7i7 Ijaet0703731 Design and Analysis of Multidielectric

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International Journal of Advances in Engineering & Technology, March 2012.

IJAET ISSN: 2231-1963

55 Vol. 3, Issue 1, pp. 55-68

DESIGN AND ANALYSIS OF MULTIDIELECTRIC LAYER

MICROSTRIP ANTENNA WITH VARYING SUPERSTRATE

LAYER CHARACTERISTICS

Samir Dev Gupta1, Amit Singh

2

1Department of Electronics and Communication Engineering, JIIT Noida, U. P., India.

2Agilent 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. With proper choice of the thickness

of substrate and superstrate layer, significant increase in gain can be achieved for practical applications. Thispaper 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.

KEYWORDS:Microstrip Antenna, Multidielectric Layer, Resonant Frequency, Permittivity, Superstrate

Layer

I. INTRODUCTIONMicrostrip antennas have inherent limitation of narrow bandwidth. When 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 [1- 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 [1] have described the significance of determining accuracy

of the resonant frequency in the design of a microstrip antenna with multidielectric layers. Thereforein 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 [10]. 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. With quasi-TEM wave propagating in the microstrip line, a quasistatic value of the effective

permittivity 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

Wheeler [11] 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


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56 Vol. 3, Issue 1, pp. 55-68

dependence of 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 [12] 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

[12]. 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 10 GHz. The muldielectriclayer design considers the effect of the cover layer on antenna performance.

(ii) Studies on performance analysis of the multidielectric antenna based on parametersinvolving combination of the superstrate layer permittivity and thickness.

(iii) Finally analyzing the characteristics of the designed antenna with/without superstratelayer.

(iv) Analysis related to axial ratio on the basis of superstrate thickness. In additionimprovement in bandwidth with changes in the superstrate thickness.

II. EFFECT OF CHANGING SUPERSTRATE LAYER THICKNESS ON THEANTENNA PARAMETERS

The microstrip antenna under consideration is designed to operate at a frequency of 10 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.

2.1 Substrate selection in the design of the patch antennaSuitable dielectric substrate of appropriate thickness and loss tangent is chosen. A thicker substrate is

mechanically strong with improved impedance bandwidth [13]. However it will increase weight and

surface wave losses. The substrates dielectric constant rplays an important role similar to that of the

substrate thickness. A low value ofr 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 r = 2.2 and the

loss tangent tan = 0.0009. The bottom layer dielectric is RT Duroid 5870 with a thickness of

0.787mm, permittivity r= 2.33 and the loss tangent tan = 0.0012.

2.2 Element width and lengthThe selection criteria for an efficient radiator with patch size which is not too large are:

(i) A low value of the patch width Wand (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 WandL should lie between 1< LW< 2 [14], [15]. The patch dimensions determine the

resonant frequency. Various parameters in design of microstrip antenna are critical because of the

inherent narrow bandwidth of the patch. Using the algorithm [12], we first calculate antenna design

parameters. Calculated length and width of the patch obtained is 8.69638mm and 9.6 mm

respectively. Effective permittivity obtained is 2.15644, for the net height of the substrate 1.574 mm.

So accordingly, the calculatedRin (the value of the patch resistance at the input slot) comes out to be

326. 8508ohms. The condition10

1


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57 Vol. 3, Issue 1, pp. 55-68

( )( )hkW

B e 00

1 log636.01120

=

(2)

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 [17].

2

0

3

2

0

3120

sincos

cos2

sin

=

d

Wk

G... (3)

( )( )

dLkJ

Wk

G

=

0

3

00

2

0

212sinsin

cos

cos2

sin

120

1

... (4)

Therefore the calculated input resistance of the patch is( )1232

1

GGRin

+=

... (5)

( )( )

100

10041log122

1205.05.0

+

+

+

=

BAW

h

Zer

... (6)

where ee WWW += and

+

=2

11

eff

e WW

... (7)

We obtain W from the following equation

+

+

=

2

2

1.1

1

4log

t

Wh

t

etW

... (8)

Also the parametersA andB are obtained using the following equations.


