DESIGN AND ANALYSIS OF MULTIDIELECTRIC LAYER MICROSTRIP ANTENNA WITH VARYING SUPERSTRATE LAYER CHARACTERISTICS

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1 International Journal of Advances in Engineering & Technology, March 0. IJAET ISSN: DESIGN AND ANALYSIS OF MULTIDIELECTRIC LAYER MICROSTRIP ANTENNA ITH VARYING SUPERSTRATE LAYER CHARACTERISTICS Samir Dev Gupta, Amit Singh Department of Electronics and Communication Engineering, JIIT Noida, U. P., India. Agilent Technologies, Manesar, Haryana, India ABSTRACT The multidielectric layer microstrip antenna structure involves addition of a superstrate layer over the substrate. It is important that the superstrate layer must act as a part of the antenna. Design of the multidielectric layer microstrip patch antenna based on different thickness and permittivity of the superstrate layer has significant effect in gain and antenna efficiency. The designer must however ensure that the superstrate layer does not adversely affect the performance of the antenna. ith proper choice of the thickness of substrate and superstrate layer, significant increase in gain can be achieved for practical applications. This paper discusses a set of closed form expressions for the resonant frequency for the general case of multidielectric layers. Multidielectric layer microstrip antenna designed for applications where various physical properties of antenna viz. permittivity, patch dimensions, height of the substrate and superstrate layer parameters which significantly affect accuracy of the resonant frequency is analyzed. Considering the effect of superstrate layer, a method for accurately determining the resonant frequency of such structures have been obtained using variation of the patch dimension. The antenna performances have been evaluated for variety of cases of permittivity and thickness of the superstrate layer. K EYORDS: Microstrip Antenna, Multidielectric Layer, Resonant Frequency, Permittivity, Superstrate Layer I. INTRODUCTION Microstrip antennas have inherent limitation of narrow bandwidth. hen a microstrip antenna is covered with a superstrate (cover) dielectric layer, its properties like resonance frequency, gain and bandwidth are changed which may seriously degrade the system performance [- 4]. By choosing the thicknesses of substrate layers and the superstrate layer, a very large gain can be realized [5-9]. Shun- Shi Zhong, Gang Liu, and Ghulam Qasim [] have described the significance of determining accuracy of the resonant frequency in the design of a microstrip antenna with multidielectric layers. Therefore in view of the inherent narrow bandwidth of the microstrip patch antennas, the antenna with multidielectric layered structure must be designed to ensure that there is a minimum drift in the resonant frequency [0]. Theoretical methods for calculating the resonant frequency of such structures have been reported using the variation technique, the multiport network approach, the spectral domain analysis and other full-wave analysis methods. Numerical methods are highly accurate but too laborious and time consuming for direct use in CAD programs. Generalization of the transmission line model treats a rectangular microstrip antenna with several dielectric layers as a multilayer microstrip line. ith quasi-tem wave propagating in the microstrip line, a quasistatic value of the effective permittivity ε eff is derived by means of the conformal mapping technique. Relatively simple expressions based on the conformal mapping technique and the transmission line model is therefore used even for more complicated multidielectric structures. The conformal transformation used by heeler [] and by Svacina [4] has been used. For the general case of multidielectric layers, it has been suggested to obtain a set of closed form expressions for the resonant frequency. The frequency 55 Vol. 3, Issue, pp

