Cross polarisation reduction of a circular polarised ... · rectangular microstrip antenna with L...

Cross polarisation reduction of a circularpolarised microstrip antenna with two L

slot DGS for wireless applications

Mrs.Anita R1, Dr.Chaitanya Kumar M.V2,1Assoc. Professor, 2Dean

1Dept. of ECE, 2School of Engineering andTechnology.

1East Point College of Engineering and Technology2CMR University1,2Bangalore,India.

1Ani [email protected]@yahoo.com

November 1, 2018

Abstract

This paper is to design a circular polarized antenna bytruncating a pair of corners of rectangular microstrip an-tenna. The corner truncated rectangular microstrip antennais designed at an operating frequency of 2.4 GHz. To reducethe cross polarization level of circular polarized antenna,two L slot defective ground structures are introduced on theground plane. Simulation of the corner truncated rectangu-lar micostrip antenna with and without defective groundstructures(DGS) is carried out using High frequency struc-ture simulator. The performance of the antenna is anal-ysed in terms of return loss, bandwidth, gain and VSWR.The simulation results have shown that the corner trun-cated rectangular microstrip patch antenna with two L slotDGS has reduced the cross polarization of antenna whichmakes it suitable for wireless applications. The return loss

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International Journal of Pure and Applied MathematicsVolume 120 No. 6 2018, 1173-1188ISSN: 1314-3395 (on-line version)url: http://www.acadpubl.eu/hub/Special Issue http://www.acadpubl.eu/hub/

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of the corner truncated rectangular microstrip antenna is-35.53 dB and the bandwidth is obtained as 85 MHz.Thebandwidth of the corner truncated rectangular microstripantenna is improved to 120 MHz by inserting two L slotDGS on the ground plane.

Key Words::Rectangular microstrip, Defective groundstructures(DGS),L slot

1 Introduction

There is a great demand of microstrip antennas which has have beenused widely in wireless communication due to their light weight, lowprofile, low cost and ease of fabrication and excellent compatibilitywith planar integrated circuits and even non planar surfaces.Dueto the demand of the small systems that has increased in recentyears, small size antennas at low frequency have drawn interestfrom researchers[1].The evolution of modern wireless communica-tion systems has increased dramatically the demand for antennas,capable to be embedded in portable devices . The need for smaller,broadband and reliable antennas has been fully demonstrated incurrent advancements in communication industry and significantgrowth in wireless communication market and consumer demand[2], [3]. The disadvantages of microstrip patch antennas are narrowbandwidth, low efficiency, large ohmic loss, capability of handlinglow RF power and low gain[4].There are many different techniquesto improve the performance of the microstrip patch antenna . Onesuch technique is defective ground structure(DGS)[5]. DGS can berealized by introducing a shape defect on a ground plane that dis-turbs the shielded current distribution depending on the shape anddimension of the defect [6].Many techniques have been developedto broaden the impedance bandwidth of microstrip patch anten-nas.In recent years,circularly polarized (CP) antennas have receivedmuch attention for wireless applications.The most common polar-ization scheme in wireless communication system is circular polar-ization..The circularly polarized antenna could have many differenttypes and structures,where the basic operation principle is to radi-ate two orthogonal field components with equal amplitude but inphase quadrature.[7]For circular polarization antenna, patch anten-nas are preferred owning to its characteristics of wide bandwidth,

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high gain, low profile and low cost.The two conventional ways togenerate circularly polarised radiation are truncating a pair of cor-ners and cutting a diagonal slot on a rectangular microstrip antenna[8]. Rectangular microstrip antenna (RMA) primarily linearly po-larized fields along its broadside direction. But some degree ofcross-polarized (XP) radiation takes place,which is more significantin H-plane than in E-plane [9], [10].The H-plane XP radiations aredue to higher-order orthogonal modes[11]. Several techniques toimprove XP radiation from RMA is reported in [12][15]. Thesetechniques include stacked patches with dual-polarized aperture-coupled microstrip antenna, T-shaped microstrip line with annularring slot, and rat race feeding technique with differential feed sys-tem. Since all these structures follow complex feeding methods,thesimplest method to improve XP radiation is the employment of de-fective ground structures. [16]. In this proposed work,a circularpolarised antenna is designed by truncating a pair of corners ofrectangular microstrip antenna.The cross polarization level of cir-cular polarised antenna is reduced by inserting two L-slot DGS isintroduced on the ground plane .The corner truncated rectangularmicrostrip antenna is designed and simulated with and without twoL slot DGS.The analysis of the antenna is carried out in terms ofreturn loss,bandwidth,gain and VSWR.A.Proposed design of corner truncated rectangualr mi-crostrip antennaThe corner truncated rectangular microstrip antenna is designedat an operating frequency of 2.4 GHz.The substrate material usedis FR4 epoxy with a dielectric constant of 4.4.The patch is de-signed with a width of 26.71 mm and a length of 24.84mm.Thecorner of the rectangular microstrip is truncated with a length of L=5mm.The ground plane is designed with a width of 59.5mm anda length of 64.5mm.The simulation is carried out using HFSS.Thefront view of simulated corner truncated rectangular microstrip an-tenna is shown in fig1 and the back view of the rectangular mi-crostrip antenna is shown in fig2.

