 Open Access
 Total Downloads : 449
 Authors : V. Saidulu, K.Srinivasa Rao, K.Kumarswamy, P.V.D.Somasekhar Rao
 Paper ID : IJERTV2IS90882
 Volume & Issue : Volume 02, Issue 09 (September 2013)
 Published (First Online): 28092013
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
Dielectric SUPERSTRATE Thickness VARIATIONON the Characteristics of Circular Patch Antenna
,
,
V. Saidulu 1*, K.Srinivasa Rao 2, K.Kumarswamy 3 P.V.D.Somasekhar Rao 4
1Department of Electronics and Communication, MGIT,Hyderabad, AP, India 2 Department of Electronics and Communication, VIF, Hyderabad, AP, India 3Department of Electronics and Communication, BIET,Hyderabad, AP, India 4Department of Electronics and Communication,JNTUH,Hyderabad,AP,India
Abstract:This paper focused about the coaxial probe fed circular patch of microstrip antenna characteristics have been studied with and without dielectric superstrate. The dielectric constant of the substrate and Superstrate material is used same for designing of circular microstrip patch antenna. The antenna designed frequency is 2.4GHz(ISM band) using cavity model. In this paper experimentally investigated the effect of single microstrip patch antenna(without Superstrate) and varying various thicknesses of Superstrate (radome) and same dielectric constant of the substrate on the patch antenna studied on the parameters such as bandwidth, beamwidth, gain, resonant frequency, input impedance, returnloss and VSWR etc. Measured results shows when placing the superstrates material thickness above the substrate the antenna parameter will be changed and antenna resonant frequency will be shifted lower side, while other parameters have slight variation in their values. In particular, the resonant frequency increases with the dielectric constant of the Superstrates thickness. In addition, it has also been observed that return loss and VSWR increases, however bandwidth and gain decreases with the dielectric constant of the superstrates. Impedance characteristics are that both input impedance and the reactance are increased as superstrate become thick and its increases.
KEYWORDS: Circular patch microstrip antenna, dielectric superstrate, VSWR, Gain, Beam width, Bandwidth, Resonant frequency etc.

INTRODUCTION:
Circularmicrostrip antenna consists of radiating patch on the one side of the substrate having the ground plane on other side. The major advantages are light weight, low profile, conformable to planar and nonplanar surfaces and easy to fabricate. The antenna is suitable for high speed vehicles, aircrafts,
space crafts and missiles because of low profile and conformal nature of characteristics [2].
Microstrip antenna has inherent limitation of narrow bandwidth. So, Superstrate (radome) is used on a microstrip antenna as a cover to protect the antenna from external environmental conditions like temperature, pressure etc. When microstrip antenna covered with a dielectric Superstrate (radome) its properties like resonance frequency, gain, bandwidth and beam width are changed which may seriously degrading the antenna performance[14]. By choosing the thickness of the substrate superstrate layer, a very large gain can be achieved [59]. Coaxial probe fed circular microstrip antenna characteristics have been investigated using High Frequency Structure Simulator (HFSS) software and measured experimentally. When microstrip antennas are covered with protective dielectric Superstrates thickness are subjected to icing conditions, or come into contact with plasma, then the resonant frequency is altered and shifted to lower sides, causing detuning which may seriously degrading the antenna performance. In this paper experimentally investigated the effect of single microstrip patch antenna (without Superstrate) and varying various thicknesses of Superstrate (radome) on the patch antenna with same dielectric constant andstudied on the parameters such as bandwidth, beamwidth, gain, resonant frequency, input impedance, returnloss and VSWR etc.

ANTENNA SPECIFICATION AND SELECTION OF SUBSTRATE MATERIALS:
The geometry of a probe fed circular patch microstrip antenna is shown in Fig1. The antenna under investigation is a patch with diameter (D) = 47.1mm which is fabricated on Arlondiclad 880 dielectric substrate, whose dielectric constant (1) is 2.2, loss tangent (tan) is 0.0009, thickness (1 ) is 1.6mm and
substrate dimension is 100mmÃ—100mm. The dielectric superstrate is Arlon diclad 880 dielectric substrate, whose dielectric constant (2) is 2.2, loss tangent (tan) is 0.0009, thickness (2) is 1.6mm and substrate dimension is 100mmÃ—100mm. The antenna center frequency is 2.4GHz(ISM band) and corresponding feed location is X=0 and Y=5.5mm is shown in Figure1 and Figure 2 .Suitable dielectric substrate of appropriate thickness and loss tangent is chosen for designing the circular patch microstrip
antenna. A thicker substrate is mechanically strong with improved impedance bandwidth and gain [10]. However it also increases weight and surface wave losses. The dielectric constant ( ) will play an important role similar to that of the thickness of the substrate. A low value of for the substrate will be increase the fringing field of the patch and thus the
Figure3: Circular patch antenna with Superstrate thickness

