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Compensation of dielectric cover effects on cp hexagonal microstrip antenna

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  • 1. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN INTERNATIONAL JOURNAL OF ELECTRONICS AND 0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 1, January- February (2013), © IAEMECOMMUNICATION ENGINEERING & TECHNOLOGY (IJECET)ISSN 0976 – 6464(Print)ISSN 0976 – 6472(Online)Volume 4, Issue 1, January- February (2013), pp. 43-54 IJECET© IAEME: www.iaeme.com/ijecet.aspJournal Impact Factor (2012): 3.5930 (Calculated by GISI) ©IAEMEwww.jifactor.com COMPENSATION OF DIELECTRIC COVER EFFECTS ON CP HEXAGONAL MICROSTRIP ANTENNA Ravindra Kumar Yadav1, Jugul Kishor1 and Ram. Lal Yadava2 1,2 Department of Electronics and Communication Engineering, 1 I.T.S Engineering College, Greater Noida, Uttar Pradesh, India, 2 Galgotias college of Engineering and Technology, Greater Noida, Uttar Pradesh, India ravipusad@gmail.com, er.jugulkishor@gmail.com and rly1972@gmail.com ABSTRACT This communication describes the design and analysis of a dielectric layer loaded circularly polarized (CP) hexagonal patch antenna in the frequency range 2.4000-2.4835 GHz. The obtained results indicate that there are significant changes in the performances of the antenna. In particular the axial ratio at resonant frequency 2.43 GHz is around 1.245 dB followed by the axial ratio bandwidth around 1.41 % hence the proposed antenna confirms the circularly polarized behaviour. Therefore the change in various response parameters due to such loading is compensated by introducing an air gap between the ground plane and the substrate of patch antenna. The thickness of the air gap is chosen such that the shifted responses are brought in the desired range. Due to air gap, the resonant frequency of dielectric loaded antenna shifted from 2.39 GHz to 2.44 GHz which is within the operating range of antenna and other performance characteristics of the antenna like input impedance, VSWR, return loss etc. also get improved, and the impedance bandwidth improved up to around 1.51 %. INDEX TERM - Hexagonal Patch Antenna, Circular Polarization, Superstrate loading I. INTRODUCTION In any communication system, matching the polarization in both the transmitter and receiver antennas is very important in terms of decreasing transmission losses. The use of circularly polarized antennas presents an attractive solution to achieve this polarization match which allows more flexibility in the angle between transmitting and receiving antennas. It also reduces the effect of multipath reflections and enhances weather penetration. Circular 43
  • 2. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 1, January- February (2013), © IAEMEpolarization is beneficial because current and future commercial as well as militaryapplications require the additional design freedom of not requiring alignment of the electricfield vector at the receiving and transmitting locations. Single feed circularly polarizedantennas are currently receiving much attention, because it allows a reduction in thecomplexity, weight and the RF loss of any antenna feed and is desirable in situations where itis difficult to accommodate dual orthogonal feeds with a power divider network. Circularlypolarized microstrip antennas have the additional advantage of small size, weight, suitabilityin conformal mounting and compatibility with microwave and millimeter wave integratedcircuits, and monolithic microwave integrated circuits (MMICS) [1-3]. A single patch antenna can be made to radiate circular polarization if two orthogonalpatch modes are simultaneously excited with equal amplitude and ± 90o out of phase with thesign determining the sense of rotation. A patch with a single point feed generally radiateslinear polarization, however in order to radiate CP, it is necessary for two orthogonal patchmodes with equal amplitude and in phase quadrature to be introduced. This can beaccomplished by slightly perturbing a patch at appropriate locations with respect to the feed.Designing a circularly polarized microstrip antenna is challenging; as it requires acombination of design steps. The first step involves designing an antenna to operate on agiven