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Fig1.Front view of Simulated corner trunated rectangularmicrostrip antenna

Fig 2.Back view of rectangular microstrip antenna

B.Proposed design of corner truncated rectangualr mi-crostrip antenna with two L slot DGSThe corner truncated rectangular microstrip antenna is introducedwith two L slot DGS on the ground plane.The L slot is designedwith a length of 28.98mm and a width of 2mm.The simulated rect-angular microstrip antenna with two L slot DGS is shown in fig3.

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Fig 3. Simulated two slot DGS on the ground plane

2 RESULTS AND DISCUSSION

The corner truncated rectangular microstrip antenna is designedand simulated using HFSS. HFSS is a finite element method solverfor electromagnetic structures.The corner truncated rectangular mi-crostrip antenna is designed with a width of 26.71mm and a lengthof 24.84mm.The ground plane is designed with a width of 59.5 mmand a length of 64.5mm.The substrate material used is FR4 epoxywith a dielectric constant of 4.4.The substrate thickness is consid-ered as 1.6mm.The feed for the antenna is a microstrip line feedwith a length of 22.65 mm and a width of 1.2 mm. The simulatedcorner truncated rectangular microstrip antenna with two L slotDGS is shown in fig4.

Fig4.Simulated rectangular microstrip with two L slot DGS

The simulated results of the return loss of corner truncated rectan-gular micrsotrip antenna is shown in fig5.

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Fig5.Return loss plot of corner truncated rectangular microstripantenna

From fig.5,we can observe that the corner truncated rectangularmicrostrip antenna provides circular polarization with the excita-tion of two orthogonal modes.The return loss is obtained as -35.53dB.The bandwidth of the corner truncated rectangular microstripantenna is obtained as 85 MHz. Fields due to higher order TM02mode are well known source of the XP radiations for the rectangularpatch. There is a distribution of modal fields due to the dominantand the first higher order mode under a rectangular patch and theTM02 mode is responsible for creating XP fields. Fig6.shows thesimulated results of the radiation pattern of corner truncated rect-angular microstrip antenna.

Fig6.Radiation pattern of corner truncated rectangular microstripantenna

From fig6,we can observe that the maximum gain is obtained as3.1073dB with respect to E plane and 3.1089 dB with repect to Hplane .The beamwidth in E plane is 92.27380◦ and the beamwidth

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in H plane is 87.82240◦. Fig7.shows the 3D radiation pattern ofcorner truncated rectangular microstrip antenna.

Fig7.3D radiation pattern of corner truncated rectangularmicrostrip antenna

From fig7,we can observe that the gain of the corner truncatedrectangular microstrip antenna is 3.1738 dB. Fig8.shows the returnloss plot of corner truncated rectangular microstrip antenna withtwo L slot DGS.

Fig 8.Return loss plot of corner truncated rectangular microstripwith two L slot DGS.

From fig8,we can observe that the return loss is obtained as -30.88dB.The bandwidth of the corner truncated rectangular microstripwith two L slot DGS is obtained as 120 MHz .From the above re-sults,we can observe that there is an improvement in bandwidthwith L slot DGS when compared with a corner truncated rectan-gular microstrip antenna without DGS. When DGS is introduced

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on the ground plane , there is a disturbance in the shielded currentdistribution which depends on the shape and dimension of the de-fect..There is an improvement in bandwidth because the shieldedcurrent distribution will influence the input impedance and cur-rent flow of the antenna. Fig9.shows the return loss plot of cornertruncated rectangular microstrip antenna with and without DGS.

Fig9.Return loss plot of corner truncated rectangular microstripantenna with and without DGS.

Fig 10.Cross pol and Copol pattern of Corner truncatedrectangular microstrip antenna

Fig 10 shows the crosspol and copol pattern of corner truncatedrectangular microstrip antenna.From the fig,we can observe thatthe cross polarization level is obtained as -16.63dB.

