International Journal of Engineering Trends and Technology (IJETT) – Volume23 Number 5 - May 2015
G. Mahendra babu
#
1, J. Ravindranadh
*
2
1
#
M.Tech Student, ECE Department, RVR&JC CE, Guntur
2
#
Associate professor, ECE Department, RVR&JC CE, Guntur
Abstract—In this paper a wideband circularly polarized capacitive fed microstrip patch antenna is proposed. It is a square patch antenna truncated in opposite corners, suspended above the ground plane. The antenna structure contains capacitive feed strip which is fed by coaxial probe. The proposed structure is designed and simulated to cover the entire frequency of GPS, i.e., L1 (1.575 GHZ), L2 (1.227 GHZ) and L5 (1.176
GHZ). The design and simulation of antenna is done using CST
MW STUDIO. The proposed structure is designed at center frequency of 1.5 GHZ. The impedance bandwidth is 37% at design frequency of 1.5 GHZ, ranging from 1.13GHZ -1.68 GHZ is obtained. Performance of antenna is discussed using parameters like return loss, VSWR and radiation pattern.
L1 band and a stacked shorted elliptical patch antenna was designed to operate in both L1 and L2 band.
The modern GPS signal contains an additional third frequency L5 (1.176 GHZ) along with L1 and
L2 bands. In this paper, a wideband circularly polarized truncated square microstrip patch antenna with capacitive feeding is designed and simulated to cover the L1, L2& L5 frequencies in single band
Keywords—Capacitive fed, circular polarization, air gap, CST II. DESIGN APPROCH
I.
INTRODUCTION
In recent years circularly polarized (CP) planar antennas have received much attention for wireless applications. The main advantage of CP is that regardless of receiver orientation, it will receive a component of signal. Microstrip antennas are resonant type, possessing narrow impedance bandwidth. But wireless applications such as GPS, mobile phones, satellite communications require more bandwidth due to integration of various services in single antenna. The demand for global positioning systems (GPS) has increased for various applications. The GPS signal is transmitted in L1
(1.575 GHZ), L2 (1.227 GHZ) and L5 (1.176 GHZ) and it is circularly polarized. Among the wide range of GPS antennas the most successful performing antennas are the conical spiral antenna (CSA), quadrifilar helical antenna (QHA) and microstrip patch antenna.CSA exhibits a broad main lobe, wide frequency band and good front to back ratio.
Quadrifilar helical antenna relatively insensitive to mutual coupling effects. On the other hand MSA are light weight, low profile and relatively low cost but it has narrow impedance bandwidth. A shorted elliptical patch antenna was designed to operate in
The microstrip patch antenna is generally fed by coaxial probe, microstrip line, aperture coupling and proximity coupling. In this paper we are using capacitive feeding method. This is a recent method.
The designed model is as shown in fig.1
Fig . 1. Cross sectional view of capacitive feed patch antenna with probe feed
.
In this a small feed patch is placed very close to actual radiating patch. The feed patch is fed with
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International Journal of Engineering Trends and Technology (IJETT) – Volume23 Number 5 - May 2015 coaxial probe. The energy from the feed patch is electromagnetically coupled to the radiating patch.
This method is advantageous because it compensates the reactance produced by the inductance of probe feeding and better impedance matching at desired frequencies.
(5)
The equivalent dielectric constant for the above air
–dielectric geometry antenna is calculated by
(6)
The above equations from (1) to (5), the dimensions are calculated by replacing the є r with
єeq Use of air gap enhances the bandwidth. The design expression for air gap to enhance the impedance bandwidth is
(7)
Where, g=height of air gap h=thickness of substrate
=relative permittivity of the dielectric
=effective dielectric constant
=normalized line extension
=equivalent dielectric constant
=width of radiating patch
Fig. 2. Top view of radiating patch
L=length of radiating patch
The antenna is designed to operate with a center frequency of 1.5 GHZ. The radiator patch dimensions are calculated from standard design expressions of patch antenna after making necessary corrections for suspended (g + h)
=effective length
=wavelength
C
0
=free space velocity. dielectric. All these parameters are optimized using
CST simulation software. The design parameters of antenna include air gap separation between radiator patch and feed patch length (t) and width (s) feed strip. The ROGERS RT5880 (duroid) with dielectric constant 2.2 is used as substrate.