2 International Journal of Advances in Engineering & Technology, March 0. IJAET ISSN: dependence of ε eff and the open-end extension of a patch, results in determination of resonant frequency of such antenna structures with good accuracy. Samir Dev Gupta et. al [] have shown that the dimension of substrate and the patch of the microstrip antenna, and the corresponding calculated resonant frequency are such that a very small variation in the dimension of the antenna parameters results into a very significant change in the actual resonant frequency. Since these calculations are repetitively carried out, the errors are cumulative at every step, thereby resulting in a notable change in the frequency. To minimize the compounding errors an algorithm has been used []. The algorithm minimizes errors at each step thereby providing a result which is highly accurate. In the following sections the discussions and analysis are devoted to (i) The design of a multidielectric layer microstrip antenna at 0 GHz. The muldielectric layer design considers the effect of the cover layer on antenna performance. (ii) Studies on performance analysis of the multidielectric antenna based on parameters involving combination of the superstrate layer permittivity and thickness. (iii) Finally analyzing the characteristics of the designed antenna with/without superstrate layer. (iv) Analysis related to axial ratio on the basis of superstrate thickness. In addition improvement in bandwidth with changes in the superstrate thickness. II. EFFECT OF CHANGING SUPERSTRATE LAYER THICKNESS ON THE ANTENNA PARAMETERS The microstrip antenna under consideration is designed to operate at a frequency of 0 GHz. The design is based on various selection criteria such as the thickness of the substrate and the superstrate, width and length of the element. Effects on antenna parameters with respect to the change in thickness of the superstrate layer have also been analyzed in the following subsections.. Substrate selection in the design of the patch antenna Suitable dielectric substrate of appropriate thickness and loss tangent is chosen. A thicker substrate is mechanically strong with improved impedance bandwidth [3]. However it will increase weight and surface wave losses. The substrate s dielectric constant ε r plays an important role similar to that of the substrate thickness. A low value of ε r for the substrate will increase the fringing field of the patch and thus the radiated power. A high loss tangent increases the dielectric loss and therefore reduces the antenna efficiency. The substrate parameters so chosen are as follows: The top layer chosen is RT Duroid 5880 having a thickness of 0.787mm, permittivity ε r =. and the loss tangent tanδ = The bottom layer dielectric is RT Duroid 5870 with a thickness of 0.787mm, permittivity ε r =.33 and the loss tangent tanδ = Element width and length The selection criteria for an efficient radiator with patch size which is not too large are: (i) A low value of the patch width and (ii) The ratio between the width and the length of the patch leading to antenna with a good radiation efficiency. For the antenna to be an excellent radiator, the ratio between and L should lie between < < [4], [5]. The patch dimensions determine the L resonant frequency. Various parameters in design of microstrip antenna are critical because of the inherent narrow bandwidth of the patch. Using the algorithm [], we first calculate antenna design parameters. Calculated length and width of the patch obtained is mm and 9.6 mm respectively. Effective permittivity obtained is.5644, for the net height of the substrate.574 mm. So accordingly, the calculated R in (the value of the patch resistance at the input slot) comes out to be ohms. The condition h < is taken into account, we obtain the values of self conductance 0 G and susceptance B [6] using equation () and () respectively. G = 0λ0 4 ( k ) 0h () 56 Vol. 3, Issue, pp

3 International Journal of Advances in Engineering & Technology, March 0. IJAET ISSN: Vol. 3, Issue, pp ( ) ( ) h k B e 0 0 log = λ () Treating element as two narrow slots, one at each end of the line resonator, the interaction between the two slots is considered by defining a mutual conductance. Considering far fields expressions, the directivity of a patch and the mutual conductance between patches are calculated [7] sin cos cos sin = d k G... (3) ( ) ( ) d L k J k G = sin sin cos cos sin 0... (4) Therefore the calculated input resistance of the patch is ( ) 3 G G R in =... (5) ( ) ( ) log = B A h Z e ε r... (6) where e e = and = eff e ε... (7) e obtain from the following equation =. 4 log t h t e t... (8) Also the parameters A and B are obtained using the following equations.

