INTERNATIONAL JOURNAL OF ELECTRONICS AND COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET) ISSN 0976 6464(Print) ISSN 0976 6472(Online) Volume 3, Issue 3, October- December (2012), pp. 303-313 IAEME: www.iaeme.com/ijecet.asp Journal Impact Factor (2012): 3.5930 (Calculated by GISI) www.jifactor.com IJECET I A E M E DESIGN OF RADIATING-EDGE GAP-COUPLED BROADBAND MICROSTRIP ANTENNA FOR GPS APPLICATION Rahul T. Dahatonde 1, Shankar B. Deosarkar 2 1. Assistant Professor, Electrical Engineering, Sardar Patel College of Engineering, Andheri (W), Mumbai, INDIA. 2. Professor & Head, Dept. of E & TC, Dr. Babasaheb Ambedkar Technological University, Lonere, Mangaon, INDIA. ABSTRACT This paper discusses design and testing of gap-coupled broadband Microstrip antenna (MSA) for Global Positioning System (GPS) application. A simple Rectangular Microstrip Patch Antenna (RMSA) was designed and tested at GPS frequency of 1.57 GHz. This RMSA was found to have bandwidth (BW) of 26 MHz. The bandwidth of this RMSA was increased up to 35.5 MHz, by placing two parasitic patches along both the radiating edges of this RMSA. Both the MSA configurations were simulated using Zeland s MoM based EM Simulation Package IE3D. The simulation results were experimentally verified by fabricating these configurations using FR4 substrate. The gap-coupled MSA yielded better gain and 36% more BW than basic RMSA. The other performance parameters of the RMSA, such as return loss, VSWR and input impedance were also improved in the proposed design. The effects of finite ground plane on the performance of gap coupled MSA were also studied and experimentally verified. It was found that, gap coupled MSA with finite ground plane performs similar to MSA with infinite ground plane and achieves 33% over all size reduction. Keywords: Rectangular Microstrip Antenna, gap-coupled Microstrip antenna, Bandwidth Enhancement, Gain Enhancement, Finite Ground plane, Size reduction. I. INTRODUCTION These days, MSAs are widely used in many applications due to their inherent advantages such as low profile, light weight, planer configuration and ease of fabrication. However, main limitation of MSAs is their inherently narrow bandwidth (BW) [1]. Most of the Wireless Communication Applications need antenna with broad bandwidth. Therefore, most of the recent research activities in MSA are aiming towards development of 303
MSAs with wide impedance BW without sacrificing return loss of the antenna. Many such techniques are proposed in literature [2]. However, techniques such as use of an electronically thick substrate also introduce a large inductance due to increased length of the probe feed, resulting in a maximum BW of less than 10% of the resonance frequency. Also, though the designs consisting of stacked patches yield little higher bandwidths (10% to 20% of the resonance frequency); these designs are complex for fabrication. The easiest way to increase BW of MSA would be to place a parasitic patch near the radiating patch. This patch is placed sufficiently close to the active patch so that it gets excited through the coupling between the two patches. Both the patches are designed such that their resonance frequencies are close to each other, yielding broad BW. The overall input VSWR is superposition of the responses of both the patches resulting in broad bandwidth [1]. In this paper, we have presented a MSA with two parasitically coupled patches placed along both the radiating edges of simple RMSA. This configuration yielded approximately 30% more BW than simple RMSA resonating at the same frequency. Both these MSA configurations, (i) simple RMSA and (ii) MSA with gap-coupled parasitically excited patches placed along radiating edges of RMSA, were designed, simulated and tested. It was observed that the second configuration, yielded considerable improvement in BW without much sacrifice on other performance parameters of MSA such as, return loss, VSWR and its input impedance. II. DESIGN OF RMSA According to Transmission Line Model, MSA is represented as two slots separated by a transmission line. The Microstrip separates two dielectrics, i.e. air and substrate. Hence most of the electric field lines reside inside the substrate and some extend to air. This transmission line cannot support pure TEM mode of propagation since the phase velocities would be different in air and the substrate. Hence, effective dielectric constant must be obtained in order to account for fringing fields. The value of effective dielectric constant is less than dielectric constant of the substrate, because the fringing fields around the periphery of the patch are not confined in the dielectric substrate, but are also spread in the air. The value of this effective dielectric constant is given by [4], ε eff = ( ε + 1) ( ε 1) 12h 2 2 where, ε eff is effective dielectric constant and r and width of the substrate, respectively. r + 1 r 1 2 + W ε, h, W represent dielectric constant, height For RMSA to be an efficient radiator, W should be taken equal to a half wavelength corresponding to the average of the two dielectric mediums (i.e., substrate and air) [1] c W = ε r +1 2 f 0 2 304