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Fig 11.Cross pol and Copol pattern of Corner truncatedrectangular microstrip antenna with two L slot DGS

Fig11.shows the crosspol and copol pattern of corner truncatedrectangular microstrip antenna with two L slot DGS.From fig,wecan observe that the cross polarization level is reduced to -27.49dB.When slot defects have been introduced at the position of nonradiating edges, there is a suppression of XP radiation . Due tothis,there is an improvement in co-XP ratio and by inserting twoL slot DGS on the ground plane,the cross polarization level of thecircular polarised antenna is reduced to a considerable level. Fig12.Shows the axial ratio Vs frequency plot of corner truncated rectan-gular microstrip antenna.

Fig12.Axial ratio Vs frequency plot of corner truncatedrectangular microstrip antenna.

From fig12,we can observe the axial ratio Vs frequency plot of cor-ner truncated rectangular microstrip antenna.The axial ratio is ob-tained as 1.6229 dB which shows that the corner truncated rect-angular microstrip antenna is a circular polarised antenna. Fig13shows the axial ratio Vs frequency plot of corner truncated rectan-gular microstrip antenna with two L slot DGS.

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Fig13.Axial ratio Vs frequency plot of corner truncatedrectangular microstrip antenna with L slot DGS.

From fig13,we can observe that the axial ratio is obtained as 0.4040dB for a corner truncated rectangular microstrip antenna with twoL slot DGS.Since the cross polarization levels are reduced by in-serting two L slot DGS,the axial ratio is reduced compared to thecorner trunated rectangular microstrip antenna without any defecton the ground plane.

Fig13..Radiation pattern of corner truncated rectangularmicrostrip with two L slot DGS

Fig13.shows the radiation pattern of corner truncated rectangularmicrostrip with two L slot DGS. From fig 13,we can observe theradiation pattern of corner truncated rectangular microstrip withtwo L slot DGS.From the above results,we can observe that thetotal gain is 3.5205 dB . Fig14.shows the 3D radiation pattern ofCorner truncated rectangular microstrip with two L slot DGS.

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Fig 14.3D radiation pattern of corner trunated rectangularmicrostrip with two L slot DGS

From fig 14,we can observe the 3D radiation pattern of corner trun-cated rectangular microstrip wth two L slot DGS.The gain of thecorner truncated rectangular microstrip with two L slot DGS isobtained as 3.599 dB. Fig 15.shows the simulated results of theelectric field Distribution of the corner truncated rectangular mi-crostrip with two L slot DGS.

Fig 15.Electric field distribution of Corner truncated rectangularmicrostrip with two L slot DGS

The simulated VSWR of the rectangular microstrip antenna withtwo L slot DGS is shown in fig16.

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Fig16.Simulated VSWR of rectangular microstrip antenna with Lslot DGS

From fig16,we can observe that there is a shift in resonant frequencyfrom 2.4 to 2.44 GHz due to the ntroduction of the defect in theground plane.The VSWR is obtained as 1.1187. The simulatedVSWR of rectangular microstrip antenna without DGS is shown infig17.

Fig17..Simulated VSWR of rectangular microstrip antennawithout DGS

From fig17,we can observe that there is no shift in frequency whenthere is no defect on the ground plane and the VSWR is obtained as1.882 dB. Table1. shows the simulated results of return loss,bandwidthand Gain of a corner truncated rectangular microstrip with andwithout two L slot DGS.

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Table1.Simulated results of return loss,bandwidth,gain,VSWR

From table 1,we can observe that the performance of the antenna isimproved by introducing two L slot DGS on the ground plane.Thegain of the antenna is improved from 3.173 to 3.599 dB and thebandwidth is enhanced from 85 to 120 MHz.The rectangular mi-crostrip antenna with T slot DGS that operates at a resonant fre-quency of 2.45GHz has a bandwidth of 87 MHZ and the returnloss is -15.10 dB [17].Also a rectangular microstrip antenna withDGS has a bandwidth of 78.84 MHz with a return loss of -26.0135dB.[18] But by inserting two L slot DGS on the ground plane inour work,the bandwidth is improved to 120 MHz.

3 Conclusion

In this paper,a circular polarized antenna is designed at an op-erating frequency of 2.4 GHz by truncating a pair of corners ofrectangular microstrip antenna.The return loss for the corner trun-cated rectangular micrstrip antenna is obtained is -35.53 dB andthe bandwidth is obtained as 85 MHz.The cross polarization of thecircular polarized antenna is reduced by introducing two L slot DGSon the ground palne.The introduction of L slot DGS reduces theXP component to a considerable level when compared with a cor-ner truncated rectangular microstrip antenna.The return loss is ob-tained as -35.53 dB and the bandwidth is obtained as 120MHz.Thisshows that there is an improvement in bandwidth of corner trun-cated rectangular microstrip antenna with two L slot DGS whencompared to a corner truncated rectangular microatrip without Lslot DGS.This circular polarized antenna is well suited for wirelessapplications.

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