frequency. However in the second step circular polarization is achieved by eitherintroducing a perturbation segment to a basic single fed microstrip antenna, or by feeding theantenna with dual feeds equal in magnitude with 90° physical phase shift. The shape anddimensions of the perturbation have to be optimized to ensure that the antenna achieves anaxial ratio < 3 dB at the desired design frequency. Various perturbation techniques forgenerating CP have been reported in the literatures, which operate on the same principle ofdetuning degenerate modes of a symmetrical patch by perturbation segments. A well-knownmethod of producing a single feed circular polarization operation of the square microstripantenna by truncating a pair of patch at two opposite corners has also been presented. It isalso found that this method can also be applied to a modified square microstrip patch withfour semi-circular grooves along the four edges of the patch of equal dimensions to achieve aCP operation with compact design along with relaxed manufacturing tolerances. Thecompactness of the proposed CP design is achieved due to the semicircular grooves at thepatch edges of the square patch. It was also found that the required size of the truncatedcorners of CP operation increases with increasing antenna size reduction. This behavior givesthe proposal of designing a relaxed manufacturing tolerance for achieving a compactcircularly polarized microstrip antenna [4-6]. On the other hand an additional dielectric layer on top of the microstrip patch mayoccur as a result of physical condition changes such as snow and ice or may be directlyintroduced as a radome in the manufacturing stage for the purpose of protection from theenvironmental hazards. The performance characteristics of the antenna structure may beadversely affected if relative permittivity and thickness of the dielectric substrate are notchosen properly. It has been also observed that the resonant frequency of the microstripantennas is shifted to a lower value as a result of dielectric shielding on the antennas. In suchcases, this shift may cause unexpected changes in the behavior of the antenna structure and,hence the operations of the supporting electronic circuitry are also affected. So the resonantfrequency shift needs to be compensated without disturbing the original configuration anddegrading its performances. In a study, the dielectric layers of different thickness were loaded on the square-ringmicrostrip antenna and found that the antenna performances such as centre frequency;bandwidth and radiating efficiency are reduced. The axial ratio data show that material with 44
  • 3. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 1, January- February (2013), © IAEMElower dielectric constant is more preferable if thicker dielectric is chosen for design [7].However, in order to compensate the shielding effects on the resonant characteristics of amicrostrip ring structure, air-gap tuning is used and found that in order to avoid degradationin the operating performances, air-gap thickness must be adjusted by taking the geometricalparameters of both substrate and dielectric layers into consideration. In addition, it is alsofound that there is the possibility of controlling the bandwidth of antennas useful in thespace-communication applications specially to minimize the interference caused. Theproposed approach will also be useful in the biomedical, geophysical, and millimeter waveintegrated circuit applications providing flexibility in the adjustment of the desiredcharacteristics without altering the original structure and not adding nay new components [8].Therefore in this paper, an attempt has been made to achieve CP radiation from the hexagonalmicrostrip antenna as well as to compensate the dielectric cover effects on the performancesof the antenna. The selection of such antennas leads to the advantages of compact structureand, ease of designing and a simple feeding technique as well.II. DESIGN SPECIFICATIONSDesign parameters of proposed hexagonal patch antenna are as follows;Feeding technique : Coaxial feedSubstrate material : RT DuroidRelative permittivity of the substrate ( ߝ௥ ሻ: 2.32Operating frequency range : 2.4-2.4835 GHzThickness of dielectric substrate : 1.575 mmElemental side : 26.94 mmFeed location (x, y) : (-4.3 mm, -4.3 mm)Inner radius a : 0.635 mmOuter radius b : 2.0445 mm Fig. 1a. HFSS geometry of hexagonal patch antenna Fig. 1b. Fabricated Hexagonal patch antenna 45