DESIGN OF CIRCULA PATCH ANTENNA:
In the most basic form, a circularmicrostrip patch antenna consists of a radiating patch on one side of the dielectric substrate, which has ground plane on the other side and ground plane and radiating patch separated by dielectric substrate. The resonant length of the antenna can determine its resonant frequency. In fact the patch is electrically a bit larger than its physical dimension. The patch antenna can be
designed at 2.4GHz and fabricated on Arlondiclad
radiated power. A high loss tangent (tan) increases the dielectric loss and therefore reduce the antenna
substrate, whose dielectric constant
1
is 2.2.The
performance.
Figure1: The structure of circular patch antenna
Figure2:Microstrip antenna with superstrate geometry
substrate and Superstrate material is same whose
dimensions are 100Ã—100mm of the patch antenna.The coaxial probe feeding is given to the substrate at particular location of the point where input impedance is approximately 50 . The main advantage of the coaxial probe feeding technique is that the feed can be placed at any desired location inside the patch in order to match with its input impedance. This feed method is easy to fabricate and also has low spurious radiation.

THEORITICAL FORMULATION:
The circular microstrip antenna can be analyzed using cavity model in cylindrical coordinates [18]. The cavity is composed of two perfect electric conductors at top and bottom to represent the patch and the ground plane.
4.1 Electric and magnetic fields:
To find the fields with in the cavity, we use the vector potential approach. For TM mode analysis need to calculate magnetic vector potential . The cylindrical coordinates the homogeneous wave equation of[15]
2 , , + 2 , , = 0 (1)
The electric and magnetic fields related to the vector
potential [15]
= 1
2 ,
= 1 1 ,
= 1
1 2
= 1
= 1
2 +
2
2 , = 0
(2) Subject to the boundary conditions of
0 1 , 0 1 2, 1 = 0 = 0
0 1 , 0 1 2, 1 = = 0, 0
= 1 + 2 + 1.7726
2
1 2
1 , 0 1 2, 1 = = 0 (3)
Therefore the resonant frequency of (13) for the dominant should be modified by usin (14)
The magnetic vector potential reduces to [15]
= 1 2 1 +
1 1
110
and expressed as [15]
110 = 1.8412
(15)
2
(4)
2
With the constraint equation of
2 + 2 = 2 = 2 (5)
4.3. Resonant input impedance:
The input impedance of a circular patch antenna at
resonance is real and the input power is independent
The primed cylindrical coordinates 1 , 1 , 1 are used to represent the fields with in the cavity while the Bessel function of the first kind of order m, and
= 1 / (6)
of the feed point position along the circumference. Taking the feed as a reference point, the radial distance = from the center of the patch for the dominant 11 mode is [15]:
1 2
=
(7)
1 =
1
1
1
2
(16)
m= 0, 1, 2 (8)
n= 1, 2, 3 (9)
p= 0, 1, 2 (10)
In1 represents the zeros of the derivative of the Bessel function , and they determine the order of the resonant frequencies. The first four values of1 , in ascending order, are
11
11
1 = 1.8412
21
21
1 = 3.0542 (11)
01
01
1 = 3.8318
31
31
1 = 4.2012
4.2. Resonant Frequencies:
Since for most typical microstrip antennas the substrate height is very small (typically h<0.05 ), the fields along z are essentially constant and are represented in (10) by p= 0 and in (7) by = 0.
0
0
Therefore the resonant freqencncies for the
modes can be written using (5) as [15]
Since the resonant input impedance of a circular patch with coaxial probe fed is expressed as [15]:
1 = = 1 (17)
4.4.Fields radiated:
Applying the Equivalence principle to the circumferential wall of the cavity, the equivalent maganetic current density can be obtained and assuming a 11 mode the field distribution under the patch. Since the thickness of the substrate is very small, the filamentary maganetic current becomes[15]: (18)
= = 2 1
Using equation( 5), the patch antenna can be treated as a circular loop and using the radiation equation the expression is given[15]
= 0 (19)
= 1 (20)
1 1
2 02
0 = (12)
2
Based on the values of (11), the first four modes in
=
(21)
110
110
ascending order are whose resonant frequency
2 02
is[15]
110
= 1.8412
2
= 1.8412
2
(13)
4.5 Designequations:
110
110
Based on the cavity model formulation, a design
Where is the speed of light in free space.
The resonant frequency of (13) does not take into account fringing. As was shown for the rectangular patch. Fringing makes the patch look electrically larger. The circular patch a correction is introduced by using an effective radius [15]
procedure is outlined which leads to practical designs of circular microstrip patch antennas for the dominant mode. The procedure assumes that the specified information includes the dielectric constant of the substrate ( ), the resonant frequency ( ) and height of the substrate h.
4.5.1 Circular patch radius and effective radius: Since the dimension of the patch is treated a circular loop, the actual radius of the patch is given by[15]
a=
(22)
2
1 2
1+ 2 +1.7726
Where F = 8.791Ã—109
Equation (1) does not take into considerations the fringing effect. Since fringing makes the patch electrically larger, the effective radius of patch is used and is given by[15]
2
1 2
Figure4: Fabricated Porto type circular patch with
= 1 + 2 + 1.7726
(23)
feed point location
Hence, the resonant frequency for the dominant
110
110
is given by[15]
110
110
= 1.8412 2
(24)
Where vo is the velocity of light