4 International Journal of Advances in Engineering & Technology, March 0. IJAET ISSN: ε eff A = 4 where ε eff h e and B ε eff = A ε r ε r = h h... (9)... (0) Thus applying the formula for the impedance matching we get the matching line impedance to be ( *90) ^0.5=7.5 ohm. Calculation of the strip impedance process based on the microstrip patch antenna parameters width, height h of the substrate having relative permittivity ε r and t the thickness of the patch involves the following sets of equations. Considering the condition /h>, the characteristic impedance of the strip is obtained based on the equation 6. The ADS based calculator determines the impedance matching feed line width which works out to be around 0.3 mm. Design changes incorporated based on the calculated values of the patch length and width is mm and 9.6 mm respectively..3 Impedance Calculation The matching transformer impedance =78.83 ohm and width = 0.6 mm, therefore the calculated (Impedance matching transformer impedance) ( ) Patch Impedance= = = Feed line impedance ohm. The input impedance is obtained from the ADS design. The patch with width 9.6 mm and length 8.58 mm corresponds to input impedance= ohm. Corresponding to the effective permittivity ε eff is equal to.5644, the input resistance of the patch is equal to ohm. Impedance of the port used is 90 ohm. Hence the impedance line parameters are: width = 0.6 mm and height = 7.5 mm, Feed line impedance= ohm and width =.5 mm, this comes fairly same as the calculated value obtained through the design. The multidielectric microstrip antenna designed is seen to radiate.034 m power, having directivity of 6.77 db and gain of 5.95 db thereby achieving antenna efficiency of 87.88%. Antenna is seen to resonate at the designed frequency of 0 GHz at a return loss of db as depicted in Figure. Considering the frequency of resonance at the designed frequency along with significantly low return loss as shown in Figure, it confirms accuracy of feed design to achieve impedance match along with perfect radiation pattern E φ and E as shown in Figure and Figure 3 respectively. In addition, veracity of algorithm used [], ensures multidielectric layer microstrip antenna design to accuracy overcomes anomalies likely to occur during fabrication along with measurement errors. Importantly operation in X-band at 0 GHz for defence application demands design accuracy for operational efficacy. 58 Vol. 3, Issue, pp

5 International Journal of Advances in Engineering & Technology, March 0. IJAET ISSN: Figure. Return Loss of the Multidielectric Antenna without Superstrate layer Figure. Radiation Pattern E φ both front and back Figure 3. Radiation Pattern E both front and back III. EFFECT OF CHANGING SUPERSTRATE LAYER THICKNESS ON THE ANTENNA PARAMETERS 3. Superstrates Selection 59 Vol. 3, Issue, pp

6 International Journal of Advances in Engineering & Technology, March 0. IJAET ISSN: The addition of a cover layer over the substrate can also result in structural resonance referred to as the resonance gain method [8]. Superstrates are selected to compare the effects of its permittivity and its thickness on various antenna parameters. The two superstrate selected are from the data sheet of Roger s Corporation are: High Permittivity: RT/Duroid 600LM having relative permittivity of 0. and loss tangent = Low Permittivity: RT/Duroid 5880LZ having relative permittivity of.96 and loss tangent = Low and high thickness of the substrate under consideration are 0.54mm and.54mm respectively. Analysis of the antenna structure is based on method of moments utilizing Momentum tool in Advanced Design System (ADS) of Agilent Technologies. The Momentum based optimization process varies geometry parameters automatically to help us achieve derived antenna structure. 3. Analysis based on Superstrate Layer Properties The effect of the superstrate layer on antenna parameters including radiation pattern involves selection of combination of superstrate layer viz. high/low permittivity and thick/thin superstrates. Table. Comparative Chart Depicting Effect of Superstrate Layer on Antenna Parameters Parameter High Permittivity Thick Superstrate Low Permittivity Thick Superstrate High Permittivity Thin Superstrate Low Permittivity Thin Superstrate Cover thickness.54 mm.54 mm 0.54 mm 0.54 mm Frequency GHz 9.39 GHz GHz 9.7 GHz Return Loss db db -.33 db db Power Radiated 0.99 m m m.05 m Directivity 8.84 db db 7.55 db db Gain 3.39 db db 6.45 db db Efficiency % % 86.0 % 86.9 % The resonance gain method for the practical application has been studied using moment method [9]. This resonance gain method involves a limited structural geometry, resonant frequency drift [8]. As described in section 3., superstrate relative permittivity chosen is either 0. or.96 corresponding to high or low relative permittivity respectively. Thickness of the superstate considered.54 or.54 mm corresponds to thick or thin superstrate respectively. Table shows the effect on antenna parameters due to change in permittivity and thickness of superstrate layer. 3.. Case High Permittivity Thick Superstrate has effect on antenna parameters. Poor gain accompanied by very low antenna efficiency of the order of 37.65%. In addition for the Case, return loss is also very poor and at frequency of GHz, it is db as shown in Figure 4. Figure 4. Return Loss for High Permittivity Thick Superstrate Antenna 60 Vol. 3, Issue, pp

