Stacked Configuration of Rectangular and Hexagonal Patches with Shorting Pin for Circularly Polarized Wideband Performance

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1 Cent. Eur. J. Eng. 4(1) DOI: /s Central European Journal of Engineering Stacked Configuration of Rectangular and Hexagonal Patches with Shorting Pin for Circularly Polarized Wideband Performance Research Article Sanyog Rawat 1, K. K. Sharma 2 1 Amity School of Engineering and Technology, Amity University Rajasthan, Jaipur , India 2 Malaviya National Institute of Technology, Jaipur , India Received 15 August 2013; accepted 30 October 2013 Abstract: The radiation characteristics of a stacked microstrip antenna geometry proficient of providing circular polarization along with wide impedance bandwidth is simulated by using IE3D software and later on this antenna was fabricated on FR-4 substrate with an air gap and testing is done in free space. The feed location, location of applied shorting pin and width of air gap introduced between driver element and parasitically coupled element were optimized to obtain best results. The measured impedance bandwidth better than 31.72% and axial ratio bandwidth close to 1.68% were achieved with the proposed geometry. The simulated and measured results obtained are in good match with each other. Keywords: Stacked patches Air gap Shorting pin Return loss Bandwidth and gain Versita sp. z o.o. 1. Introduction Microstrip antennas are finding extensive application in modern communication systems because they offer several advantages over conventional microwave antennas. These antennas are robust structures, have relatively low manufacturing cost, small size, lightweight, can be directly integrated with the other circuit components, have conformability with the hosting surfaces and polarization diversity [1 4]. The miniaturizations in electronic designs have generated remarkable demand for compact and efficient antenna geometries. However, sanyog44@yahoo.com these antennas have several limitations including single frequency operation at dominant mode, low gain, impurity in polarization, poor bandwidth and low radiation efficiency [2]. The bandwidth and gain of antenna may be appreciably enhanced by introducing substrate materials having low permittivity and loss tangent or by putting an air gap between the patch and the ground plane [5, 6]. Numerous such antennas exploiting thick air substrates are described in recent times to achieve broadband operation and circular polarization [7 9]. The miniaturization of antennas size is important for their applications in mobile handsets and other wireless devices. The reduction in overall size can be achieved by using shorting pin [10]. The use of shorting pin or shorted wall to trim down the patch size for particular operating frequency has been observed in past [11, 12]. 20

2 S. Rawat, K. K. Sharma In the present communication, radiation characteristics of a compact stacked configuration of hexagonal and rectangular patches having a shorting pin along with an air gap introduced between driver element and parasitic element is reported. The performance of antenna is tested in free space and compared with that achieved through method of moment based IE3D software. 2. Antenna geometry and results Initially, we considered a simple hexagonal patch antenna having length of each arm equal to 12 mm as shown in Figure 1. The patch lies in XY plane over an infinite ground plane with substrate thickness (h) is very less as compared to free space wavelength (λ 0 ), substrate relative permittivity ε r and relative permeability µ r = 1 as shown in Figure 1. The magnetic field has two components in x and y direction and since the thickness is very less than wave length, the fields variation along the z direction may be considered negligible and the component of the current; orthogonal to the edge of the microstrip antenna vanishes at the boundaries. This shows that the geometry supports TM mn modes [3]. Considering the above mentioned supposition, the structure is assumed as a hexagonal resonant cavity with sidewalls magnetic in nature, top and base sides electric in nature. The hexagonal geometry is than designed and simulated by using IE3D software [13] with relative permittivity ε r = 4.4, substrate thickness h = cm and loss tangent tan δ = The coaxial or probe feed is used in the geometry having 50 ohm characteristic impedance. The simulated variation of reflection coefficient (S 11 ) of this single layer hexagonal geometry with respect to frequency is shown in Figure 2. Within the frequency range of 3 GHz to 5 GHz, this antenna resonates at a single frequency (f r = 3.61 GHz). The simulated VSWR value corresponding to the resonant frequency 3.61 GHz is while the impedance bandwidth of this antenna is nearly 3.5% as shown in Figure 2. The simulated gain of this antenna is around 2.43 dbi while input impedance at the resonance frequency is close to ( j1.806) ohm. As antenna is operating at a single frequency and offering low gain and small bandwidth values, this geometry in its present configuration is unsuitable for applications related to communication systems. This antenna is next modified in steps to obtain improved performance. An additional rectangular patch of dimension (length l = 24 mm and width w = 21 mm is introduced just over the hexagonal patch geometry through Teflon screws as shown in Figure 3. The patch lying on upper substrate is little larger in size than Figure 1. Top and side view of simple hexagonal geometry. Figure 2. Simulated reflection coefficient (S 11 ) of hexagonal antenna geometry as a function of frequency (GHz); Simulated VSWR of hexagonal antenna geometry as a function of frequency (GHz). 21