DIELECTRIC SUPERSTRATE EFFECT ON CIRCULAR MICROSTRIP PATCH ANTENNA:
When circular patch microstrip antenna with the dielectric Superstrate or Radom is shown in Figure (3). The characteristics of antenna parameters change as a function of the dielectric Superstrate layer. The properties of a microstrip antenna with dielectric Superstrate layer have been studied theoretical formulation using cavity model analysis. The resonant frequency of a microstrip antenna covered with dielectric Superstrate layer can be determined when the effective dielectric constant of the structure is known. The change of the resonant frequency by placing the dielectric Superstrate thickness has been calculated using the following the expression [1].
Figure5: Structure of dielectric substrate and Superstrate materials

EXPERIMENTAL MEASUREMENTS:
The geometrical structure under consideration is shown in Figure1. A circular patch antenna, designed patch with diameter (D) =47.1mm was fabricated on thick dielectric substrate whose dielectric constant 2.2, loss tangent is 0.0009, thickness is 1.6mm and the dielectric superstrate is same substrate specification. The patch was fed through probe of 50 cable. The location of feed probe had been found theoretically and chosen as x=0, y= 5.5mm.
=
(25)
Then the patch was covered with dielectric
If = + and 0.1 , then
Superstrate thickness material such as ArlonDiclad 880 whose dielectric constant (2) is 2.2, loss
tangent (tan) is 0.0009 and thickness ( ) is 1.6mm.
= 1 2
1+1 2
2
The impedance characteristics were measured by
Where,
= Effective dielectric constant with dielectric superstrte
=Effective dielectric constant without dielectric superstrate
= Change in dielectric constant due to dielectric superstrate
=Fractional change in resonance frequency
=Resonce frequency
means of HP 8510B network analyzer. The radiation pattern measurements were performed in the anechoic chamber by the use of automatic antenna analyzer. The measured results were shown in Table4, Table5, Table6 and Table7.The measured far field radiation patterns and VSWR, returnloss, input impedance plots for various thickness such as 0.2mm,0.5mm,0.8mm,1.0mm, 1.3mm, 2.2mm, 2.4mm and 3.2mm is shown in Figure8 to Figure 20
Dielectric constant(1 )
Loss tangent(tan)
Thickness (1),mm
2.2
0.0009
1.6
Dielectric constant(1 )
Loss tangent(tan)
Thickness (1),mm
2.2
0.0009
1.6
and the corresponding data Tables is shown in Table4 to Table7.

RESULT OF CIRCULAR PATCH ANTENNA WITHOUT SUPERSTRATE:
In order to present the design procedure of antenna achieving impedance matching for the case, the first prototype of the antenna was designed using Arlondiclad 880 substrate resonating at 2.4GHz and corresponding the results are shown in Figure 6. The obtained results show that the value of VSWR is 1.4666, Bandwidth is 4.6GHz, the Gain is 4.8dB, half power beamwidth is 108.160 in horizontal polarization and 105.450 in vertical polarization, input impedance is 36.244 +j8.9070 and returnloss is 13.848dB.The corresponding datatable is shown in Table4 and Table5.