The fringing fields along the width can be modeled as radiating slots increasing electrical length of patch than physical length. This increase in length is given as, W ( ε eff + 0.3) + 0.264 h L = 0.412h W ( ε 0.258) + 0.813 eff h Thus at resonance frequency, effective length of the patch is, L e = L + 2 L From these equations, dimensions of RMSA for GPS application frequency of 1.57 GHz were obtained. The optimized length and width of the RMSA was found to be 58 mm and 44.5 mm, respectively. This design was simulated using Zeland s MoM based EM Simulation Package, IE3D [5]. For simulations, the FR4 substrate with dielectric constant of 4.47 with thickness of 1.59 mm was considered. This patch was fed by a 50Ω coaxial feed line. The feed point location was optimized using IE3D for better performance of this RMSA. Figure 1-a indicates that the minimized value of return loss, 15.73 db, occurs at resonance frequency of 1.57 GHz. The impedance BW of this RMSA is around 26 MHz. Figure 1-b shows that the VSWR BW of this RMSA is around 25 MHz which is very close to impedance BW of RMSA. At resonance frequency of 1.57 GHz VSWR is almost 1, which shows close to perfect matching of antenna with the feed line. Figure 1-a. Return loss Vs Frequency for RMSA (Simulated) Figure 1-b. VSWR Vs Frequency for RMSA (Simulated) From Smith Chart obtained using software IE3D, the simulated value of input impedance for this RMSA was found to be 39.1Ω. III. Design of Gap-coupled RMSA [6-8] The RMSA designed in earlier section can be used for GPS application. However, it has comparatively narrow BW. The BW of this RMSA can be increased by placing either one or two parasitic patches along one or both of the radiating edges of the active patch with 305
a small spacing between them [1]. The edges along the width and length of the RMSA are known as radiating and non-radiating edges, respectively. The parasitic patches get excited due to coupling with fringing fields along the width of the active rectangular patch. Using commercially available software IE3D various configurations of MSA with gap coupled parasitic patches, were analyzed. It was observed that performance of MSA with two gap coupled patches along both the radiating edges is better than MSA with one gap coupled patch along one of the radiating edge of MSA. The further details about these observations are discussed in the section of results and discussion. Based on these observations, a MSA with two gap coupled patches placed along both the radiating edges of RMSA was proposed. The dimensions of the RMSA were kept same as that of RMSA discussed in earlier section. The length and width of parasitically coupled patches was kept as 10 mm and 58 mm, respectively. The spacing between fed RMSA and parasitic patches on both the sides was kept 5.75 mm. Keeping the substrate parameters same as RMSA, this modified design was analyzed using IE3D. It can be seen from Figure 2-a that, for gap coupled MSA, the return loss is almost 37 db, at resonance frequency of 1.57 GHz, is much better than simple RMSA, indicating better radiation from proposed configuration. The impedance BW of this configuration is around 35.5 MHz. Figure 2-b shows that the VSWR BW of this configuration is 33.21 MHz which is very close to its impedance BW. At resonance frequency of 1.57 GHz VSWR is almost 1, which shows close to perfect matching of antenna with the feed line. Figure 2-a. Return loss Vs Frequency for Gap-coupled MSA (Simulated) Figure 2-b. VSWR Vs Frequency for Gap-coupled MSA (Simulated) From Smith Chart obtained using software IE3D, the simulated value of input impedance for this gap coupled MSA was found to be 49.87Ω. IV. EXPERIMENTAL VERIFICATION Both the MSA configurations discussed in earlier sections were fabricated on FR4 substrate, since it s easily available and not much expensive for experimental purpose. Figures 3-a and 3-b, show the photographs of fabricated RMSA and gap coupled MSA, respectively. 306