7 International Journal of Advances in Engineering & Technology, March 0. IJAET ISSN: Figure 5. Radiation Pattern E φ both front and back Figure 6. Radiation Pattern E both front and back The lossy nature of the antenna combined with poor return loss is substantiated by the radiation pattern in both φ and plane highlighting minor lobes and distorted pattern as can be seen in Figure 5 and Figure 6. It is therefore concluded that combination of thick superstrate of high relative permittivity will result in antenna behavioural pattern not conforming to the design. 3.. Case Antenna parameters in case of Low Permittivity Thick Superstrate show improvement in antenna gain and efficiency. Return loss at 9.39 GHz is db shows marginal improvement as seen in Figure 7. Radiation patterns seen in Figure 8 and Figure 9 shows significant improvement so also radiated power output as compared to Case. 6 Vol. 3, Issue, pp

8 International Journal of Advances in Engineering & Technology, March 0. IJAET ISSN: Figure 7. Return Loss for Low Permittivity Thick Superstrate Antenna Figure 8. Radiation Pattern E φ both front and back Figure 9. Radiation Pattern E both front and back 6 Vol. 3, Issue, pp

9 International Journal of Advances in Engineering & Technology, March 0. IJAET ISSN: Case 3 High Permittivity Thin Superstrate when used, shows a significant improvement in antenna parameters viz. antenna gain and efficiency. Return loss at GHz is -.33 db, shows good improvement as seen in Figure 0. Radiation pattern seen in Figure and Figure, shows similar radiation plots as seen in Case. Marginal increase in radiated power output is seen as compared to Case. Figure 0. Return Loss for High Permittivity Thin Superstrate Antenna. Figure. Radiation Pattern E φ both front and back Figure. Radiation Pattern E both front and back 63 Vol. 3, Issue, pp

10 International Journal of Advances in Engineering & Technology, March 0. IJAET ISSN: Case 4 Low Permittivity Thin Superstrate used as shown in Table shows drop in antenna gain and marginal increase in efficiency. Return loss at 9.7 GHz, is db, shows significant improvement and as seen in Figure 3. Figure 4 and Figure 5, shows radiation plots. Radiation pattern is seen to be with perfect null in both the plane. Increase in radiated power output is.0m which is the best among all the other three cases discussed above. Hence for antenna to resonate close to the desired frequency with return loss better than -30 db, radiated power is around m and pattern with no sidelobes and perfect null. Case 4 viz. Low Permittivity Thin Superstrate is the best choice for multi-dielectric antenna design. Figure 3. Return Loss for Low Permittivity Thin Superstrate Antenna Figure 4. Radiation Pattern E φ both front and back Plots shown in Figures 4,7,0 and 3 indicate changes in resonant frequency, effect on return loss. There are variations in antenna directivity, gain, efficiency and finally the radiated power due to change in superstrates characteristics. To minimize losses and resonate close to the desired designed frequency, choice of thin and low permittivity superstrate having sufficient mechanical strength to withstand stress and weather vagaries is recommended for aerospace applications. 64 Vol. 3, Issue, pp

11 International Journal of Advances in Engineering & Technology, March 0. IJAET ISSN: Combined Result Figure 5. Radiation Pattern E both front and back Table gives the bird s eye view to analyse multidielectric antenna and effect of relative permittivity and thickness of Superstate (Cover) layer on antenna parameters and radiation pattern in both the planes. Table. Multidielectic Layer Antenna Parameters with and without Superstrate Layer Permittivity Superstrate Types Resonant Frequency (fz) (GHz) S (db)/ (Normalised) Power Radiated (milliwatt) Gain (db)/ (Normalised) Directivity (db)/ (Normalised) Efficiency / (Normalised) ithout Superstrate Low Permittivity Thin Superstrate High Permittivity Thin Superstrate Low Permittivity Thick Superstrate High Permittivity Thick Superstrate 0-3.8/(0.9) /(0.97) 6.77/(0.77) 87.89/(.0) /(.0) /(0.97) 6.88/(0.78) 86.9/(0.99) /(0.6).0 6.4/(.0) 7.3/(0.8) 86.0/(0.98) /(0.4) /(0.96) 7.44/(0.84) 79.04/(0.90) /(0.) /(0.53) 8.84/(.0) 36.63/(0.4) Though the results are reasonably attractive for low permittivity dielectric of the superstrate layer thickness of the order of 0.54mm, this choice may lead to fragile structure. Hence it is desirable to go for designs with low permittivity superstrate layer thickness of.54mm with good gain, antenna efficiency and radiated power output implying low losses. IV. Effect on Axial Ratio due to Superstrate Thickness & Improvement in Bandwidth A very important parameter is the polarization of an antenna. The axial ratio helps to quantify the polarization. The axial ratio is the relationship between major and minor axes of an elliptically polarized wave and it varies between one and infinity. A linearly and a circularly polarized antenna, the axial ratio tends to infinity and respectively. Aspect ratio observed in case of multidielectric antenna without and with superstrate layer is of the order of. It is a linearly polarized multidielectric antenna. Similarly with the superstrate layer incorporated in a multidielectric layer microstrip antenna, it is observed that there is a bandwidth enhancement of the order of 5% to 43.8% with a 65 Vol. 3, Issue, pp

