INVESTIGATIONS ON THE EFFECT OF
DIELECTRIC SUPERSTRATES ON THE
CHARACTERISTICS OF MICROSTRIP
PATCH ANTENNA
V.Saidulu
ECE Department, Mahatma Gandhi Institute of Technology Affiliated to JNTUH
Gandipet, Hyderabad, Telangana State. [email protected]
Abstract
This paper describes the effect of the superstrates on resonant frequency, bandwidth, gain and axial ratio of rectangular microstrip patch antenna without and loaded with dielectric superstrates. It is found that there is degradation in the performance of the patch antenna when the superstrate is touching the patch antenna. The microstrip patch antenna without dielectric superstrate achieves an impedance bandwidth of 0.038 GHz (SWR 2) at 2.40 GHz, gain is 8.77 dB. and loaded with dielectric superstrates which shows that the resonate frequency is decreased to 2.34GHz from 2.40 GHz and achieved impedance bandwidth is 0.032 GHz (SWR 2) at 2.30 GHz. As the dielectric constant of the superstrate is increases, it has been observed that the center frequency decreased to 2.34 GHz from 2.40 GHz, bandwidth is decreased to 0.032 GHz from 0.038GHz. This antenna produce an axial ratio is >40dB (102=100) over the operating frequencies (2.35 to 2.45 GHz) which is indicate
that the antenna produce linear polarization. In addition, it has also been observed that the return loss and VSWR increase, however bandwidth and gain decreases with the dielectric constant of the superstrates. This antenna is generally employed in wireless communication, Bluetooth and Wi-fi applications. The frequency generally employed in these applications is 2.4 GHz. There is a good agreement between simulated and measured results.
Keywords: Rectangular patch, Superstrate, Resonant frequency and Gain etc.
1. Introduction
A microstrip patch antenna consisting of a radiating patch on one side of dielectric substrate, which has ground plane other side is shown in Fig.1, microstrip antenna have been employed in airborne and spacecraft systems because of their low profile and conformal nature. In many of these applications require dielectric superstrate on the radiating element to provide protection against icing, snow, heat, physical damage and other environmental hazards. The dielectric superstrate can seriously alter the performance of rectangular microstrip patch antenna [1-19].When microstrip antenna with dielectric superstrate; the resonant frequency is altered, causing detuning which may seriously degrading system performance. As bandwidth and gain of microstrip is low, it is important to determine the effect of dielectric superstrate on the resonant frequency of microstrip antenna. The present paper deals that the effect of dielectric superstrates on the parameter such as bandwidth, gain, resonant frequency etc. The obtained results shows that the resonant frequency will be shifted to lower side by placing superstrate above the patch antenna, while other parameter have slight variation in their values. In particular, the resonant frequency increases with dielectric constant of the superstrates. In addition, it has also been observed that the return loss and VSWR increase, however bandwidth and gain decreases with the dielectric constant of the superstrates.
2. Specifications of Substrate, Superstrates and Design of Rectangular Patch Antenna
bandwidth and radiation [1-3, 6]. Keeping these aspects in mind, the rectangular MPA is fabricated on Arlon diclad 880 dielectric substrate, whose dielectric constant ( ) is 2.2, loss tangent (tan ) is 0.0009, thickness ( is 1.6 mm and appropriate substrate dimensions. The specifications of dielectric substrate and superstrate materials are shown in Table 1 and Table 2.
Table 1: Specification of dielectric substrate material used in the design of rectangular patch antenna
Dielectric constant( Loss tangent(tan ) Thickness ( ),mm
2.2 0.0009 1.6
Table 2: Specification of dielectric superstrate materials used in the design of rectangular patch antenna
Dielectric constant( Loss tangent(tan ) Thickness ( ),mm
2.2 0.0009 1.6
3.2 0.003 3.2
4.8 0.02 1.6
10.2 0.0035 0.8
3. Design of Rectangular Microstrip Patch Antenna
The rectangular microstrip antenna is designed at a frequency of 2.4GHz on Arlon diclad 880 substrate ( =2.2, =1.6 mm), which lies in the ISM band using the standard formula available in the literature [1-19]. The parameters of this antenna are evaluated using HFSS simulation, Version 13.0 and measured. The geometry of rectangular microstrip antenna with and loaded with dielectric superstrate is shown in Fig.1. The design and analyzed of rectangular patch antenna using the transmission line model .The calculated dimensions of rectangular microstrip patch antenna are shown in Table 3.
Table 3: The Design Dimensions of Patch Antenna
Type of Patch Width (Wp),mm Length (Lp),mm Feed Point (Fx, Fy),mm
Rectangular patch 49.50 40.30 10.5, 0
(a) (b)
Fig: 1 (a) Geometry of rectangular microstrip patch antenna (Top view). (b) The schematic of a patch antenna loaded with a superstrate at height H (in mm) above the patch (side view).
4. Dielectric Superstrate Effects
When microstrip antennas are covered with protective dielectric superstrate, the antenna resonant frequency, bandwidth, gain and other parameters are altered, resulting in detuning, which may seriously degrade the system performance. The change of the resonant frequency by placing the dielectric superstrate has been calculated using the following the expression [1, 16].
(1)
(2)
Where,
= Effective dielectric constant with dielectric superstrate =Effective dielectric constant without dielectric superstrate
Change in dielectric constant due to dielectric superstrate Fractional change in resonance frequency
Resonance frequency
5. Simulated Experimental Results
5.1. Experimental Measurements
The radiation pattern measurements were performed in the anechoic chamber by the use of automatic antenna analyzer (Agilent E8363B). The measurement setup for return-loss measurement (without superstrate) is shown in Fig.2.