  • 4. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 1, January- February (2013), © IAEME However the Figures 1a & b show the geometry of the hexagonal patch antenna. Thereason behind selecting the hexagonal microstrip antenna that, it has smaller size compared tothe square and circular microstrip antennas, as well as better impedance bandwidth overrectangular and square microstrip antennas for a given frequency. Therefore, authors havedesigned a coaxial fed hexagonal patch antenna and circularly polarized radiation has beenachieved by adjusting the position across the antenna.Since a circular disc is the limiting case of the polygon with large number of sides, theresonant frequency for the dominant as well as for the higher order modes can be calculatedfrom the formula of the circular disc by simply replacing radius a by equivalent radius ܽ௘௤. ଡ଼′౤౦ .ୡ f୬୮ ൌ (1) ଶπୟ౛౧ √ε౨Where ܺ௡௣ are the zeros of the derivative of the Bessel function Jn(x) of the order n. ′The equivalent radius ܽ௘௤. is determined by comparing areas of a regular hexagon and acircular disk of radius ܽ௘௤. ଷ√ଷ ୗమ πaୣ୯ ଶ ൌ ଶ (2)or aୣ୯. ൌ 0.9094 S (3)Thus the resonant frequency of a hexagonal element may be written: ଡ଼′౤౦ .ୡ ଵ.ଵଡ଼′౤౦ .ୡ f୰ ൌ ଶπ.ሺ଴.ଽ଴ଽସୗሻ. ൌ ଶπୗ√ε౨ (4) √ε౨For the lowest order mode ܶ‫ܯ‬ଵଵ X ୬୮ ൌ 1.84118 ′ (5)Using above design parameters and design expressions, the proposed antenna has beendesigned and performances are examined using HFSS, and the obtained results are describedin the following sections. 0 -2 dB(Return loss) -4 -6 -8 -10 -12 -14 -16 -18 -20 1 1.5 2 2.5 3 3.5 Frequency(GHz) Fig. 2. Return loss of the hexagonal microstrip antennaThe resonant frequency of the conventional hexagon antenna of side length of 26.94 mm, isfound to be 2.43 GHz with a return loss around -18.52 dB as shown in Figure 2. Whereas thevalue of VSWR is 2.068 at 2.43 GHz, and corresponding values of VWSR with frequency isplotted is Figure 3. 46
  • 5. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 1, January- February (2013), © IAEME 60 50 40 VSWR 30 20 10 0 0 1 2 3 4 Frequency (GHz) Fig. 3. VSWR of the hexagonal patch antenna Fig. 4. Radiation pattern of the hexagonal antennaThe radiation pattern of the antenna shows that it is omni-directional as well as linearlypolarized with small levels of cross polarization as shown in Figure 4.. The gain for theoptimized antenna is 5.861 dB, and shown in Figure 5, however the input impedance of theantenna is 46 at 2.43 GHz (Figure 6). Axial ratio with respect to frequency is shown inFigure 7, and found that axial ratio at the resonant frequency (2.43 GHz) is around 1.245 dBand axial ratio bandwidth is about 1.41 %. 10 5 0 dB(gain) -200 -100 -5 0 100 200 -10 -15 -20 -25 theta(deg) Fig. 5. Gain of the proposed antenna 47
  • 6. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 1, January- February (2013), © IAEME 60 50 Impedence(ohm) 40 30 20 10 0 0 1 2 3 4 Frequency(GHz) Fig. 6. Impedance response of the proposed antenna 10 8 dB(axial ratio) 6 4 2 0 1 1.5 2 2.5 3 3.5 Frequency(GHz) Fig.7. Axial ratio plot of the proposed antennaIII. HEXAGONAL MICROSTRIP ANTENNA WITH DIELECTRIC COVER The geometry of a dielectric loaded hexagonal patch antenna is shown in Figure 8, wherePlexiglas, ሺߝ௥ ൌ 3.4) have been used as dielectric covers and the effects on the differentantenna parameters are analyzed and shown in Figures 9-13. Fig.8. Structure of proposed antenna with dielectric cover 48
  • 7. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 1, January- February (2013), © IAEME 0 -5 -10 S11 (dB) -15 -20 1.5 2 2.5 3 Frequency(GHz) Fig.9. Return loss of proposed antenna with dielectric cover 60 50 40 VSWR 30 20 10 0 0 1 2 3 4 Frequency(GHz) Fig.10. VSWR of the proposed antenna with dielectric cover 60 50 Impedence(ohm) 40 30 20 10 0 0 1 2 3 4 Frequency(GHz) Fig.11. Impedance of proposed antenna with dielectric cover 49