12 International Journal of Advances in Engineering & Technology, March 0. IJAET ISSN: exception related to case (viz. Thick superstrate with high permittivity dielectric constant). Figure 6 shows combined plot of all antenna parameters normalized. The multidielectric layer antenna with low permittivity thin superstrate achieves best result vis-à-vis all other combinations of permittivity and superstrate thickness. However a discussed in the previous sections, the combination with next best results that suits for practical applications is the multidielectric layer microstrip antenna with low permittivity and thick superstrate. Table 3. Aspect Ratio and Bandwidth variations in Multidielectic Layer Antenna with and without Superstrate Layer Permittivity Superstrate Types Aspect Ratio(dB) Normalized Bandwidth (MHz)/ (Normalized) ithout Superstrate 80/(0.7) Low Permittivity Thin Superstrate /(.0) High Permittivity Thin Superstrate /(0.88) Low Permittivity Thick Superstrate /(.0) High Permittivity Thick Superstrate /(0) Figure 6. Antenna Parameters plot for Multidielectric Antenna without and with Superstrate Layers. V. CONCLUSION Parameters of microstrip antenna which inherently limits the gain, directivity, returns loss and radiated power is improved upon. Considering the effect of superstrate layer method for accurately determining the resonant frequency of such structures have been reported using the variation of patch dimension. To overcome the time consuming and laborious accurate numerical methods a direct use in algorithm for the design of the antenna is suggested. Data obtained from simulation with variation of the height of the transformed antenna and its effect can be used to predict the antenna parameters including resonant frequency, return loss, power radiated, directivity and gain for a multilayer microstrip antenna subjected to the limits for the thickness of the superstrate layer (0.54mm-.54mm). Gain of a multilayered structure increases as the height of the cover layer is decreased. As regard thin cover layer dielectric, conductor losses are dominant while for thicker cover layer surface wave losses 66 Vol. 3, Issue, pp