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59 Vol. 3, Issue 1, pp. 55-68

Figure 1. Return Loss of the Multidielectric Antenna without Superstrate layer

Figure 2. Radiation PatternE both front and back


III. EFFECT OF CHANGING SUPERSTRATE LAYER THICKNESS ON THEANTENNA PARAMETERS

3.1 Superstrates Selection


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60 Vol. 3, Issue 1, pp. 55-68

The addition of a cover layer over the substrate can also result in structural resonance referred to as

the resonance gain method [18]. 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

Rogers Corporation are: High Permittivity:RT/Duroid 6010LM having relative permittivity of 10.2

and loss tangent = 0.0023. Low Permittivity:RT/Duroid 5880LZ having relative permittivity of 1.96

and loss tangent = 0.0019. Low and high thickness of the substrate under consideration are 0.254mm

and 2.54mm 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.

3.2Analysis based on Superstrate Layer PropertiesThe 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 1. 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 2.54 mm 2.54 mm 0.254 mm 0.254 mmFrequency 8.537 GHz 9.392 GHz 8.607 GHz 9.721 GHz

Return Loss -3.574 dB -14.405 dB -21.331 dB -35.041 dB

Power Radiated 0.1929 mW 0.8413 mW 0.9895 mW 1.015 mW

Directivity 8.842 dB 7.443 dB 7.1255 dB 6.876 dB

Gain 3.329 dB 5.883 dB 6.1425 dB 5.977 dB

Efficiency 37.65 % 79.04 % 86.20 % 86.92 %

The resonance gain method for the practical application has been studied using moment

method [19]. This resonance gain method involves a limited structural geometry, resonant

frequency drift [18]. As described in section 3.1, superstrate relative permittivity chosen is either10.2 or 1.96 corresponding to high or low relative permittivity respectively. Thickness of the

superstate considered 2.54 or .254 mm corresponds to thick or thin superstrate respectively. Table 1shows the effect on antenna parameters due to change in permittivity and thickness of superstrate

layer.

3.2.1 Case 1High 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 1, return loss is also very poor

and at frequency of 8.537 GHz, it is -3.754 dB as shown in Figure 4.

Figure 4. Return Loss for High Permittivity Thick Superstrate Antenna


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The lossy nature of the antenna combined with poor return loss is substantiated by the radiation

pattern in both 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.

3.2.2 Case 2

Antenna parameters in case of Low Permittivity Thick Superstrate show improvement in antenna gain

and efficiency. Return loss at 9.392 GHz is -14.405 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 1.


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Figure 7. Return Loss for Low Permittivity Thick Superstrate Antenna




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3.2.2 Case 3High Permittivity Thin Superstrate when used, shows a significant improvement in antenna

parameters viz. antenna gain and efficiency. Return loss at 8.607 GHz is -21.331 dB, shows good

improvement as seen in Figure 10. Radiation pattern seen in Figure 11 and Figure 12, shows similar

radiation plots as seen in Case 2. Marginal increase in radiated power output is seen as compared to

Case 2.

Figure 10. Return Loss for High Permittivity Thin Superstrate Antenna.


Figure 12. Radiation Pattern E both front and back


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64 Vol. 3, Issue 1, pp. 55-68

3.2.4 Case 4

Low Permittivity Thin Superstrate used as shown in Table 1 shows drop in antenna gain and marginal

increase in efficiency. Return loss at 9.721 GHz, is -35.041 dB, shows significant improvement and as

seen in Figure 13. Figure 14 and Figure 15, shows radiation plots. Radiation pattern is seen to be with

perfect null in both the plane. Increase in radiated power output is 1.01mW which is the best among

all the other three cases discussed above. Hence for antenna to resonate close to the desired frequencywith return loss better than -30 dB, radiated power is around 1 mW 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 13. Return Loss for Low Permittivity Thin Superstrate Antenna


Plots shown in Figures 4,7,10 and 13 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.


