negative permittivity (ε). Hence, metamaterials are termed as double negative (DNG) or single negative materials (SNG). In single negative metamaterial, the permeability (μ) negative materials are mu negative (MNG) and permittivity (ε) negative materials are termed as epsilon negative (ENG) materials. Metamaterial structure consists of split ring resonators (SRRs) to produce negative permeability and thin wire elements to generate negative permittivity. These materials have negative refractive index that is a reversal of Snell's law, hence, called as negative index materials (NIM). Due to negative refractive index, the group and phase velocities of electromagnetic wave appear in opposite direction such that the direction of propagation is reversed with respect to the energy flow direction. It also exhibits the reversal of Doppler's shift ,,, . These interesting properties of metamaterial play a prominent role in designing antenna structures to enhance their gain, directivity and bandwidth, while satisfying the condition of miniaturization.
Richard W. Ziolkowski and his group proposed the work on the dipole antenna enclosed in a double negative (DNG) or a single negative (SNG) metamaterial spherical shell in the year 2005  . In 2006, the same group had reported a metamaterial-based efficient ESA using an infinitesimal electric dipole with ENG spherical shell  . In their proposed work, the radiated power and gain of the dipole antenna is considerably increased. In 2008, Aycan Erentok and Richard W. Ziolkowski reported a planar two-dimensional metamaterial inspired ESA to achieve the advantages of simple fabrication, low cost and ease of testing. Due to these numerous advantages, they referred this as EZ antennas having high radiation efficiencies and good impedance matching  . Howard R. Stuart and Alex Pidwerbetsky, in 2006, reported ESA elements using negative permittivity resonators  . In 2007, Kamil Boratay Alici and Ekmel Ozbay presented an electrically small SRR antenna using monopole and circular SRRs having low profile, high gain, radiation efficiency of 43.6% and larger estimated radiation Q than the minimum Q Chu limit. It is reported that the MNG materials are good candidates to obtain ESAs , . Duan et al. (2009) reported an ESA inspired by spired SRR of size ka = 0.6745 by Chu limit having high gain, low cost and good relative bandwidth  . Pattnaik et al. presented a dual band antenna which is a hybrid structure of planar rectangular microstrip slotted antenna and planar metamaterial rectangular SRR to achieve multiresonance and high gain  .
In the present paper, the authors have proposed a planar metamaterial loaded electrically small microstrip patch antenna. A planar metamaterial square SRR (MNG structure) is used to load the electrically small microstrip patch antenna. In the proposed antenna structure, the inductance of rectangular microstrip patch antenna is magnetically coupled to the MNG structure that is SRRs. The entire antenna dimensions are fitted inside a radian sphere of radius "a".
This paper is systematically organized into four sections as follows. The detailed geometrical structure and design of the electrically small rectangular microstrip patch antenna is presented in Section 2. In Section 3, the unit cell of square-shaped SRRs and its metamaterial characteristics are discussed. The results of ESA structure are also presented and analyzed. Finally, the paper is concluded in Section 4.
| 2. Antenna Design|| |
[Figure 1] and [Figure 2] respectively depicts the geometry and cross-sectional view of the proposed electrically small rectangular microstrip patch antenna. This is the composite structure of planar rectangular microstrip patch antenna and square-shaped SRRs. In this composition, the rectangular microstrip patch antenna is loaded by planar metamaterial SRRs made up of square-shaped SRRs. The rectangular microstrip patch antenna is placed at the distance d = 0.50 mm from the square SRRs. The dimensions of the rectangular patch antenna are length Lr = 5 mm and width Wr = 0.5 mm which is excited by the coaxial feed at x = −3.2 mm and y = −2.2 mm. This antenna structure is designed and simulated on RT Duriod 5880 substrate having thickness h = 3.175 mm and dielectric constant εr = 2.2. The dimensions of the square SRR structure are Ls = 5 mm, the gap at the split of the rings is g = 0.2 mm, the separation between inner and outer rings is s = 0.2 mm and the width of the rings is w = 0.2 mm. This structure is simulated using method of moment based IE3D electromagnetic simulator of Zeland Software Incorporation, Fremont, USA.
|Figure 1: Geometrical structure of electrically small rectangular microstrip patch antenna loaded with planar metamaterial SRRs.|
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Initially, an unloaded rectangular microstrip patch antenna is designed and simulated, which has a resonating frequency of f r = 23 GHz. The antenna is then loaded with the proposed SRR and the wavelength of the loaded structure is calculated to be 31.54 mm. Therefore , = 0.775 < 1, which satisfies the condition that the proposed antenna structure is an ESA ,,,,, .