3 Stacked Configuration of Rectangular and Hexagonal Patches with Shorting Pin for Circularly Polarized Wideband Performance Figure 3. Top and side views of stacked antenna geometry without airgap. that of hexagonal patch and the upper structure does not have any metallic ground plane. The inset feedline is applied on the lower hexagonal patch and in the first step of modification, separation between lower patch (driver element) and upper patch (parasitic patch) d is maintained equal to zero. In this way the upper patch is parasitically coupled to the lower patch or the driver element. The simulated variation of reflection coefficient with respect to frequency is shown in Figure 4 signify that antenna now has resonant frequency of 3.13 GHz. The resonance frequency of this modified antenna is slightly less as compared to hexagonal geometry reported earlier (3.61 GHz) perhaps due to increase of antenna thickness. As shown in Figure 4, the simulated VSWR value at the resonant frequency is 1.08 which shows fine matching between antenna and probe feed. The bandwidth of antenna is now increased to 7.66% which is twice more than that of earlier reported simple hexagonal patch geometry. The simulated gain obtained for the this antenna geometry is 3.25 dbi while input impedance at the resonance frequency is close to (48.04 j3.36) ohm. These results again indicate that this modified patch geometries is still unsuitable for modern communication systems. Hence we separated the two substrate materials in steps and finally distance d = 1.59 mm with Teflon screws is maintained. This separation in lower and upper substrates has reduced the resultant effective permittivity (ε eff ) and loss tangent of substrate material. 3. Modified stacked arrangement with shorting pin and air gap The antenna geometry described in preceding section is further modified in steps by applying a shorting pin in the driven element and by varying airgap d = 1.59 mm between driven and parasitic elements. Both these microstrip elements are designed on a FR-4 substrate and divided through an air gap of thickness 1.59 mm by Figure 4. Variation of simulated reflection coefficient (S 11 ) of stacked geometry without airgap with respect to frequency; Simulated VSWR of stacked geometry without air gap as a function of frequency. using teflon screws at the corner, as shown in Figure 5. With proposed introductions, it is realized that radiation properties of antenna is improved largly. The layout of the design of this proposed antenna geometry is shown in Figure 5 and Figure 5(c). On placing an air gap of 1.59 mm between the two layers of microstrip elements, the simulated resonant frequency of modified stacked geometry is shown in Figure 6 are 3.29 GHz and 4.24 GHz while the measured resonant frequencies of this geometry shown in Figure 6 are 3.41 GHz and 4.40 GHz. It may be observed that the impedance bandwidth of antenna has now approached to GHz (31.72%) corresponding to central frequency of 3.94 GHz. The measured VSWR values at two resonating frequencies are approaching to unity (1.05 and 1.08 respectively) as shown in Figure 7 which signify that still antenna geometry has excellent match with the 22

4 S. Rawat, K. K. Sharma (c) Figure 6. Simulated reflection coefficient (S 11 ) of stacked geometry with air gap h a = 1.59 mm and shorting pin; Measured reflection coefficient (S 11 ) of stacked geometry with air gap h a = 1.59 mm and shorting pin. Figure 5. Side view of stacked antenna structure with shorting pin. Top view of designed rectangular patch. (c)top view of designed hexagonal patch with feed and shorting pin. coaxial feed line. The measured input impedance related to two resonant frequencies are (52.36 j1.61) ohm and (51.38 j3.69) ohm respectively as shown in Figure 7 which are nearly reaching to 50 ohm impedance of the coaxial feed line. The existence of a small loop in the Figure 7 implies the possibility of circular polarization (CP). In order to radiate CP radiations, the presence of two modes which are orthogonal to each other with equal amplitude and phase quadrature are desired. The possible presence of two orthogonal patch modes or for confirmation of circular polarization condition, axial ratio of antenna with frequency is simulated as shown in Figure 8. It is observed that the axial ratio presented by antenna within frequency range to GHz (73 MHz) is well within prescribed 3 db range. The minimum value of axial ratio is db at frequency GHz which indicates pure circular polarization at this frequency. For analyzing the type of polarization, simulated E-plane right and left circularly polarized 23