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3.2.5 Combined ResultTable 2 gives the birds 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 2. Multidielectic Layer Antenna Parameters with and without Superstrate Layer

Permittivity

Superstrate

Types

Resonant

Frequency

(fz) (GHz)

S11

(dB)/

(Normalised)

Power

Radiated

(milliwatt)

Gain (dB)/

(Normalised)

Directivity

(dB)/

(Normalised)

Efficiency /

(Normalised)

WithoutSuperstrate

10 -31.8/(0.91) 1.03 5.95/(0.97) 6.77/(0.77) 87.89/(1.0)

Low Permittivity

Thin Superstrate9.721 -35/(1.0) 1.02 5.98/(0.97) 6.88/(0.78) 86.92/(0.99)

High

Permittivity

Thin Superstrate

8.607 -21/(0.61) 1.0 6.14/(1.0) 7.13/(0.81) 86.20/(0.98)

Low Permittivity

Thick

Superstrate

9.392 -14.4/(0.4) 0.84 5.88/(0.96) 7.44/(0.84) 79.04/(0.90)

High

Permittivity

Thick

Superstrate

8.537 -3.6/(0.1) 0.2 3.24/(0.53) 8.84/(1.0) 36.63/(0.42)

Though the results are reasonably attractive for low permittivity dielectric of the superstrate layer

thickness of the order of 0.254mm, this choice may lead to fragile structure. Hence it is desirable to gofor designs with low permittivity superstrate layer thickness of 2.54mm with good gain, antenna

efficiency and radiated power output implying low losses.

IV. Effect on Axial Ratio due to Superstrate Thickness & Improvementin 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 1 respectively. Aspect ratio observed in case of multidielectric

antenna without and with superstrate layer is of the order of 1. It is a linearly polarized multidielectric

antenna. Similarly with the superstrate layer incorporated in a multidielectric layer microstripantenna, it is observed that there is a bandwidth enhancement of the order of 25% to 43.8% with a


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66 Vol. 3, Issue 1, pp. 55-68

exception related to case 1 (viz. Thick superstrate with high permittivity dielectric constant). Figure

16 shows combined plot of all antenna parameters normalized. The multidielectric layer antenna with

low permittivity thin superstrate achieves best result vis--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)

Without

Superstrate 1 280/(0.7)

Low Permittivity

Thin Superstrate 0.988949735 400/(1.0)

High Permittivity

Thin Superstrate 0.980851123 350/(0.88)

Low PermittivityThick Superstrate 0.899375385 400/(1.0)

High Permittivity

Thick Superstrate 0.416799696 0/(0)

Figure 16. Antenna Parameters plot for Multidielectric Antenna without and with Superstrate Layers.

V. CONCLUSIONParameters 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 heightof 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 (0.254mm-2.54mm).

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


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67 Vol. 3, Issue 1, pp. 55-68

are significant. It is foundchoice of low permittivity dielectric both thin and thick as cover layer is

suitable for applications requiring high antenna efficiency.

REFERENCES

[1] Shun-Shi Zhong, Gang Liu & Ghulam Qasim, (1994) Closed Form Expressions for Resonant

Frequency of Rectangular Patch Antennas With Multi-dielectric Layers, IEEE Transactions onAntennas and Propagation, Vol. 42, No. 9, pp. 1360-1363.

[2] H. A. Wheeler, (1964) Transmission line properties of parallel wide strips by a conformal mapping

approximation, IEEE Trans. Microwave Theory Tech., Vol. M1T-12, pp. 280-287.

[3] M. Kirschning & R. H. Jansen, (1982) Accurate model for effective dielectric constant of microstrip

with validity up to millimeter-wave frequencies, Electronic Letter., Vol. 18, pp. 272-273.

[4] J. Svacina, (1992) Analysis of multilayer microstrip lines by a conformal mapping method, IEEE

Trans. Microwave Theory Tech., Vol. 40, No. 4, pp.769- 772.