The proposed ESA structure is a combination of rectangular microstrip patch and square SRR unit cell. In this structure, the microstrip patch is loaded with SRR metamaterial. Initially, the metamaterial characteristics of the square SRR unit cell are verified. In loading condition, the rectangular microstrip patch is excited by coaxial feed. The SRR unit cell is closely placed near to the rectangular microstrip patch. Hence, due to magnetic coupling, the electric field gets induced in the SRR unit cell and both the inner and outer split rings get excited. This excitation makes the SRR unit cell to exhibit the metamaterial characteristics. After loading the patch antenna by metamaterial square SRR unit cell, the resonant frequency of the rectangular microstrip patch is reduced, thus making the entire structure as an ESA.
In the past, Smith et al.  proposed a metamaterial structure using FR4 substrate. In this work, a copper wire of width 0.14 mm was used along with the split ring, whereas no such wire is used in the proposed structure. It is purely planar square SRR. Hence, it is a quite different SRR geometry.
| 3. Results and Discussion|| |
In this section, the metamaterial characteristics of proposed square-shaped SRR are verified. The square SRR unit cell consists of inner and outer square SRRs. Both the rings are separately excited using coaxial feed at each split ring. This is similar to coax feeding of a microstrip line. The central conductor is connected to one side of the strip at the split and the outer of the connector is connected to ground.
The results of rectangular microstrip patch antenna with unloaded and loaded conditions are presented, compared and analyzed. [Figure 3]a and b shows the reflection coefficient (S11 ) and transmission coefficient (S21 ) characteristics, respectively, of the square SRRs. [Figure 3]b shows the zoomed resonance behavior of the structure at 4-6 GHz band. It shows that the SRRs resonate at 5 GHz.
|Figure 3: (a) S-parameters (S011) and (S21) of metamaterial square-shaped SRRs (b) zoom of S-parameters.|
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The effective medium theory is used to extract the permeability (μr) and permittivity (εr) from the reflection and transmission coefficient parameters (S-parameters) using Nicolson-Ross-Weir (NRW) approach ,,. The expressions of Equations (2) and (3) are used to determine the effective parameters.
In this work, IE3D is used as a simulator to get the S-parameters, and subsequently using the mathematical equations and MATLAB code, the metamaterial characteristics have been verified.
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. The values of V1 and V2 are estimated using Equations (4) and (5).
In SRRs, the factor k0d = 0.33, which is <<1 ,.
This structure exhibits real negative permeability (μr) as shown in [Figure 4] which indicates that the SRR structure is single negative that is mu negative (MNG) metamaterial.
|Figure 4: Real part of extracted permeability (μr) for square-shaped SRRs.|
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The value of permeability (μr ) is negative in the frequency range of 4.8-5.3 GHz. From [Figure 3]a and b, strong reflectivity is observed near the resonant frequency of SRRs, that is at 5 GHz in the range of 4.8-5.3 GHz. In the same frequency band, the magnetic permeability [Figure 4] is negative, thus exhibiting negative refractive index. The square split ring structure used to load the rectangular microstrip patch antenna reveals metamaterial behavior.
The return loss (S11 ) characteristic of an unloaded rectangular microstrip patch antenna is shown in [Figure 5]. The unloaded rectangular microstrip patch antenna resonates at fr = 23 GHz.
|Figure 5: Return loss (S11) characteristics of unloaded rectangular microstrip patch antenna.|
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[Figure 6] shows the return loss (S11 ) characteristics of an electrically small rectangular microstrip patch antenna loaded with square-shaped SRRs. The loaded antenna structure resonates at 9.51 GHz with a bandwidth of 512 MHz (−10 dB). The fractional bandwidth (FBW) is 5.37%.
