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IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS
1
A Novel Compact Butler Matrix
Without Phase Shifter
Ge Tian, Jin-Ping Yang, and Wen Wu
Abstract—A novel compact 4 4 Butler matrix using only microstrip couplers and a crossover is proposed in this letter. Compared with the conventional Butler matrix, the proposed one avoids
the interconnecting mismatch loss and imbalanced amplitude introduced by the phase shifter. The measurements show accurate
with an ampliphase differences of
and
tude imbalance less than 0.4 dB. The 10 dB return loss bandwidth
is 20.1%.
Index Terms—Butler matrix, coupler, phase shifter.
I. INTRODUCTION
M
ULTIPLE-BEAM antenna technology is an attractive
candidate for mobile and satellite communication systems [1]. The Butler matrix is used as beam-forming network
for a multiple-beam antenna system due to its simplicity and
low power loss [2]–[5]. A lumped-element unit cell is used to
realize a compact Butler matrix with a strong size reduction in
[2]. In [3], a Butler matrix is proposed using slotline technology
and lumped elements. However, the phase and amplitude performance of these Butler matrices are influenced by the effect
of the lumped elements. In [4], a classical branch-line coupler
and a Schiffman phase shifter are used to build a Butler matrix. A composite right/left handed transmission line is applied
to engineer a Butler matrix in [5]. This novel transmission line
is composed of a Wunderlich-shaped dentiform capacitor and a
meandered-line short-circuited stub inductor.
However, phase shifters and lumped elements in the above
Butler matrices usually degrade the performance. In order to get
a better performance, a novel 4 4 Butler matrix employing
only microstrip couplers and a crossover is proposed in this
letter. With the help of a
phase difference coupler, the
phase shifter is not needed in this Butler matrix. Cross-slot patch
couplers are used because they have advantages of miniaturized size and lower radiation loss [6]. This Butler matrix is designed, fabricated, and measured for verification. Further more,
it is connected to a patch array antenna to demonstrate its performance as a beam-forming network.
Manuscript received October 22, 2013; accepted February 01, 2014.
G. Tian and W. Wu are with the Ministerial Key Laboratory of JGMT, School
of Electronic Engineering and Optoelectronic Technology, Nanjing University
of Science and Technology, Nanjing, China.
J.-P. Yang is with the Key Lab of Radio Astronomy, Purple Mountain Observatory, CAS, Nanjing, China.
Color versions of one or more of the figures in this letter are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LMWC.2014.2306898
Fig. 1. Block diagram of the proposed Butler matrix.
TABLE I
PHASE RELATIONS OF COUPLERS WITH
AND
PHASE DIFFERENCES BETWEEN OUTPUTS
II. DESIGN OF NOVEL BUTLER MATRIX
The 4 4 Butler matrix has four input ports and four output
ports. As different input ports are excited, the Butler matrix provides four output signals with equal amplitude and phase differences of 45 ,
, 135 , and
, respectively. As a consequence, four beams with different directions are obtained, one
for each input.
The proposed Butler matrix is shown in Fig. 1. It contains
couplers with
and
phase difference and a crossover.
The phase relations of the couplers are listed in Table I. It is
noted that, this novel Butler matrix employs a
coupler replacing the combination of quadrature coupler and phase shifter
in the conventional Butler matrix.
When the signal is excited on Port1, it goes through the path
of A-B-C-D to Port5 with 135
phase shift.
Similarly, a phase shift of 90
is realized
between Port1 and Port6, when the signal goes through the path
of A-F-G-H. Also the signal goes through the path of A-B-C-E
to support a phase shift of 45
between
Port1 and Port7. The phase shift between Port1 and Port8 is 0
, when the signal goes through the path of
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2
IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS
TABLE II
THE PHASE RELATIONS OF THE BUTLER MATRIX
Fig. 2. (a) Structure of coupler with
phase difference between outputs.
phase difference between outputs.
(b) Structure of coupler with
TABLE III
PHYSICAL DIMENSIONS OF COUPLERS WITH
AND
PHASE DIFFERENCES
A-F-G-I. Thus the phase difference between the output ports is
45
. When the
signal is excited on other input ports, the phase shifts between
the input ports and output ports can be calculated in the similar
way. The phase relations of the proposed Butler matrix are listed
in Table II.
III. RESULTS AND DISCUSSIONS
Fig. 3. Simulated and measured S-magnitudes of the proposed Butler matrix.
The Butler matrix is fabricated on Rogers RO4003 substrate
with a dielectric constant of 3.55 and a thickness of 0.813 mm.
Simulations are carried out with CST Microwave Studio and
the target frequency is 6 GHz. A cross-slot patch coupler and
a crossover in [7] are used to compose this Butler matrix. As
shown in Fig. 2(a), the structure of the
coupler takes the
form of a chamfer patch, which is etched by a pair of dumb-bell
slots. A square patch with crossed slots for the
coupler
is shown in Fig. 2(b). Their physical dimensions are listed in
Table III. As shown in Fig. 1, the input Port3 and Port2 are
symmetric, and so are input Port4 and Port1. Thus, only the
characteristics for exciting Port1 and Port2 are shown in this
letter.
Fig. 3 shows the simulated and measured S-magnitudes of
the Butler matrix. When Port1 is excited, the transmission
characteristics are
,
,
and
at the target frequency.
When Port2 is excited, the transmission characteristics are
,
,
and
. The return loss is greater than 24.1 dB and
isolation is greater than 21.4 dB at the target frequency. The
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TIAN et al.: A NOVEL COMPACT BUTLER MATRIX WITHOUT PHASE SHIFTER
3
Fig. 6. Radiation pattern of the proposed Butler matrix connected to a patch
array antenna.
Fig. 4. Simulated and measured phase differences of the Butler matrix.
TABLE IV
COMPARISON OF PERFORMANCE OF BUTLER MATRICES
Table IV compares the performance of the proposed Butler
matrix with several previous Butler matrices. The proposed one
has the widest 10 dB return loss bandwidth among the Butler
matrices, with minimum phase error and amplitude imbalance.
Fig. 5 shows the photograph of the fabricated Butler matrix.
The proposed Butler matrix is connected to a patch array antenna to demonstrate its performance as a beam-forming network. Four patches of the array antenna are placed with a distance of
( is wavelength at
). The simulated
and measured radiation patterns are shown in Fig. 6. The measured beam directions are at
,
, 43.8 ,
(theoretical beam directions are 14.5 ,
, 48.6 ,
).
IV. CONCLUSION
In this letter, a novel 4
4 Butler matrix is proposed by
employing couplers with
and
phase differences. A
phase shifter is not needed in this Butler matrix as in conventional ones. As expected, this novel Butler matrix allows for significant improvement of phase and amplitude performance in a
compact topology.
REFERENCES
Fig. 5. Photograph of the fabricated Butler matrix.
10 dB return loss bandwidth is 20.1% and the 3 dB transmission
bandwidth is 24.4% (bandwidth with transmission characteristic between 6 dB and 9 dB).
Fig. 4 shows the simulated and measured phase differences
of the Butler matrix. By feeding signal at Port1 and Port2, the
measured phase differences of
and
are
obtained with an amplitude imbalance of less than 0.4 dB. Thus,
the overall phase error is less than 0.9 at the target frequency.
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