The minimum possible width of feed strip is 2.4 mm so that we can connect probe pin. The minimum separation between the two patches is
0.5mm.The sizes of feed patch („t‟ and „s‟) and the gap between the two patches are varied to adjust input impedance. The feed patch dimensions are optimized to get better performance by antenna.
(1)
The final dimensions for the proposed antenna are shown in Table1 below
(2)
(3)
(4)
TABLE I. Dimensions of proposed antenna
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International Journal of Engineering Trends and Technology (IJETT) – Volume23 Number 5 - May 2015 f
2
=upper frequency limit (1.68 GHz)
Parameter
Size of original square patch
Size of truncated corner square
Length of feed strip, s
Width of feed strip, t
Separation between two patches
Air gap, g
Value
70 mm
25 mm
12 mm
2.4 mm
0.5
16 mm
And the three bands i.e. L1, L2 and L5 are covered in this range (1.13 GHz to 1.68 GHz).
The VSWR plot of the designed antenna is as shown in fig5.
Substrate thickness, h
Dielectric constant
1.4 mm
2.2
Fig. 4. VSWR plot of antenna
III. SIMULATION RESULTS AND DISCUSSION:
The simulated return loss, VSWR and radiation pattern results are shown in fallowing figures.
The VSWR of the designed antenna at central frequency is 1.05. The VSWR of the designed antenna at L1, L2 and L5 bands are nearly equal to
2.The antenna which having VSWR in the range of
1 to 2 is treated as good antenna.
The radiation pattern of antenna is shown in figures fig5 and fig6.
Fig. 3. Return loss plot
From the return loss plot the impedance bandwidth
(37%) is calculated using the formula
(8)
Where, f
0
=central frequency (1.5 GHz) f
1
=lower frequency limit (1.13 GHz)
ISSN: 2231-5381 http://www.ijettjournal.org
Fig. 5. polar plot of radiation pattern
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International Journal of Engineering Trends and Technology (IJETT) – Volume23 Number 5 - May 2015
[2]. G.S.N Raju –antennas and wave propagation, Pearson Edition
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Fig. 6. 3D radiation pattern of antenna
IV. CONCLUSION
Hence, a novel wideband circularly polarized antenna is designed and simulated at 1.5 GHz. It radiates upper hemisphere of antenna, which is required for circularly polarized antenna. So, this antenna is suitable for wideband GPS applications.
[7]. Alak majumder, “ Rectangular microstrip patch antenna using coaxialprobe feeding technique to operate in S-band” IJETT,
Volume 4, Issue 4, April-2013.
[8]. Makesh. R.Solanki, ushakiran.K and K.J.Vinoy, “Broadband design of a triangular microstrip antenna with a capacitive feed” , journal of microwaves, optoelectronics and electromagnetic applications, 2008, 7(8):44-53.
[9]. Kasabegoudar, V.G, Vinoy, K.J., “ Coplanar capacitive fed microstrip antenna for wideband applications”,
IEEE Trans.
Antennas and propagation, 2010, 58(10); PP.3131-3138.
[10]. Veeresh.G, Kasabegowdar and K.J.Vinoy,
“A wideband microstrip antenna with symmetric radiation pattern”, microwave and optical technology letters, 2008, 50(8):1991-95.
ACKNOWLEDGMENTS
We wish to gratitude the efforts, and I am very much thankful to RVR&JC College of engineering,
Chowdavaram, Guntur for guidance which helped us work hard towards producing this work.
REFERENCES
AUTHORS BIOGRAPHY
First author I am doing my M.Tech in RVR & JC
College of engineering, Guntur. I have done my project on antennas and wave propagation.
[1]. C .
A. Balanis –antenna theory, 2 nd
edition john weily & sons,
Inc., New York 1995
Second Author has done M.Tech in (DSCE), and now he is pursuing PhD in microwave antennas, under JNTU, Kakinada.
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