Figure 3-a. Photograph of fabricated RMSA Figure 3-b. Photograph of fabricated gap coupled MSA These fabricated antennas were tested on Agilent Marconi Scalar Network Analyzer 6204 available at Antenna Laboratory of Dr. Babasaheb Ambedkar Technological University, Lonere, Maharashtra. Figure 4-a shows measured values of return loss Vs frequency for RMSA. It can be seen that the measured value of impedance BW for RMSA is 25.82 MHz which is very close to simulated value of 26 MHz. Figure 4-b, shows the measured value of VSWR Vs frequency. It can be observed that the measured value of VSWR BW of RMSA is 27.12 MHz. Figure 4-a. Return loss Vs Frequency of RMSA (Measured) Figure 4-b. VSWR Vs Frequency of RMSA (Measured) Figure 5-a and 5-b, show the measured values of return loss Vs frequency and VSWR Vs frequency, respectively, for gap coupled MSA. It can be observed that the measured value of impedance BW and VSWR BW for gap coupled MSA is exactly same, i.e. 31.1 MHz. This value is slightly less than simulated value due to practical constraints during fabrication of MSA. 307
Figure 5-a. Return loss Vs Frequency for Gap coupled MSA (Measured) Figure 5-b. VSWR Vs Frequency for Gap coupled MSA (Measured) The value of input impedance for simple RMSA and gap coupled MSA was measured to be 40.10Ω and 51.23Ω, respectively from the Smith Chart shown in Figure 6-a and b, respectively. It can be observed that the measured values of input impedance for both the MSAs are fairly close to the simulated values. Figure 6-a. Smith Chart of RMSA (Measured) Figure 6-b Smith Chart of gap-coupled MSA (Measured) Table 1 summarizes simulated and measured values of various performance parameters for these two configurations of MSA. It can be observed that the simulated and measured values of all the performance parameters of MSA for both the configurations are matching fairly well. Also the gap coupled MSA shows considerable improvement in impedance BW over RMSA. Other performance parameters such as return loss, VSWR and input impedance are also improved. 308
Table 1. Summary of simulated and measured values of various performance parameters of RMSA and gap coupled MSA MSA Configuration Return Loss (db) at 1.57 GHz VSWR At 1.57 GHz Input Impedance (Ω) At 1.57 GHz Impedance Bandwidth (MHz) Simulated Measured Simulated Measured Simulated Measured Simulated Measured Basic RMSA -15.74-17.12 1.39 1.31 39.10 40.10 26.00 25.82 MSA with gap-coupled parasitic patches -36.98-36.36 1.02 1.03 49.87 51.23 35.50 31.10 V. EFFECT OF FINITE GROUND PLANE For all the simulations and measurements discussed in earlier sections, we have considered MSAs with infinite ground plane, because the transmission line model used for designing basic RMSA is based on assumption of infinite ground plane [1]. However, in practice, MSAs are designed and fabricated with ground plane of sufficiently larger size but still this is not infinite. It is proved in [1] that when the size of the ground plane is greater than the patch dimensions by approximately six times the substrate thickness all around the periphery, the results are similar to that of the infinite ground plane. The advantage of having finite ground plane for MSA is that the over all size of MSA reduces since the dimensions of finite ground plane are comparatively smaller than that of (assumed to be) infinite ground plane. So as to reduce the size of the gap coupled MSA shown in Figure 3-b, another configuration of gap coupled MSA with finite ground plane was designed and fabricated. The photograph of this gap coupled MSA with finite ground plane is shown in Figure 7. Figure 7. Photograph of fabricated gap coupled MSA with finite ground plane The length and width of the finite ground plane are chosen as 115 mm and 100 mm, respectively. Thus for the same gap coupled MSA, the area of finite ground plane is 33% less than infinite ground plane. The dimensions of fed and parasitic patches and location of 309
feed point are not change. This gap coupled MSA with finite ground plane was analyzed using IE3D and the simulation results were verified experimentally. The measured values of return loss Vs frequency, VSWR Vs frequency and Smith Chart are shown in Figure 8-a, -b, - c, respectively. Figure 8-a. Return loss Vs Frequency for gap coupled MSA with finite ground plane (Measured) Figure 8-b. VSWR Vs Frequency for gap coupled MSA with finite ground plane (Measured) Figure 8-a and b show that the measured value of return loss and VSWR for gap coupled MSA with finite ground plane, at the resonance frequency of 1.57 GHz is 29.04 db and 1.07, respectively. Its impedance BW and VSWR BW are exactly equal to 31.80 MHz. Figure 8-c. Smith Chart for gap coupled MSA with finite ground plane (Measured) The Smith Chart in Figure 8-c shows that the measured value of input impedance for gap coupled MSA with finite ground plane, at resonance frequency of 1.57 GHz is 47.5 Ω. The simulated and measured values of various performance parameters of gap coupled MSA with finite and infinite ground plane are summarized in Table 2. 310