RESULT OF CIRCULAR PATCH WITH SUPERSTRATE THICKNESS
In order to observe the effect of dielectric Superstrate thickness varying on the circular patch antenna characteristics such as bandwidth, gain, beamwidth, resonant frequency, input impedance, returnloss andVSWR etc. The proposed antenna has been analyzed using various dielectric Superstrate thickness such as0.2mm,05mm, 0.8mm,1.0mm,1.3mm,1.5mm,2.2mm and 3.2mm, corresponding resonating frequency will be shifted at 2.40GHz, 2.40GHz, 2.38GHz, 2.369GHz,
2.87GHz,2.40GHz, 2.28GHz,2.31GHz and 2.21GHz.
The gain is varied from 0.47GHz to 3.43GHz, the bandwidth is varied from 1.58GHz to 2.49GHz, the half power beamwidth in vertical polarization is varied from 98.160 to105.330 , the half power beam width in vertical polarization is varied from 74.860 to90.200 , the impedance is varied from 25.387j16.690 to 53.759j45.307, and the
returnloss is varied from 8.286 dB to 12.142dB. The VSWR is varied from 1.656 to 2.253 based upon the various thickness of the dielectric Superstrates. The obtained the characteristics are shown in Figure 9 to Figure 20 and corresponding data are tabulated in Table 6 and Table 7 and also shown in Figures 7 to Figure 17.
TABLE1: Specification of dielectric substrate materials used in the design of circularpatch antenna
TABLE2: Specification of dielectric Superstrate material used in the design of circular patch antenna
Dielectric constant(2 )
Loss tangent(tan)
Thickness (2),mm
2.2
0.0009
1.6
TABLE 3: Calculated Diameter and Feed point location of circular patch antenna:
Type of Patch
Diameter(mm)
Feed Point(mm)
Circular patch antenna
47.1
5.5
TABLE 4: Experimental data for Gain, Bandwidth (BW)and half power beamwidth (HPBW) of circular patch antenna without Superstrate:
1
,GHz
BW(GHz
Gain dB
HPBW HP,(Deg)
HPBW
(Deg)
2.2
2.40
0.03091
6.7
98.77
90.73
TABLE 5: Experimental data for impedance, return loss and VSWR of circular patch antenna without Superstrate:
1
(GHz)
IMPDANCE()
RL(dB)
VSWR
2.2
2.40
60.696+j15.164
15.558
1.5654
2
(GHz)
Gain (dB)
BW GHz)
HPBW(HP)
,Deg
HPBW(VP),
Deg
0.2
2.41
3.92
0.012
84.26
77.47
0.5
2.419
4.01
0.031
85.70
73.02
0.8
2.41
3.64
0.012
84.32
76.99
1.0
2.419
5.88
0.012
88.33
75.49
1.3
2.419
5.29
0.031
90.0
76.84
1.5
2.419
5.21
0.012
90.0
76.80
2.2
2.394
2.87
0.033
89.06
74.51
2
(GHz)
Gain (dB)
BW GHz)
HPBW(HP)
,Deg
HPBW(VP),
Deg
0.2
2.41
3.92
0.012
84.26
77.47
0.5
2.419
4.01
0.031
85.70
73.02
0.8
2.41
3.64
0.012
84.32
76.99
1.0
2.419
5.88
0.012
88.33
75.49
1.3
2.419
5.29
0.031
90.0
76.84
1.5
2.419
5.21
0.012
90.0
76.80
2.2
2.394
2.87
0.033
89.06
74.51
TABLE6: Experimental measured data for gain, bandwidth (BW) and half power beamwidth (HPBW)of circular patch antenna with Superstrate thickness (mm).
2.4
2.341
3.91
0.023
92.89
75.84
3.2
2.213
3.29
0.023
92.78
79.34
TABLE7: Experimental measured data for inputimpedance(IMP), returnloss(RL) and VSWR of circular patch antenna with Superstrate thickness (mm).
2
(GHz)
IMP()
RL(dB)
VSWR
0.2mm
2.41
34.427 j11.039
12.857
1.567
0.5mm
2.419
27.784 j7.3993
10.423
1.846
0.8mm
2.41
21.980 j12.968
9.956
5.581
1.0mm
2.419
24.635 j2.850
9.11
2.021
1.3mm
2.419
21.582
+j1.3726
10.075
2.355
1.5mm
2.419
21.248
+j3.7056
7.673
2.497
2.2mm
2.394
19.24
+j3.215
10.231
2.213
2.4mm
2.341
20.342
+j2.231
12.211
1.912
3.2mm
2.321
25.212 j1.2123
13.231
2.341


