An improved Design of a printed antenna array for GSM
applications
FADLALLAH N.1, RAMMAL M.2, ABOUCHAHINE S.2, VAUDON P.1
1
Limoges University, IRCOM- Equipe Electromagnétisme
123, Avenue Albert Thomas – 87060 Limoges – France
2
CEDRE project, Lebanese University, Equipe Radiocom
Saida – Liban
ABSTRACT: This paper describes low cost, wideband resonant antennas for GSM
system applications. A thick substrate is used to increase the bandwidth of such antenna.
A new design method is suggested to solve this problem due to the inductance created by
probe feed. The 3D FDTD is used for the simulation of the antenna. A wide impedance
matching bandwidth (8%) was reached. A good agreement is obtained between
theoretical and practical results.
KEY WORDS: GSM system, Antenna arrays, Wideband
1.
Introduction
Recently, most of the research on
microstrip antennas focused on methods to
increase their bandwidth [1]. The U-slot
antenna, which achieves a relatively broad
bandwidth without a parasitic patch, has
been reported [2,3]. A broader bandwidth
was obtained using an improved feeding
method [4]. In this paper we discuss the
design of a wideband coax-feed array
antenna using a thick substrate. But this
gave new challenges for the design due to
the inductance created by the probe feed.
This problem is solved by exciting the
antenna at the edge of the patch and by
adjusting its dimensions. The 3D FDTD
method [5,6] is used successfully to obtain
the radiation pattern, the impedance and
the corresponding bandwith.
2.
The rectangular patch antenna was made
using plexy-glass substrate (r=2.55).
3.
Simulation Results
Finite Difference Time Domain Method
(FDTD) is a mathematical tool for
solving
Maxwell’s
equations
of
electromagnetic problem. Usually the
space of the system under test is
subdivided in small subcells and we
have absorbing boundaries for free space
calculation. As consequence, the FDTD
program permits to calculate the electric
and magnetic fields at every point and at
any time. A 80 cell margin was used in
the horizontal dimension and 30 cell one
in vertical directions resulting in a
120 x120x100 cell grid, (Fig. 1).
Absorbing
boundaries
Antenna structure
The antenna was designed for the GSM
Band (890MHZ-960MHZ) (with return
loss Lret 10 dB). The polarization
isolation was planned to be comparable
with other similar antennas.
Lossy
medium
v(t)
Fig. 1. Antenna and space modeling
The FDTD simulator step is 2 mm for
each direction. The excitation is a TEM
wave generated between the bottom end of
the feeding probe, which is a line of 2 mm
thickness, and the ground plane. The
feeding signal consists of a wide Gaussian
pulse in order to get wideband impedance
calculation for the frequencies between 0.8
and 2 GHz.
The impedance calculation of this antenna
is done using the Fourier Transform of
temporal voltage and current components.
Huygens principle was used to obtain the
far field components and the radiation
patterns of the antenna. So, a relatively
high number (3000) of iteration steps is
used in order to sufficiently attenuate the
pulse amplitude and reduce the simulation
errors.
patch with higher dimensions such that
the resonant frequency of the mode will
be around 800 MHz. The dimensions of
the modified element patch are 9.6cm x
9.6 cm with a tolerance of ±0.5 mm and
the height of the substrate is 2.5cm, (Fig.
3).By exciting the antenna at the edge of
the patch and by adjusting the thickness
of the feeding line, the imaginary part
will be highly reduced between 880
MHz and 1 GHz and the created
problem is resolved. In addition the real
part of the impedance still has accepted
value in this frequency band (Fig. 4).
Ground
plane
Radiating
element
Excitation
point
4. Impedance matching
Initially, the antenna was designed with
centre resonant frequency at 925 MHz in
the middle of the band where the point of
excitation is adjusted in order to obtain a
50 Ohms impedance matching. Results
obtained (Fig. 2) shows that the inductance
created by the feeding line mentioned above
deteriorates completely the impedance
matching of the antenna.
