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Investigation on the Resonant Frequency of Suspended Microstrip Antenna With Various Ground Plane Shapes for Wireless Application
D.Thiripurasundari1, D.S.Emmanuel2
School of Electronics Engineering, VITUniversity,Vellore-14,Tamil Nadu
,
Abstract— The resonant frequency of rectangular suspended patch antenna with rectangular, inverted V shaped and W shaped ground planes are analyzed for varying conductor thicknesses. It is observed that increasing metal thickness lowers the fundamental resonant frequency for the V shaped and W shaped ground plane and remains same for rectangular ground plane. Increase in the patch metallization from .0002 mm to 1 mm lowers the resonant frequency by 16% -20% for W-shaped ground plane for 1:1.5 VSWR bandwidth. Increased gain and reduced cross polarization is observed for W-shaped ground plane compared to that of V-shaped and rectangular ground plane due to the constructive interference between the patch and ground plane in the far field.
Index Terms—microstrip antenna, metallization, ground plane
I.INTRODUCTION
Wireless communication has been developed rapidly in the past decade and it already has dramatic impact in our life. In the last few years, the development of wireless local area networks (WLAN) represented one of the principal interests in the information and communications field. WLAN takes advantage of a license free frequency bands, Industrial, Scientific and Medical (ISM) bands which have frequency span from 2.412 GHz to 2.482 GHz and from 5.15 GHz to 5.825 GHz Furthermore, many Internet Service Providers (ISPs) looks convenient to apply 802.11 based point-to-point links in order to provide the wireless coverage of the last mile towards the client. Thus, specific antenna system was recently developed to comply with requirements dictated by such applications. Specifically, low-cost solutions for antenna design becomes critical and required since both market and technology are so far ready to mass production [1].
In recent years there has been increased interest in microstrip antennas, especially in the context of microwave and millimeter wave MMIC, WLAN, WiMAX, cellular base station, due to its attractive features such as light weight, small volume low profile and low production cost which widely have been researched and developed in the recent twenty years [2-6]. Microstrip antennas are planar resonant cavities that leak from their edges and radiate. Printed circuit techniques are utilized to etch the antenna on soft substrates to produce low cost and repeatable antennas in low profile. The antennas fabricated on compliant substrates withstand tremendous shock and vibration environments. Manufacturers of mobile communication base stations often fabricate these antennas directly in metal sheet and mount them on dielectric posts or foam in a variety of ways to eliminate the cost of substrates and etching. This also eliminates the problem of radiation from surface waves excited in a thick dielectric substrate. To achieve a broadband operation, conventional patch antenna is designed using thick air substrates incorporating a U slotted patch, an E shaped patch and soon [3-5]. In addition to the obtained wide impedance bandwidths excellent radiation characteristics with much reduced cross polarization have been obtained [5-6]. Vast literature on microstrip antenna concentrates on the design of microstrip antenna bandwidth enhancement and size reduction. Not much attention had been paid on the influence of conductor thickness on the resonant frequency.
In this paper we cling to structure suggested in [5], but explore effect of patch metallization on resonant frequency by comprehensive parametric analysis. By choosing suitable metallization optimum bandwidth can be achieved. The simulated and measured result of return loss over the frequency band is presented.
II.Antenna geometry
The geometry of broad band patch antenna with W shaped ground plane is similar to that proposed in [5,6].The geometry of antenna is shown in Figure.1. The antenna consists of rectangular half wavelength resonant radiating structure designed for a center frequency of 2.1 GHz and a inverted V shaped or W shaped ground plane. Patch dimension is 6090mm2 (LW), ground plane dimension is 120 194mm2, height of the substrate 3mm, feeding position of the substrate (0,22)mm.
III.result and discussion
A parametric study is performed to investigate the influence of conductor thickness on the resonant frequency for various ground plane shapes.
Effect of rectangular patch with inverted V shaped ground plane.
A rectangular patch with a rectangular ground plane is as shown in Fig.1 is simulated. As the height of the substrate increases, probe pin length increases and its reactance become inductive and hamper the impedance matching. In this study the height of the substrate is maintained constant and only the effective height of the substrate is varied by bending the ground plane angle α in steps of 10° up to 40. Since ground plane is bent,it is essential that the conductor should have finite thickness. Conductor thickness is varied from .002mm to 1mm. A shift in the resonant frequency from 2.3 GHz to 2 GHz is observed from Table.1. This result shows that for a patch of minute thickness, current path is only on one side, whereas for finite thickness two current paths(upper and lower surfaces) are considered, which cause shift in the frequency.
