Switched Beam Antenna using Omnidirectional Antenna Array
Siti Zuraidah Ibrahim and Mohamad Kamal A.Rahim
Wireless Communication Centre,
Faculty of Electrical Engineering,
Universiti Teknologi Malaysia,
81300 Skudai, Johor, Malaysia
[email protected], [email protected].
Abstract – This paper presents the result of using
omnidirectional antenna array on the Switched Beam
Antenna System. The 4x4 conventional Butler Matrix
has been used in this project as the beamforming
network to provide four different values of the phase
difference that coupled to antenna elements. By
integrating the omnidirectional antenna array with 4x4
Butler Matrix, this system has the capability to
generate four different beams at four different
directions with the coverage area of 360º. Two types
of beam can be generated by the system which are
narrow beam and broader beam. The comparison
between the measured and computed radiation pattern
of the switched beam antenna also presented.
Keywords: Switched Beam Antenna; 4x4 Butler Matrix;
Microstrip; Antenna Array
1. Introduction
The topic of switched beam antenna as a smart
antenna has been discussed vigorously as the
implementation of it is simple and requires less cost as
compared to adaptive antenna array. Unlike the
adaptive antenna, switched beam antenna only
constructed by a number of radiating elements, a
beamforming network and RF switch [7] while
adaptive array systems provide more intelligent
operation and needs more advanced signals processing
to function. For simplicity, this paper presents the
switched beam antenna that comprises of antenna array
and beamforming network only since the combination
of these blocks are good enough in order to observe
the radiation pattern characteristics of the system.
Recently, most of the papers were discussing
about switched beam antenna that uses microstrip
antenna array which only accomplished to serve 120º
of angular coverage area [2, 3, and 5]. In this project,
omnidirectional antenna has been used as the radiating
elements and the radiation pattern characteristics of the
system have been observed. The conventional Butler
matrix is used as the beamforming network of the
system. The measured radiation patterns attained by
this project are then compared to the theoretical
calculation.
2. Project Development
The development of the project involves three
stages which are implementation of the antenna array,
design and implementation of Butler Matrix and the
integration between antenna array and Butler Matrix.
2.1 Implementation of Antenna Array
The antenna array is consists of four radiating
antenna spaced at half-wavelength apart at the carrier
frequency and arranged in a linear form. The antenna
elements spaced at d= λ/2 = 6.25 cm apart. This half
wave spacing ensures that the array will have the
largest gain and directivity that does not have gating
lobes [1].
The radiating elements that have been used in this
project are commercial dipole antennas manufactured
by D-Link Company. The antenna is designed to
operate at 2.4 GHz frequency band, and is aimed for
WLAN applications.
2.2 Design and Implementation of Butler
Matrix
The Butler Matrix is a 2n x 2n network consisting
in 2 inputs and 2n outputs, 2n-1 log2 2n hybrids and
some phase shifters [6]. In this project, the designed
Butler Matrix consists of four 90° hybrid coupler, two
0 dB crossover and two -45° phase shifter. Each
component is designed at the operating frequency of
2.4GHz and simulated using Agilent ADS schematic.
The prototype is implemented using microstrip
transmission line technique and fabricated on FR4
board with relative permittivity 4.5, dissipation factor
tan δ = 0.019, and thickness of 1.6 mm.
The Butler Matrix has four inputs 1R, 2L, 2R and
1L, and four outputs A1, A2, A3 and A4. The four
outputs are used as inputs to antenna elements to
produce four beams. The input ports of the Butler
Matrix are named according to the beam position
which will be generated by activating one of the input
ports of the Butler Matrix. Figure 1 shows the block
structure and layout of the Butler Matrix.
Butler Matrix also known as beamforming
network as it has a capability to steer a beam
electronically to a certain direction by providing
multiple phase differences, β through different
n
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transmission paths. The conventional Butler Matrix
can provide four different value of β which are -45º,
+135º, -135º, +45º. It was designed in such a way so
that when current excited to any input ports, the phase
different between adjacent output ports will only has
one constant β as shown in Table 1.
Table 2: Simulated results of the corresponding phase
shifts between the inputs and the outputs of the Butler
Matrix.
Port
1R
2L
2R
1L
A1
1º
-90º
-44.4º
-134.1º
A2
-44.8º
42.2º
-179.7º
-93º
A3
-93º
-179.7º
42.2º
-44.8º
A4
-134.1º
-44.4º
-90º
1º
Table 3: Computed phase error.
Port
1R
| Error|
2L
| Error|
2R
| Error|
1L
| Error|
Figure 1: The block structure and layout of the Butler
Matrix.
β1
β2
β3
(A2-A1)
(A3-A2)
(A4-A3)
-45.8º
0.8º
132.2º
2.8º
-135.3º
0.3º
41.1º
3.9º
-48.2º
3.2º
138.1º
3.1º
138.1º
3.1º
48.2º
3.2º
-41.1º
3.9º
135.3º
0.3º
-132.3º
2.8º
45.8º
0.8º
Target
-45º
+135º
-135º
+45º
The radiation characteristics of the beams are
measured using far-field method in the anechoic
chamber. At first, the radiation pattern of the single
antenna is measured. The obtained radiation pattern of
the single antenna can be used later to calculate the
predicted radiation pattern of the integrated project.
The measured radiation pattern of the single antenna is
shown in Figure 2.