13 International Journal of Advances in Engineering & Technology, March 0. IJAET ISSN: are significant. It is found choice of low permittivity dielectric both thin and thick as cover layer is suitable for applications requiring high antenna efficiency. REFERENCES [] Shun-Shi Zhong, Gang Liu & Ghulam Qasim, (994) Closed Form Expressions for Resonant Frequency of Rectangular Patch Antennas ith Multi-dielectric Layers, IEEE Transactions on Antennas and Propagation, Vol. 4, No. 9, pp [] H. A. heeler, (964) Transmission line properties of parallel wide strips by a conformal mapping approximation, IEEE Trans. Microwave Theory Tech., Vol. MT-, pp [3] M. Kirschning & R. H. Jansen, (98) Accurate model for effective dielectric constant of microstrip with validity up to millimeter-wave frequencies, Electronic Letter., Vol. 8, pp [4] J. Svacina, (99) Analysis of multilayer microstrip lines by a conformal mapping method, IEEE Trans. Microwave Theory Tech., Vol. 40, No. 4, pp [5] N. G. Alexopoulos, & D. R. Jackson, (984) Fundamental superstrate (cover) effects on printed circuit antennas, IEEE Trans. Antennas Propagation, Vol. AP-3, pp [6] G. Alexopoulos & D. R. Jackson, (985) Gain enhancement methods for printed circuit antennas, IEEE Trans. Antennas Propagation, Vol. AP-33, pp [7] H. Y. Yang & N. G. Alexopoulos, (987) Gain enhancement methods for printed circuit antennas through multiple substrates, IEEE Trans. Antennas Propagation, Vol. AP-35, pp [8] X. Shen, G. Vandenbosch & A. Van de Capelle, (995) Study of gain enhancement method for microstrip antennas using moment method, IEEE Trans. Antennas Propagation, Vol. 43, pp [9] X. Shen, P. Delmotte & G.Vandenbosch, (00) Effect of superstrate on radiated field of probe fed microstrip patch antenna, Proc. Inst. Elect.Eng.-Microwave Antennas Propagation, Vol. 48, pp [0] Zhong, S.Z., Liu, G. & Qasim, G. (994) "Closed Form Expressions for Resonant Frequency of Rectangular Patch Antennas with Multidielectric Layers," IEEE Transactions on Antennas and Propagation, Vol. 4, No. 9, pp [] H. A. heeler, (964) Transmission line properties of parallel wide strips by a conformal mapping approximation, IEEE Trans. Microwave Theory Tech., Vol. MT-, pp [] Samir Dev Gupta, Anvesh Garg & Anurag P. Saran (008) "Improvement in Accuracy for Design of Multidielectric Layers Microstrip Patch Antenna, International Journal of Microwave and Optical Technology (IJMOT), Vol.3, No. 5, pp [3] U.K. Revankar & K.S.Beenamole, (003) Low Sidelobe Light eight Microstrip Antenna Array For Battlefield Surveillance Radars, IEEE Radar Conference, pp [4] Richards.F., Y.T. Lo & D.D. Harrison, (98) An Improved Theory for Microstrip Antennas and Applications, IEEE Trans on Antennas and Propagation, Vol. AP-9, pp [5] Lo Y.T., D. Solomon &.F. Richards, (979) Theory and Experiment on Microstrip Antennas, IEEE Trans. on Antennas and Propagation, Vol. AP-7, pp [6] C. A. Balanis, (997) Antenna Theory Analysis and Design, John illy & Sons, nd Edition, Chapter 4, pp [7] Anders G. Derneryd, (978) A Theoretical Investigation of the Rectangular Microstrip Antenna Element, IEEE Transactions on Antennas and Propagation, Vol. AP-6, No. 4, pp [8] Chisang You & Manos M. Tentzeris, (007) Multilayer Effects on Microstrip Antennas for Their Integration ith Mechanical Structures, IEEE Transactions on Antennas and Propagation, Vol. 55, No. 4, pp [9] X. Shen, G. Vandenbosch & A. Van de Capelle, (995) Study of gain enhancement method for microstrip antennas using moment method, IEEE Transactions on Antennas and Propagation., Vol. 43, pp Authors Samir Dev Gupta received his B.E. (Electronics) from U.V.C.E. Bangalore, M.Tech (Electrical Engg.) from I.I.T. Madras and M.Sc.(Defence Studies) from Madras University. His current area of research is Conformal Microstrip Antenna Design. Experience of 9 years in teaching profession at Post Graduate and Graduate level including three years teaching at Institute of Armament Technology, Pune (Defence Research and Development Organisation) then affiliated to Pune University, now Defence Institute of Advanced Technology (Deemed University). His areas of specialization include Antennas, Microwave Communication and Radar Systems. He was a recognized Post Graduate Teacher in Microwave Communication at Pune University. 67 Vol. 3, Issue, pp

14 International Journal of Advances in Engineering & Technology, March 0. IJAET ISSN: ork experience of about 5 years in maintenance, operations and modifications of Radar, Microwave Communication, Aircraft Simulators and Avionics related systems. Amit Singh received B.Tech degree in Electronic and Communication Engineering from the Jaypee Institute Of Information Technology, Noida, India, in 00. He is a Research and Development Engineer at the EEs of, Agilent Technologies, Gurgaon, India. His main research interests include electromagnetic and its applications, in particular conformal microstrip antennas. 68 Vol. 3, Issue, pp