Table 2. Summary of simulated and measured values of performance parameters of gap coupled MSA with finite and infinite ground plane MSA Configuration Gap-coupled MSA with infinite ground plane Gap-coupled MSA with finite ground plane Return Loss (db) at 1.57 GHz VSWR at 1.57 GHz Input Impedance (Ω) at 1.57 GHz Impedance Bandwidth (MHz) Simulated Measured Simulated Measured Simulated Measured Simulated Measured -36.98-36.36 1.02 1.03 49.87 51.23 35.50 31.10-31.12-29.04 1.09 1.07 48.73 47.50 32.00 31.80 It can be observed from Table 2, that if dimensions of finite ground plane are chosen properly then the overall size reduction of MSA can be obtained without much sacrifice on performance parameters of MSA, such as return loss, VSWR, input impedance and BW. VI. RESULTS AND DISCUSSION In order to increase BW of RMSA, its various configurations with one or two parasitic patches along one or both of the radiating and non-radiating edges of the active patch with a small spacing between them, were analyzed using software IE3D. It was observed that, if the parasitic patches are placed along the non-radiating edges, the field coupling between fed patch and parasitic patch is very small, since the field variation along non-radiating edge of MSA is sinusoidal. Therefore, to achieve better coupling, the spacing between fed patch and parasitic patch has be sufficiently small. If the parasitic patches are placed along the radiating edges of MSA, the coupling between fed patch and parasitic patches is better since the field is uniform along the radiating edges of the MSA. In this case, the parasitic patches get excited due to coupling with fringing fields along the width of the active patch. Therefore, a configuration with parasitic patches along the radiating edges was chosen. When one parasitic patch is placed along one of the radiating edges of the RMSA, the BW of the antenna increases. However, the radiation pattern is not symmetrical with respect to the broadside direction, since the radiation from parasitic patch shifts beam maxima to the side where it s placed. If MSA configuration is made geometrically symmetric with two parasitic patches along the two radiating edges of RMSA, the radiation pattern becomes symmetric in the broadside direction. In this configuration, since both the parasitic patches are on the opposite sides of the fed patch, they shift the beam maxima in the direction opposite to each other. The overall pattern of three patches will be the superposition of the individual pattern, and hence it will remain symmetrical with the broadside direction. Due to addition of two parasitic patches along with single rectangular patch, size of gap coupled MSA is more than that of simple RMSA. However, due to increase in effective 311
aperture of MSA, its gain increases. Thus the gap coupled MSA gives better gain and BW than that of simple RMSA. However, increase in gain could not be verified experimentally, since gain measurement facilities are very expensive and were not available at the time of experimentation. In order to reduce over all size of antenna, effects of finite ground plane on the performance of gap coupled MSA were studied and experimentally verified. It was observed that software IE3D assumes infinite ground plane by default since it s based on Method of Moment (MoM) which considers only infinite ground plane. Therefore, during simulations, with infinite ground plane, software performs meshing only on radiating patch and simulates it. Therefore, the simulation time for gap coupled MSA with infinite ground plane is small compared to simulation time for gap coupled MSA with finite ground plane. In case of the MSA with finite ground plane, the ground plane with desirable dimensions is to be defined in the software, which is treated as a patch on the opposite side of the actual radiating patch. Therefore, during simulation, meshing is performed on radiating patch as well as on the finite ground plane and both are simulated together, hence simulation time increases. Also due to this, there are back lobes in the radiation pattern of MSA with finite ground plane. However, advantage of finite ground plane is reduction in overall size of MSA compared to MSA with infinite ground plane. CONCLUSION A RMSA and a gap coupled MSA with finite and