[5] N. G. Alexopoulos, & D. R. Jackson, (1984) Fundamental superstrate (cover) effects on printed circuit

antennas, IEEE Trans. Antennas Propagation, Vol. AP-32, pp. 807816.

[6] G. Alexopoulos & D. R. Jackson, (1985) Gain enhancement methods for printed circuit antennas,

IEEE Trans. Antennas Propagation, Vol. AP-33, pp. 976987.

[7] H. Y. Yang & N. G. Alexopoulos, (1987) Gain enhancement methods for printed circuit antennas

through multiple substrates, IEEE Trans. Antennas Propagation, Vol. AP-35, pp. 860863.[8] X. Shen, G. Vandenbosch & A. Van de Capelle, (1995) Study of gain enhancement method for

microstrip antennas using moment method, IEEE Trans. Antennas Propagation, Vol. 43, pp. 227231.

[9] X. Shen, P. Delmotte & G.Vandenbosch, (2001) Effect of superstrate on radiated field of probe fed

microstrip patch antenna, Proc. Inst. Elect.Eng.-Microwave Antennas Propagation, Vol. 148, pp. 141

146.

[10] Zhong, S.Z., Liu, G. & Qasim, G. (1994) "Closed Form Expressions for Resonant Frequency of

Rectangular Patch Antennas with Multidielectric Layers," IEEE Transactions on Antennas and

Propagation, Vol. 42, No. 9, pp. 1360-1363.

[11] H. A. Wheeler, (1964) Transmission line properties of parallel wide strips by a conformal mapping

approximation, IEEE Trans. Microwave Theory Tech., Vol. M1T-12, pp. 280-287.[12] Samir Dev Gupta, Anvesh Garg & Anurag P. Saran (2008) "Improvement in Accuracy for Design of

Multidielectric Layers Microstrip Patch Antenna, International Journal of Microwave and Optical

Technology (IJMOT), Vol.3, No. 5, pp 498-504.[13] U.K. Revankar & K.S.Beenamole, (2003) Low Sidelobe Light Weight Microstrip Antenna Array For

Battlefield Surveillance Radars, IEEERadar Conference, pp. 97-101.

[14] Richards W.F., Y.T. Lo & D.D. Harrison, (1981) An Improved Theory for Microstrip Antennas andApplications, IEEE Trans on Antennas and Propagation, Vol. AP-29, pp. 38-46.

[15] Lo Y.T., D. Solomon & W.F. Richards, (1979) Theory and Experiment on Microstrip Antennas,

IEEE Trans. on Antennas and Propagation, Vol. AP-27, pp. 137-145.

[16] C. A. Balanis, (1997) Antenna Theory Analysis and Design, John Willy & Sons, 2nd Edition, Chapter

14, pp.730-734.

[17] Anders G. Derneryd, (1978) A Theoretical Investigation of the Rectangular Microstrip AntennaElement, IEEE Transactions on Antennas and Propagation, Vol. AP-26, No. 4, pp. 532-535.

[18] Chisang You & Manos M. Tentzeris, (2007) Multilayer Effects on Microstrip Antennas for TheirIntegration With Mechanical Structures, IEEE Transactions on Antennas and Propagation, Vol. 55,No. 4, pp. 1051-1058.

[19] X. Shen, G. Vandenbosch & A. Van de Capelle, (1995) Study of gain enhancement method for

microstrip antennas using moment method, IEEE Transactions on Antennas and Propagation., Vol.

43, pp. 227 231.

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 19 years

in teaching profession at Post Graduate and Graduate level including three years teaching atInstitute of Armament Technology, Pune (Defence Research and Development Organisation)

then affiliated to Pune University, now Defence Institute of Advanced Technology (DeemedUniversity). His areas of specialization include Antennas, Microwave Communication and

Radar Systems. He was a recognized Post Graduate Teacher in Microwave Communication at Pune University.


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Work experience of about 15 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 2010. 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 conformalmicrostrip antennas.

7i7 Ijaet0703731 Design and Analysis of Multidielectric

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