Fig.2 Prototype of fabricated microstrip antenna measurements
5.2. Result of Patch Antenna without Superstrate (Free Space Radiation Conditions
In order to present the design procedure of achieving impedance matching for this case, first prototype of the antenna was designed using a Arlon diclad 880 substrate resonating at 2.4 GHz and corresponding results are shown in Fig. 3 (a). The obtained results show that the values return loss is -18.51 dB, VSWR is 1.27 and bandwidth is 0.038 GHz. However, the gain is 8.77dB. The simulated and measured results are good agreement. The return loss and radiation pattern plot of patch antenna without superstrate which is shown in Fig. 3 (b).
(a) (b)
From the Fig, 3 (a) and 3 (b), it can be observed that there is a good agreement between simulated and measured results. The resonant frequency is 2.40 GHz, same as the design frequency, the bandwidth is 0.041 GHz (VSWR ≤ 2) and the simulated gain is 8.77 dB, where as measured result is 8.75 dB.
Fig. 4 Comparison of measured and simulated results of axial ratio plot for rectangular patch antenna without dielectric superstrate 2.2
Fig.4 shows the axial ratio vs frequency plot. From the Fig.4, it can be seen that the AR is >40dB (102=100)
over the operating frequencies (2.35 to 2.45 GHz). This indicates that the rectangular patch antenna produces linear polarization. There is a good agreement between simulated and measured results.
5.3. Results Patch Antenna Loaded with Superstrates
In order to observe the effects of dielectric covers on the antenna characteristics, the proposed antenna has
been analyzed using dielectric cover of dielectric constant 2.2, 3.2, 4.8 and 10.2. The obtained characteristics are shown in Figs 5 (a) and (b); however the corresponding data are tabulated in Table 4.
(a) (b)
Fig. 5 (a) Comparison of measured and simulated results of return-loss for rectangular microstrip patch antenna loaded with dielectric superstrate 2.2. (b) Measured and simulated radiation patterns of rectangular patch antenna in E-plane for 2.2, Φ=0,
2.34 GHz.
return-loss is increases from -18.51 to -19.91dB for various dielectric constants. The same trend is observed in the measured results for
ε
r2= 3.2, 4.8, 10.2 is shown in Table 4. There is a good agreement between the simulated and measured results.Table 4: Comparison of simulated and measured results of resonant frequency, return-loss, bandwidth, gain and VSWR of rectangular microstrip patch antenna without superstrate and loaded with superstrates.
FREQUENCY (GHz) RETURN LOSS (dB) BANDWIDTH (GHz) GAIN (dB) VSWR
Simulated Measured Simulated Measured Simulated Measured Simulated Measured Simulated Measured
1* 2.40 2.40 -18.51 -19.51 0.038 0.041 8.77 8.75 1.27 1.24
2.2
2.34 2.34 -19.91 -20.91 0.032 0.033 8.67 8.60 1.22 1.20
2.36 2.36 -18.69 -17.02 0.038 0.040 8.68 8.63 1.26 1.33
2.38 2.38 -17.46 -18.50 0.039 0.040 8.70 8.65 1.31 1.27
3.2
2.31 2.31 -20.46 -21.46 0.034 0.034 8.26 8.30 1.21 1.18
2.35 2.35 -18.45 -18.90 0.038 0.038 8.46 8.45 1.27 1.26
2.39 2.39 -16.44 -17.45 0.039 0.040 8.65 8.60 1.35 1.31
4.8
2.26 2.26 -21.29 -23.30 0.033 0.034 7.76 7.77 1.19 1.15
2.33 2.33 -17.95 -17.50 0.034 0.034 8.23 8.19 1.29 1.31
2.40 2.40 -14.60 -15.56 0.039 0.040 8.70 8.60 1.46 1.40
10.2
2.08 2.08 -25.64 -26.62 0.032 0.033 6.02 6.00 1.11 1.10
2.24 2.24 -18.40 -19.45 0.033 0.034 7.45 7.38 1.27 1.24
2.40 2.40 -11.15 -11.81 0.041 0.042 8.72 8.71 1.77 1.69
*without dielectric superstrate
6. Results and discussion
Rectangular patch microstrip antenna has been designed and fabricated at 2.4 GHz with Arlon diclad 880 substrate having εr =2.2. The effect of the superstrate with different dielectric materials having εr =2.2, 3.2, 4.8
and 10.2 has been investigated. The simulation and measurements have been carried out for studying the effect of superstrates on various parameters like resonant frequency, bandwidth, gain and VSWR (Return-loss). It has been observed that there is a slight degradation in the performance of the antenna when the superstrate is touching the patch antenna. The center frequency is decreased to 2.36 GHz from 2.40 GHz, bandwidth is decreased to 0.032 GHz from 0.038 GHz and gain is decreased to 8.67 dB from 8.77 dB. As compared to free space radiation conditions i.e without superstrate, for H=0, the resonant frequency ( ) is 2.34 GHz (2.5% less), bandwidth is 0.033 GHz (17.51% less) and gain is 8.60 dB (1.71% less).
7. Conclusion
The experimental and simulated results show that the variation of VSWR with different dielectric superstrate thickness, as dielectric superstrate thickness increases, VSWR increases. The variation of the antenna gain at different dielectric superstrate thickness as dielectric superstrate thickness increases, the gain decreases. The bandwidth of the microstrip antennas can also increases with increasing thickness of the dielectric superstrate for low dielectric constant materials, and decreases for high dielectric constant of the substrate materials. Initially the return loss increases with increasing thickness of dielectric superstrates and then decreases.
Acknowledgments
Authour express thanks to Dr. K. Srinivasa Rao, Professor of Department of ECE, DRKIET, Hyderabad and Dr. P.V.D Somasekhar Rao, Professor of ECE and Dean of GNIT, Hyderabad, who guided me to carry out this work.
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