infinite ground plane was designed and analyzed using software IE3D. It was observed that the theoretical results obtained using IE3D are in good agreement with measured results, for all three configurations of MSA. It was observed and experimentally verified that the gap coupled MSA yielded better gain and 36% more BW than basic RMSA. The other performance parameters of the RMSA, such as return loss, VSWR and input impedance were also improved in the proposed design. The effects of finite ground plane on the performance of gap coupled MSA were also studied and experimentally verified. It was found that, gap coupled MSA with finite ground plane performs similar to MSA with infinite ground plane and achieves 33% over all size reduction. REFERENCES [1] Kumar G. and Ray K.P., Broadband Microstrip Antenna, Artech House, 2003. [2] Kin Lu Wong, Compact and Broadband Microstrip Antennas, John Wiley & Sons, 2002. [3] Bhartia P., Millimeter-Wave Microstrip and Printed Circuit Antennas, Artech House, 1991. [4] Balanis C.A., Antenna Theory Analysis and Design,2nd Edi.,John Wiley & Sons,pp.730-750,1997. [5] IE3D 12.0, Zeland Software Inc., Fremont, CA, USA, 2008. [6] Kumar, G., Broadband Microstrip Antennas Using Coupled Resonators, Ph.D.thesis, Indian Institute of Technology, Kanpur, India, 1982. [7] Kumar, G., and K. C. Gupta, Broadband Microstrip Antennas Using Additional Resonators Gap-Coupled to the Radiating Edges, IEEE Trans. Antennas Propagation, Vol. AP-32, December 1984, pp. 1375 1379. 312
[8] Kumar, G., and K. C. Gupta, Nonradiating Edges and Four Edges Gap-Coupled with Multiple Resonator, Broad Band Microstrip Antennas, IEEE Trans. Antennas Propagation,Vol. AP-33, February 1985, pp. 173 178. [9] Varun Shukla, Arti Saxena and Swati Jain, A New Rectangular Dielectric Resonator Antenna Compatible For Mobile Communication Or Broadband Applications International journal of Electronics and Communication Engineering &Technology (IJECET), Volume3, Issue2, 2012, pp. 360-368, Published by IAEME [10] Jagadeesha.S, Vani R.M and P.V Hunugund, Size Reduction And Multiband Operation Of Rhombusshaped Fractal Microstrip Antenna For Wireless Applications International journal of Electronics and Communication Engineering &Technology (IJECET), Volume3, Issue2, 2012, pp. 445-450, Published by IAEME [11] Nagraj Kulkarni and S. N. Mulgi, Corner Truncated Inverted U - Slot Triple Band Tunable Rectangular Microstrip Antenna for Wlan Applications International journal of Electronics and Communication Engineering &Technology (IJECET), Volume3, Issue1, 2012, pp. 1-9, Published by IAEME [12] B.Ramarao, M.Aswini, D.Yugandhar and Dr.P.V.Sridevi, Dominant Mode Resonant Frequency Of Circular Microstrip Antennas With And Without Air Gap International journal of Electronics and Communication Engineering &Technology (IJECET), Volume3, Issue1, 2012, pp. 111-122, Published by IAEME [13] P.A Ambresh and P.M.Hadalgi,, Slotted Inverted Patch - Rectangular Microstrip Antenna For S And L - Band Frequency International journal of Electronics and Communication Engineering &Technology (IJECET), Volume1, Issue1, 2010, pp. 44-52, Published by IAEME Author Biography Rahul T. Dahatonde, received his B. E. and M. Tech. degrees in Electronics & Telecom. Engineering in the year 2001 and 2003 from North Maharashtra University, Jalgaon and Dr. B. A. Technological University, Lonere, respectively. He is currently a faculty in Dept. of Electrical Engineering at Sardar Patel College of Engineering, Mumbai and is pursuing his Ph.D. from Dr. B.A.T.U., Lonere in the area of Microstrip Patch Antennas. His research interests include antennas, microwaves and EMI/EMC. He has published around 10 research papers in various international and national journals/conferences. He is life member of ISTE, India. Dr. Shankar B. Deosarkar, obtained his M.E. degree in 1990 and Ph.D. in 2003 from S.R.T. Marathwada University, Nanded, India. He is a Professor and Head of the E & TC Department at Dr. Babasaheb Ambedkar Technological University, Lonere. Currently he is on lien and is associated with V. P. College of Engineering, Baramati as Principal. He has teaching experience of over 25 years. He has also worked as the Controller of Examinations of the Dr. B. A. T. U. Lonere from 1990 to 2007. His research interests include antennas, microwaves, EMI/EMC and signal integrity issues in high speed circuits. He has around 50 research papers in various international and national journals/conferences to his credit. Dr. Deosarkar is Fellow of IETE and life member of ISTE, India. At present four research scholars are pursuing Ph.D. under his guidance. 313