Fig. 3. Realized antenna
500
Imaginary part
400
Real part
300
Zc 200
100
450
400
0
350
-100
0.75
300
Zc
0.8
0.85
0.9
0.95
1
1.05
1.1
1.15
Frequency GHz
Imaginary part
250
200
Fig. 4. Simulated real and part of the
input impedance
150
100
Real part
50
0
0.75
0.8
0.85
0.9
0.95
1
1.05
Frequency (GHz)
1.1
1.15
Fig. 2. Real and imaginary parts of the
imaginary patch impedance
To solve this problem, instead of using the
antenna at the center resonant frequency of
the fundamental mode (TM10), we use a
The reflection coefficient presented in
(Fig. 5) shows that the measured and
simulated results are similar. The center
frequency is about 925 MHz. The
measured bandwidth is about (8%) for a
10-dB return loss. The practical
bandwidth is less than the theoretical one
due to the limited ground plane.
5. GSM antenna array design
0
-2
-4
-6
-8
dB
-10
simulated
Measured
-12
-14
-16
-18
0.7
0.75
0.8
0.85
0.9
0.95
1
1.05
1.1
Frequency (GHz)
Fig. 5. Simulated and measured reflection
coefficients
The radiation patterns of the antenna in
both E-plane and H-plane are as
theoretical results (Fig. 6), the obtained
gain is about 6 dB in he whole band.
The maximum gain available from a
single patch is limited to 6 dB. In some
GSM applications this may not be
sufficient. Our approach to improving on
this is to use arrays of the above patch.
An alignement of four element array
have been relised (Fig. 7) and a
combined T feed divider is used to
achieve an equal amplitude distribution
for each element array. A phase shift
between elements, that allows lobe
tilting, can be realized with different
coax- feed length between the divider
and the path excitation. The reflection
coefficient presented in (Fig. 8) shows
that the bandwidth covers all the GSM
band The radiated pattern realized with
an appropriate excitation with
10o
tlilting angle is showed at (Fig. 9). The
antenna has a gain about 12 dBi with a
low VSWR in the whole GSM band.
Ground
plane
Radiating
Element
Point of
excitation
Fig. 7. Four elements GSM antenna
array
0
-5
-10
dB
-15
-20
-25
Fig. 6. Measured and simulated radiation
patterns
-30
800
900
950
1000
Measured 1100
1050
Fig. 8. Measured Reflex ion Coefficient
[2] K.F. Lee, et al., “Experimental and
Simulation Studies of the Coaxially Fed
U-slot Rectangular Patch Antenna,” IEE
Proceedings- Microwave Antennas and
Propagation, Vol. 144, No. 5, 1997, pp.
354–358.
0
-5
Simulated
Measured
-10
Theoretical Gain=12.1dB
Measured Gain=11.6dB
-15
dB
-20
[3] Kin – Lu Wong, “Compact and
Broadband Microstrip Antennas”, Wiley
series in microwave and optical
enginnering, 2002.
-25
-30
-35
-100 -80
-60
-40
-20
0
20
teta,deg.
40
60
80
100
Fig. 9. Four elements arrays with 10o
tilted angle
6. Conclusion
In this paper we have presented the
simulated and measured results for a coaxfed wideband GSM antenna array. The
bandwidth obtained in the measurements
was 8%. This antenna shows good VSWR,
gain and useful radiation patterns over the
frequency band. The antenna could be
used in cellular base stations when wide
bandwidth is required.
[4] C. Mak, K.M. Luk and K.F. Lee,
“Microstrip Line-fed L-strip Patch
Antenna,” IEE Proceedings-Microwave
Antennas and Propagation, , No. 4,
1999, pp. 282–284.
[5] Karl. S. KUNZ, Raymond J.
LUEBBERs " The finite difference time
domain method for electromagnetics "
CRC press, Bora Raton Ann Arbor
London Tokyo, 1993
[6] M. Rammal and all, "Rigorous
design of omni directional antennas
using the FDTD method" , microwave
Engineering Europe, June/July 1997.
7. References
[1]. P. Garg, et al, Microstrip Antenna
Design Hand-book, Boston: Artech House,
2001.
[7] J.P. BERENGER "A perfectly
matched layer for the absorption of
electromagnetic waves ", J. Computer
Physics, vol 114, No 1, 1981.