  • 8. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 1, January- February (2013), © IAEME Fig.12. Radiation pattern of proposed antenna with dielectric cover 10 5 0 -200 -100 -5 0 100 200 dB -10 -15 -20 -25 Theta(degree) Fig.13. Gain of proposed antenna with dielectric coverFigures 9-13 show the performance characteristics of the proposed antenna with a dielectriccover of thickness 0.5 mm. The Figure 9 indicates that the return loss of the antenna is -17.24dB at 2.39 GHz. However Figure 10 shows that the VSWR is nearly equal to 2. The Figure11 shows the magnitude of the input impedance of the antenna. The radiation pattern and gainof the antenna are shown in Figures 12 and 13 respectively.IV. COMPENSATED HEXAGONAL PATCH ANTENNA As reported in reference [9], we know that due to dielectric loading, capacitanceof the antenna system increases, which decreases the overall performances of the antennasuch as resonant frequency, impedance bandwidth and radiating efficiency. Therefore, inorder to compensate dielectric loading effect, one should/decrease change the capacitance ofthe antenna system. Hence in this work, to achieve the original capacitance of the antenna, anair gap is created between ground plane and substrate of the antenna. Due to such air gap thecapacitance of the antenna system further decreases causing significant improvements inoverall performances of the antenna system. 50
  • 9. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 1, January- February (2013), © IAEME 0 dB(Return loss) -5 -10 -15 -20 1.5 2 2.5 3 Frequency(GHz) Fig.14. Return loss of compensated hexagonal patch antennaIn particular, we inserted an air gap of 0.1mm between the substrate and the ground plane. As we haveseen that using a 0.5mm thick dielectric cover over the patch causes the shifting of resonant frequencyfrom 2.43 GHz to 2.39 GHz which is beyond the operating range of antenna (i.e. 2.4-2.4835 GHz) andhence the performance of antenna get deteriorated. When we create an air gap between the groundplane and the substrate, the resonant frequency of dielectric loaded antenna shifted back from 2.39GHz to 2.44 GHz which is within the operating range of the antenna. The obtained compensatedperformance characteristic impedance bandwidth, input impedance, VSWR, return loss etc. are shownFigure 14-19. In particular, return loss with the dielectric cover decreased from -18.52 dB to -17.2407dB, again improved to around -18 dB. 60 50 40 impedence(ohm) 30 20 10 0 0 1 2 3 4 Frequency (GHz)) Fig.15. Input impedance of compensated hexagonal patch antenna 60 50 40 VSWR 30 20 10 0 0 1 2 3 4 Frequency(GHz) Fig.16. VSWR of compensated hexagonal patch antenna 51
  • 10. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 1, January- February (2013), © IAEME 10 5 0 -200 -100 -5 0 100 200 Gain(dB) -10 -15 -20 Theta(deg) -25 Fig.17. Gain of the compensated hexagonal patch antenna Fig.18. Radiation pattern of compensated hexagonal patch antennaSimilarly input impedance decreased from 53 to 42 , is improved back to 56 in case ofcompensated antenna, while VSWR is improved from 2.42 to 2.37, along with the gain improvementfrom 5.998 dB to 5.83 dB. The comparison of the obtained results of the proposed antenna are listedin Table 1 10 9 8 Axial Ratio(dB) 7 AxialRatio_Hexagonal 6 Patch_Without Dielectric cover 5 4 AxialRatio_Hexagonal with dielectric Cover 3 2 AxialRatioValue_Hexagonal 1 compensated 0 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 Frequency (GHz) Fig.19. Axial ratio of hexagonal patch antenna 52
  • 11. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 1, January- February (2013), © IAEME Table 1 Comparison of antenna parameters With Without dielectric Antenna Compensated dielectric loading parameters values loading of 0.5 mm Resonance frequency 2.43 2.39 GHz 2.44 GHz (GHz) GHz Return loss (dB) -18.52 -17.2407 -17.931 Impedance ( ) 53 42 56 VSWR 2.068 2.42 2.376 Gain (dB) 5.861 5.998 5.8307 Impedance bandwidth 1.45% 1.30% 1.51%V. CONCLUSIONS Thus a dielectric covered hexagonal patch antenna is designed and analyzed with thehelp of HFSS. And found that due to dielectric layer the resonant frequency of the antennagoes beyond the operating range; hence the performance of antenna deteriorates. In additionvarious parameters; return loss, input impedance, bandwidth, VSWR, gain also get altered. Inaddition basic antenna provides circularly polarized radiation (AR < 3dB) at the frequency2.2 GHz. However, the dielectric loading deteriorates the circular polarization