5 Stacked Configuration of Rectangular and Hexagonal Patches with Shorting Pin for Circularly Polarized Wideband Performance Figure 8. Variation of simulated axial ratio stacked antenna geometry with air gap and shorting pin. Figure 9. E-plane left and right circular polarization patterns at GHz. Figure 7. Measured Voltage Standing Wave Ratio (VSWR) of stacked geometry with air gap h a = 1.59 mm and shorting pin; Measured input impedance of stacked antenna geometry with air gap h a = 1.59 mm and shorting pin. patterns of the stacked geometry at frequency GHz are obtained as shown in Figure 9. The E-plane right circular pattern is nearly 34.3 db down as compared to E- plane left circular pattern. This signify that radiations at frequency GHz is left circularly polarized in nature. The simulated E and H plane elevation radiation patterns of antenna geometry with air gap (h a = 1.59 mm) at five different frequencies within the impedance bandwidth region have been shown in Figure 10 and Figure1 10. It may be seen that the radiation patterns at all these frequencies are perpendicular to the patch and similar in shape. The simulated co and cross polar radiation patterns of antenna geometry at resonant frequencies of 3.29 GHz and 4.24 GHz are presented in Figure 11 and Figure 11 respectively. These patterns clearly shows that copolar elevation patterns are almost 9 db higher than cross polar elevation patterns. The simulated gain of the antenna with respect to frequency is shown in Figure 12 that indicates that in the complete bandwidth; gain is uniform in nature and nearly 6 dbi. The variation in gain at the desired frequency range is well within 1 dbi. 24

6 S. Rawat, K. K. Sharma Figure 11. Variation of two-dimensional elevation patterns at 3.29 and 4.24 GHz. Figure 10. Elevation gain pattern of antenna as a function of elevation angle and frequencies Phi=0 and Phi= Conclusions In this paper the radiation parameters of a stacked configuration of circularly polarized wideband antennas having a shorting pin and air gap between driven element and parasitic element is presented. The designed antenna offers much enhanced impedance bandwidth and gain than a single. The elevation patterns of antenna over complete range of frequencies where broadband response is attained are simulated and the patterns at five frequencies signify that these are nearly similar in nature and maximum intensity is pointing towards normal to the patch in each case. These radiation properties imply that proposed antenna geometry may be a potential candidate for various applications in the field of communication systems. Figure 12. Variation of simulated gain of stacked antenna geometry with air gap and shorting pin. Acknowledgement Authors are grateful to Prof. Deepak Bhatnagar, University of Rajasthan for allowing them to use available IE3D simulation software and measurement facilities at 25

7 Stacked Configuration of Rectangular and Hexagonal Patches with Shorting Pin for Circularly Polarized Wideband Performance his center. Authors would like to also thank Mr. Brajraj Sharma and Mr. Ajay Tiwari from University of Rajasthan for their help and support in testing and measurement of antenna geometry. References [1] Kumar G., Ray K. P., Broadband Microstrip Antennas, Artech House, Boston, 2003, 2-4 [2] Garg R., Bhartia P., Bahl I. Ittipiboon, A., Microstrip Antenna Design Handbook, Artech House Publications, New York, 2001, 2-3 [3] Carver K. R., Mink J. W, Microstrip Antenna Technology. IEEE Transactions on Antenna and Propagation 29 (1), 1981, 2-24 [doi: /TAP ] [4] Carver K. R, Practical Analytical Techniques for the Microstrip Antenna. Proceedings Workshop Printed Circuit Antenna Technology, Mexico State Univversity, Las Cruces, 1979, 1-20 [5] Wong K. L., Compact and Broadband Microstrip Antennas, Wiley Publication, New York, 2003, 2-14 [6] Guha D., Resonant Frequency of Circular Microstrip Antennas with and without air gaps, IEEE Transactions on Antennas and Propagation, 49 (1), 2001, [doi: / ] [7] Yang S. L., Luk K. M, Wideband Folded-Patch Antennas Fed by L-shaped Probe, Microwave Optical Technology Letters, 45 (4), [doi: /mop.20821] [8] Shekhawat S., Sekra P., Bhatnagar D., Saxena V. K., Saini J. S., Stacked Arrangement of Rectangular Microstrip Patches for Circularly Polarized Broadband Performance, IEEE Antennas and Wireless Propagation Letters, 9 (1), 2010, [doi: /LAWP ] [9] Chaimool S., Rakluea C., Chung K. L., Akkaraekthali, P., Single-Feed Circularly Polarized Microstrip Patch Antenna Stacked with Periodic Structure, Microwave Optical Technolgy Letters, 54(1), 2012, [doi: /mop.26495] [10] Yoon C., Choi S. H., Lee H. C., Park H. D., Small Microstrip Patch Antennas with Short-pin Using a Dual Band Operation, Microwave Optical Technolgy Letters, 50 (2), 2007, [doi: /mop.23099] [11] Mishra A., Singh P., Yadav N. P., Ansari J. A., Vishvakarma B. R., Compact Shorted Microstrip Patch Antenna For Dual Band Operation, Progress In Electromagnetics Research C, 9, 2009, [doi: /PIERC ] [12] Singh A. K., Meshram M. K., Slot Loaded Shorted Patch Antenna for Dual Band Operation, Microwave Optical Technolgy Letters, 50, 2008, [doi: /pierc ] [13] IE3D software, Release 14.65, Zeland Software Inc., Freemont, USA 26