EXPERMENTAL ANALYSIS OF CIRCULAR PATCH MICROSTRIP ANTENNA:
(a) (b)
Figure6: Experimental measured results (a) impedance and (b) VSWR plot of circular patch antenna without Superstrate at dielectric constant(1) is 2.2
(a) 0.2mm (b) 0.5mm
Figure 7: Experimental measured VSWR plot of circular patch antenna with Superstrate thickness at 0.2mm and 0.5mm
(a) 1.0mm (b) 1.5mm
Figure8: Experimental measured VSWR plot of circular patch antenna with Superstrate thickness at 1.0mm and 1.5mm
(a) 1.3mm (b) 0.8mm
Figure9: Experimental measured VSWR plot of circular patch antenna with Superstrate thickness at 1.3mm and 0.8mm
(a) 0.2mm (b) 0.5mm
Figure10: Experimental measured returnloss plot of circular patch antenna with Superstrate thickness at 0.2mm and 0.5mm
(a) 1.0mm (b) 1.5mm
Figure11: Experimental measured returnloss plot of circular patch antenna with Superstrate thickness at 1.0mm and 1.5mm
(a) 1.3mm (b) 0.8mm
Figure12: Experimental measured returnloss plot of circular patc antenna with Superstrate thickness at 1.3mm and 0.8mm
(a) 0.2mm (b) 0.5mm
Figure13:Experimental measured impedance plot of circular patch antenna with Superstrate thickness at 0.2mm and 0.5mm
(a) 1.0mm (b) 1.5mm
Figur14:Experimental measured impedance plot of circular patch antenna with Superstrate thickness at 1.0mm and 1.5mm
(a) 1.3mm (b) 0.8mm
Figure14:Experimental measured impedance plot of circular patch antenna with Superstrate thickness at 1.3mm and 0.8mm
(a) 0.2mm (b) 0.5mm
Figure15: Experimental measured far field radiation pattern with and without Superstrate (radome)at 0.2mm and 0.5mm thickness in vertical polarization
(a) 1.0mm (b) 1.3mm
Figure16:Experimental measured far field radiation pattern with and without Superstrate (radome)
thickness at 1.0mm and 1.3mm invertical polarization.
(a) 2.2mm (b) 2.4mm
Figure17: Far field radiation pattern with and without Superstrate (radome)thickness at 2.2mm and 2.4mm in vertical polarization
(a) 0.8mm (b) 3.2mm
Figure18: Far field radiation pattern with and without radome at 0.8mm and 3.2mm thickness in vertical polarization.

RESULTS AND DESCUSSION:
A comparison of experimental results with and without dielectric Superstrate thicknessfor circular patch microstrip antenna is presented in Table4, Table5, Table6 and Table7. The data refer the highest gain 3.43dB is obtained for circularpatch antenna at Superstrate thickness at 2.2mm. The return loss is first increases with increasing the dielectric constant of the dielectric Superstratethickness and decreases. The band width of microstrip antennas also increases with increasing thickness of dielectric Superstrate thickness for low dielectric constant materials, and decreases for high dielectric constant materials. The variation of VSWR with different dielectric
Superstratethickness, as dielectric Superstrate thickness increases, VSWR increases.Increase with high dielectric constant of the Superstrates. It is also observed that the resonant frequency decreases monotonically with the increase in the superstrate thickness and dielectric constant of the Superstrates.
The impedance characteristics are that both input impedance and the reactance are increased as Superstrates becomes thick and its increases. TheHPBW is also increases with the increasing thickness of the dielectric Superstrates.

CONCLUSION:
In particular, the resonant frequency increases with the dielectric constant of the Superstrates thickness. In addition, it has also been observed that return loss and VSWR increases, however bandwidth and gain decreases with the dielectric constant of the Superstrates. Impedance characteristics are that both input impedance and the reactance are increased as Superstrate become thick and its increases.The
value of impedance, return loss and VSWR are
minimum, whereas BW is maximum for Superstrate thickness at 2.2mm.
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