03-09-2012, 11:26 AM
Design and Implementation of a Smart Antenna Using Butler Matrix for ISM-band
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Abstract
In this paper, an ISM-band smart antenna system of 4-element microstrip linear
array antenna with Butler matrix beamforming network is designed, analyzed and implemented
using microstrip technology in completely planar structure without su®ering from power losses or
poor antenna pattern characteristics. The performance of this smart antenna system is analyzed
and the beamforming features are monitored as function of geometrical antenna and Butler matrix
parameters in the ISM-band at frequency from 2.4 to 2.48 GHz. Smart antenna e±ciency and
directivity are improved and its side lobe level is enhanced which make it very promising.
INTRODUCTION
Recently, switched beam smart antenna systems are investigated to improve the performance of
wireless networks. Antenna arrays using microstrip fabrication technology are versatile in terms of
their geometrical shapes and implementations. The popularity of Butler matrix as a beamformer
in a switched multiple beam smart antenna is due to many advantages. First, it can be imple-
mented easily using hybrids and phase shifters. Second, the generated beams are orthogonal of the
Woodward-Lawson type and have narrow beamwidth and high directivity. Third, it has minimum
path length and number of components compared to other uniform excitation beamforming net-
works. Fourth, it has a high and almost constant beam crossover level that does not change with
frequency. This allows a good coverage pattern and full system gain at any point in the coverage
area. It can achieve continuous beam scanning without any mechanical motion in the scanning
process [1].
Several strategies for Butler matrix implementation are published. First, re°ective Butler ma-
trices are introduced in [2], but they need circulators, which add complexity to the circuit. Second,
high-permittivity substrates are used to reduce the structure's size [3], but this creates di±cul-
ties in implementation and measurements. Third, hybrid [4] and multilayered implementations [5]
are not easy as single layer structures. Therefore, planner implementation was very attractive for
researchers [6{9].
DESIGN AND IMPLEMENTATION OF BUTLER BEAMFORMING
In this section, a multiple-beam forming network using Butler matrix is designed and realized with
microstrip technology to feed the antenna array of the previous section at frequency 2.45 GHz. A
4 £ 4 butler matrix creates a set of 4 orthogonal beams in space by processing the signal from
the 4 antenna elements of an equi-spaced linear array. The Butler matrix is realized using four
directional couplers, two 0 dB cross couplers, and phase shifters. Its components are designed and
fabricated separately as shown in Figure 2. In addition, measured results are compared to simulated
ones as shown in Table 2, achieving good agreement, which validates both design and simulation
methodologies. The four-beam smart antenna generates four orthogonal beams to cover 120± area.
Careful design and optimization procedure are performed to obtain accurate phase shift (¯) results
between ports as summarized in Table 3.
CONCLUSIONS
This paper presents a planar design, simulation and implementation of a smart antenna system
using microstrip antenna array with Butler beamforming network for wireless applications in the
ISM-band at 2.45 GHz. A linear antenna array is initially designed using PCAAD software. Then,
realized and optimized using ADS Momentum simulation. The Beamforming/feeder network is
designed using a 4£4 Butler matrix, and realized using 4 quadrature hybrids, 2 phase shifters and
2 crossover circuits. To verify modeling and simulation procedures, the main basic elements such
as patch antenna, directional coupler, and cross coupler are fabricated and measured. Very good
agreement between measurements and simulations is obtained which validates the design. Finally,
the microstrip antenna array and the Butler matrix feed network are simulated and optimized to
achieve the required parameters in the ISM-band. Smart antenna parameters such as e±ciency,
directivity and maximum scan angle are improved, while minimizing the physical size. This vali-
dates antenna parameters and shows that it has an outstanding performance compared to what is
found in corresponding published literatures.