Table 1: Design Target of the Butler Matrix
Port
1R
2L
2R
1L
β
-45º
+135º
-135º
+45º
2.3 Integration of the project
The ports of the antenna array are connected to the
output ports of the Butler Matrix by using four equal
length coaxial cables.
3. Result
Figure 2: Measured radiation pattern of individual
antenna
For the measurement of the integrated project, all
input ports are fed with the same signal but only one
port is activated at one time while the other ports are
terminated with 50Ω. Figure 3 illustrates the measured
radiation patterns of 1R, 2L, 2R and 1L beams.
Table 2 shows the simulated results of the
corresponding phase shifts between the inputs and the
outputs of the Butler Matrix. The main concern about
the result of the Butler Matrix is the phase differences
between the output ports, not the value of phase at the
output ports. Thus, the phase differences between
output ports are calculated and the difference between
the obtained value and the target also being compared
as shown in Table 3.
(a)
(b)
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antenna elements and λ is the wavelength at the free
space. By substituting N=4, d=λ/2 and β = -45º, +135º,
-135º, +45º, to the equation (2), AF for this project can
be computed. The computed radiation pattern of AF
can be plotted by substituting θ from 0º to 360º to
equation (2). The computed radiation pattern of AF
corresponds to each ports can be shown in Figure 5.
(c)
(d)
Figure 3: Measured radiation patterns, (a) Port 1R, (b)
Port 2L, (c) Port 2R and (d) Port 1L
It can be observed that when port 1R and 1L is
activated, two main beams appeared and directed to
the upper (+14º, -10º) and bottom (+168º, -158º) part
of the polar plot. The beamwidth size of the main
beams is narrower while for port 2L and 2R, the
beamwidth size of the main lobe is broader. Figure 4
shows the overlapping radiation pattern of the Port 1R,
2L, 2R and 1L on the same plot. It can be shown that
the system capable to cover up to 360º of coverage
area.
(a)
(b)
(c)
(d)
Figure 5: Computed radiation pattern of AF, (a) β = -45º,
(b) β = +135º, (c) β = -135º, (d) β = +45º
Figure 4: Measured radiation pattern of the integrated
project
4. Result Analysis
The radiation pattern of the integrated system can
be predicted using pattern multiplication theorem that
is given as follows [1]:
Array pattern = Single Unit pattern x Array Factor (1)
(a)
(b)
(c)
(d)
The data of a single array unit pattern is obtained
from the measured result of the single antenna (Figure
2) while the data of the array factors can be calculated
using following equation [4]:
⎛ − βλ ⎞
(sin θ − sin(sin −1 ⎜
⎟))
⎝ 2πd ⎠
d
⎛ − βλ ⎞
Nπ (sin θ − sin(sin −1 ⎜
⎟))
λ
⎝ 2πd ⎠
sin( Nπ
AF =
d
λ
(2)
where N is the number of antenna element, d is the
distance between adjacent antenna elements, β is the
phase difference of excitation current between adjacent
Figure 6: Comparison between the computed and
measured radiation patterns (a) Port 1R, (b) Port 2L, (c)
Port 2R and (d) Port 1L
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The radiation patterns of the built project can be
predicted using equation (1). Figure 6 illustrates the
comparison between the calculated and measured
radiation patterns of the array. It can be observed that
the measured radiation has a similar pattern to the
computed pattern. This is verified that the experiment
is reliable as it has a good agreement with the
theoretical calculation.
5. Conclusion
The implementation of linear array with Butler
Matrix is presented. The Butler Matrix with four inputs
and four outputs has been designed to excite the four
units of antenna array to steer the beams in different
directions. The experimental results obtained show that
the constructed Butler Matrix is able to produce four
different beams in four different directions and
accomplish to serve 360º of coverage area. It has two
types of beam which are narrow beam (port 1R and
1L) and broader beam (port 2L and 2R). The radiation
characteristics of the antenna array are compared
between the theoretical and measured result, and they
correlates well.
References
[1] C.A. Balanis, Antenna Theory Analysis and
Design, John Wiley, Inc., New Jersey, 2005.
[2] D. C. Chang, S. H. Jou. “The study of Butler
Matrix BFN for four beams antenna system”, in
IEEE Antenna and Propagation Society
International Symposium, June 2003, pp. 176-179.
[3] N. T. Pham, G. A. Lee, F. D. Flavis. “Microstrip
antenna array with beamforming network for
WLAN applications”, in IEEE Antenna and
Propagation Society International Symposium,
July 2005, pp. 267-270.
[4] R. Tang and R.W. Burns. “Phased Array” in R. C.
Johnson (eds.), Antenna Engineering Handbook,
McGraw Hill, Inc., New York, 1993.
[5] S. R. Ahmad and F. C. Seman, “4-port Butler
Matrix for Switched Multibeam Antenna Array”,
in Proceedings of the 2005 Asia-Pacific
Conference on Applied Electromagnetics, Johor,
Malaysia, November 2005, pp. 69-73.
[6] T. A. Denidni and T. E. Liber, “Wide band Four
Port Butler Matrix for Switched Multibeam
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Indoor and Mobile Radio Communication
Proceedings, September 2003, pp.2461-2464.
[7] Y. J. Chang and R. B. Hwang “Switched beam
System for low-tier wireless communication
systems,” in Proceedings of the 2001 Asia Pacific
Microwave
Conference,
Taipei,
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