It is well known that for an ESA, radiation quality factor is a fundamental interest. Chu derived a theoretical relationship of minimum quality factor (Qchu ) in terms of an antenna and is re-validated by Mclean which is expressed as Equation (6) ,,.
|Figure 6: Return loss characteristics ( S11) of loaded rectangular microstrip patch antenna.|
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Using this formula, the minimum radiation factor for the proposed antenna structure is calculated as Qchu = 3.43. The radiation Q (Qrad ) of the ESA should be adequately large that is greater than 10 (Qrad > 10) ,, . The Qrad is derived from the bandwidth using Equation (7).
The estimated radiation factor Qrad = 18.86, which is much larger than the minimum radiation factor (Qmin = 3.43). Thus, a practically realizable bandwidth is achieved in the proposed ESA structure.
In composite structure that is after loading the microstrip patch antenna, due to electromagnetic induction, the time varying flux induces the current on the square-shaped SRRs. This current is responsible to induce large electric field across the gap capacitance at the splits and mutual capacitance between the outer and inner split rings. The inductance of rectangular microstrip patch antenna with the mutual inductance and the capacitance of SRRs form the LC resonator circuit of the loaded antenna structure.
Due to magnetic coupling, the resulting resonant frequency of the loaded microstrip patch antenna becomes lower than the resonance frequency of single uncoupled resonator and is controlled by the capacitance developed at the SRRs  . Thus, the rectangular microstrip patch antenna resonates at 9.51 GHz that is lowered down from 23 GHz. The metamaterial SRRs and rectangular microstrip patch antenna strongly resonate at the shifted resonant frequency. [Figure 7] shows the variation of Rin with frequencies. It is seen that 50 Ω matching is reached at 9.51 GHz. The feed point location selected for coax feeding provides a good impedance matching, therefore additional matching networks like capacitive loading, reactance and resistive matching network, quarter-wavelength transformer, admittance (J)-inverter arrangement to match the impedance ,,,, are not required.
|Figure 7: Input impedance characteristics of metamaterial SRR loaded ESA.|
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[Figure 8]a and b respectively shows the elevation and azimuth radiation patterns, of the proposed electrically small rectangular microstrip patch antenna loaded with metamaterial. Gain of 3.2 dBi with directivity of 7.8 dBi is achieved. The radiation efficiency of the ESA is 42%.
|Figure 8: Radiation patterns of electrically small rectangular microstrip patch antenna loaded with metamaterial square SRRs (a) elevation (b) azimuth.|
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| 4. Conclusion|| |
In this paper, a metamaterial square SRR (MNG structure) loaded electrically small (λ/6.2 × λ/5.2) rectangular microstrip patch antenna operating at 9.51 GHz is presented. This antenna structure is miniaturized by loading with metamaterial square split ring structure. The size of the designed antenna is ka = 0.775 by using Chu limit. This is a complete planar antenna structure with gain of 3.2 dBi and impedance bandwidth of 512 MHz that is of 5.3%. In this structure, good impedance matching is achieved without incorporating any additional impedance matching network. A high directivity of 7.8 dBi is also achieved at 9.51 GHz. Thus, metamaterial loading is an ideal approach for the size reduction of antenna. This type of antenna due to its reduced size, considerable good bandwidth, with reasonable better gain and with high directivity, will find its application in wearable devices, handheld devices and in mobile communication. In future, the authors intend to fabricate array of such unit cells and test the array experimentally to evaluate the performance.
| 5. Acknowledgments|| |
The authors sincerely express their gratitude to the anonymous reviewers for their valuable comments. 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, for sponsoring him to pursue the full time Ph.D. under AICTE sponsored QIP (POLY) scheme.
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| Authors|| |
J. G. Joshi received B.E. 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. S.S. Pattnaik and Dr. Swapna Devi. His research interests include Microstrip Patch and Planar Metamaterial antennas.
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. 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 Ph. Ds. Eight Ph.D. students and four M.E. Students are presently pursuing their thesis under the guidance of Prof. (Dr.) S.S. Pattnaik.
Swapna Devi received Ph.D in Engineering from Tezpur University in the year 2008. M.E. degree from Regional Engineering College (Presently NIT), Raurkala, Orissa, in 1997. B. Tech degree in Electronics and Communication Engineering 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 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 conference. Dr. Swapna Devi is a life member of ISTE, member of IEEE and IETE. She has completed 03 sponsored projects.
M. R. Lohokare received the 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. S.S. Pattnaik and Dr. Swapna Devi. His research interests include Soft computing, Microstrip antenna, and video processing.
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8]
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