characteristicsof the antenna and axial ratio values goes beyond 3dB. Therefore, the main focus has beengiven to compensate these changes by introducing an air gap between the ground plane andsubstrate of the hexagonal patch antennas. The thickness of the air gap is chosen such that theshifted responses are brought in the desired range. It is also found the proposed compensationtechnique does not play an effective role to get back the same circular polarization radiation.That is the compensation of superstrate loading effects on the CP antenna can be chosen forfurther research.ACKNOWLEDGMENT The authors express their appreciation to Dr. B. K. Kanaujia, Professor, Departmentof Electronics and Communication, Ambedkar Institute of Technology, New Delhi for allowsus to use HFSS simulation software and experimentations.REFERENCES1. M. Dubey, D. Bhatnagar, V. K. Saxena and J. S. Saini, “Broadband dual frequencyhexagonal microstrip antenna for modern communication systems,” IEEE InternationalConference on Emerging Trends in Electronic and Photonic Devices & Systems, 2009,ELECTRO 09, pp. 303-306, Dec. 2009.2. K. S. Arvind and J. R. Wolfgang, “Spectral domain analysis of a hexagonal microstripresonator," IEEE Tran. Microwave Theory and Techniques, Vol. 30, pp. 825-828, 1982. 53
  • 12. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 1, January- February (2013), © IAEME3. K. P. Ray, M. D. Pandey and S. Krishnan, “Determination of resonance frequency ofhexagonal and half hexagonal microstrip antenna,” Micro. Optical Tech. Letter, Vol. 49, No.11, pp. 2876-2879, 2007.4. K. P. Ray, D. M. Suple and N. Kant, “Perturbed hexagonal microstrip antenna for circularpolarization,” IEEE Applied Electromagnetics Conference (AEMC), pp. 1-4, Dec. 2009.5. K. P. Ray, D. M. Suple and N. Kant, “Suspended hexagonal microstrip antennas forcircular polarization,” International Journal of Microwave and Optical Technology, Vol.5,No. 3, pp. 119-123 May 2010.6. A. K. Verma and Nasimuddin, “Analysis of circular microstrip patch antenna as anequivalent rectangular microstrip patch antenna on iso/anisotropic thin substrate,” IEE Proc.-Microwave Antenna Propagation Vol. 150, No. 4, pp. 223-229, August 2003,7. C. Y. D. Sim, T. Y. Han and J. F. Wu, “Impedance matching and dielectric effects on CPsquare ring microstrip antenna,” Chienkuo Technology University, Taiwan 500, R. O. C, pp.1996.8. Çi˘gdem, Seçkin Gürel and Erdem Yazgan, “Compensation of dielectric effects on theresonant behaviour of the microstrip ring structure by using an air-gap control,” IEEETransactions on Electromagnetic Compatibility, Vol. 43, No. 2, pp. 219-223, May 2001.9. I. Bahl, P. Bhartia, S. Stuchly, "Design of microstrip antennas covered with a dielectriclayer," IEEE Transactions on Antennas and Propagation, Vol. 30, No. 2, pp.314-318, Mar1982.10. Gangadhar P Maddani, Sameena N Mahagavin and Shivasharanappa N Mulgi, “DesignAnd Development Of Microstrip Array Antenna For Wide Dual Band Operation”International journal of Electronics and Communication Engineering &Technology(IJECET), Volume1, Issue1, 2010, pp. 107 - 116, Published by IAEME.11. Suryakanth B and Shivasharanappa N Mulgi, “Design And Development Of Low Profile,Dual Band Microstrip Antenna With Enhanced Bandwidth, Gain, Frequency Ratio And LowCross Polarization” International journal of Electronics and Communication Engineering&Technology (IJECET), Volume1, Issue1, 2010, pp. 88 - 98, Published by IAEME.12. Amit Kumar Gupta ,R.K. Prasad and Dr. D.K. Srivastava, “Design And Development OfDual E-Shaped Microstrippatch Antenna For Bandwidth And Gain Enhancement”International journal of Electronics and Communication Engineering &Technology(IJECET), Volume3, Issue3, 2012, pp. 34 - 42, Published by IAEME.13. M. Veereshappa and Dr.S.N Mulgi, “Design And Development Of Triple BandOminidirectional Slotted Rectangular Microstrip Antenna” International journal ofElectronics and Communication Engineering &Technology (IJECET), Volume3, Issue1,2012, pp. 17 - 22, Published by IAEME.14. Mahmoud Abdipour, Gholamreza Moradi and Reza Sarraf Shirazi, “A Design ProcedureFor Active Rectangular Microstrip Patch Antenna” International journal of Electronics andCommunication Engineering &Technology (IJECET), Volume3, Issue1, 2012, pp. 123 - 129,Published by IAEME. 54