Fig. 1 Rectangular Microstrip patch antenna with inverted V shaped Ground Plane
Table 1 rectangular patch with inverted V shaped ground plane
(deg) / V shaped ground plane
T=0.002mm / T=1mm
fr
(GHz) / RL
(dB) / VSWR 1:1.5
BW% / fr (GHz) / RL
(dB) / VSWR 1:1.5
BW%
0 / 2.3 / -20.43 / 1.98 / 2.3 / -20.43 / 1.98
10 / 2.262 / -30 / 4.55 / 2.0928 / -15.75 / 3.2
20 / 2.224 / -32.7 / 7.14 / 2.0585 / -19.57 / 6.9
30 / 2.2 / -28.17 / 9.47 / 2.033 / -21.84 / 8.88
40 / 2.195 / -27.13 / 10.52 / 2.012 / -22.92 / 12.1
Effect of rectangular patch with W shaped ground plane
The ground plane shape is further modified to W shape as shown in Fig.2 by introducing the flange such that two degrees of freedom (bent angle () and flange length (Wf)) in the design is achieved. The angle between the two ground plane flangesβ is varied from 60 to 120 and Wf is fixed at 60mm. A increase in resonant frequency is observed with increase in flange angle. Also, a decrease in resonant frequency is observed with increase in conductor thickness as tabulated in Table.2.
Fig 2. Rectangular patch with W shaped ground palne
TABLE 2 Rectangular Patch with W shaped Ground Plane
Variation of flange angle (=40, flange length Wf=60mm)
(deg) / W shaped ground plane
T=0.002mm / T=1mm
fr
(GHz) / RL
(dB) / VSWR
BW% / fr (GHz) / RL
(dB) / VSWR
BW%
60 / 2.15 / -25.99 / 8.65 / 1.9702 / -17.08 / 6.5
90 / 2.22 / -19.6 / 12.87 / 1.9913 / -31.3 / 10.59
120 / 2.2 / -29.09 / 12.41 / 2.02 / -30.06 / 12.9
The angle between the two flanges is maintained same,β= 90° and the length of W shaped flange Wf is increased as shown in Fig.3.
Figure 3 Variation of flange length Wf (a.Wf=40mm, b.Wf =100mm)
Table 3 Rectangular Patch with W shaped Ground plane variation of flange length Wf(=40, =90)
Wf(mm) / W shaped ground plane
T=0.002mm / T=1mm
fr (GHz) / VSWR
BW% / Gain
(dB) / fr (GHz) / VSWR
BW% / Gain
(dB)
20 / 2.19 / 14.3 / 6.82 / 2.004 / 10.89 / 7.37
40 / 2.17 / 13.4 / 7.75 / 1.99 / 10.23 / 8.41
60 / 2.22 / 12.87 / 8.23 / 1.99 / 10.57 / 9.25
80 / 2.22 / 12.23 / 8.98 / 1.99 / 11.02 / 9.58
100 / 2.22 / 12.46 / 9.2 / 2.00 / 10.09 / 9.73
It is observed from Table.3 that, as the flange length increases, there is not much variation in the bandwidth (variation is between 10%-11% only) but the gain of the antenna increases. It is also noted that the increase in gain is rapid till Wf=80 mm and beyond that,there is no predominant variation as the height of the flangeexceeds the height of the patch.The vector current distribution on the patch and the ground plane is shown in Fig.4 In an inverted V-shaped ground plane, the current distribution is towards the probe, whereas for the W-shaped structure the current path in the outer flange Wf is not towards the probe instead it is flowing outwards. Hence there is a constructive interference between the patch and ground. This constructive interference in the far field increasesthe gain and decrease the cross polarization level when compared to that of an inverted V-shaped ground plane. This conforms increase in bandwidth of W-shaped ground plane without compromising on gain or on cross polarization levels. This study also demonstrates that increase in patch metallization lowers the resonant frequency by 19%-20%.
- V-shaped Ground plane
b. W-shaped Ground plane
Figure 4 Vector Current Distributions
Table 4 Comparison of cross polarization levels of various ground planes
Ground plane structure / Cross polarization Level(dBi)Rectangular patch / -10.97
V-shaped Ground / -3.43
W-shaped Ground / -15.5
IV.conclusion
Microstrip patch antenna with rectangular, inverted V shaped and W-shaped ground plane with varying thicknesses is studied. It is found that antenna with 1mm thickness resonates at 20% lower frequency for V- shaped and 22% for W-shaped than the antenna with .002mm thickness. The shift is due to the different current paths taken by the antenna with finite length thickness than antenna of minute thickness. The resonant frequency and gain of the antenna is altered by the thickness and the shape of the ground plane.
V.refrences
[1]Kin Lu Wang, Planar Antennas for Wireless Communication, John Wiley and Sons Inc 2003.
[2] Thomas A Milligan, Modern Antenna Design, IEEE Press, John Wiley and Sons Inc, 2003
[3]Sheng Horng,” The influence of Metallization thickness on a microstripline fed patch Antenna”, IEEE APS
[4]King-Lu Wang, Chia-Luan Tang, Jyh-ing Chiou, “Broad Band Probe Fed Patch Antenna With W Shaped Ground Plane”, IEEE Antennas and Propagat., vol.50, no.6, Jun 2002, pp 827-831
[5]T.Shanmuganathan, S.Raghavan, “Design of Compact Broadband Microstrip Patch Antenna With Probe Feeding For Wireless Application “, Int. J. Electron Commun.(AEU),2008
[6]Kumar, G. and Ray, K.P., Broadband Microstrip Antennas, Artech House, Inc, 2003.
[7]Garg, R., Bhartia, P., Bahl, I, Ittipiboon, A., Microstrip Antenna Design Handbook, Artech House, Inc, 2001.