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IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 19, NO. 11, NOVEMBER 2009 683
A Novel Triple-Band Microstrip Branch-LineCoupler With Arbitrary Operating Frequencies
Chong-Yi Liou, Min-Sou Wu , Student Member, IEEE , Jen-Chun Yeh , Student Member, IEEE ,Yu-Zhi Chueh , Student Member, IEEE , and Shau-Gang Mao , Senior Member, IEEE
Abstract—This letter presents a novel microstrip branch-linecoupler operating in three frequency bands. The design method-ology of the triple-band branch-line coupler is established byusing a compensation technique to improve the matching prop-erty within each passband region. The proposed coupler withcompact size is realized by folded microstrip open-circuited andshort-circuited stubs. The measured, full-wave simulated andequivalent-circuit modeled results illustrate good agreementamong them, which validates the design method and shows theadvantages of deep rejection between each operating frequency,and the dc grounded input and output ports.
Index Terms—Branch-line coupler, folded microstrip stub, mi-crostrip line.
I. INTRODUCTION
BRANCH-LINE couplers are one of the most rapidly de-
veloping passive components in modern microwave com-
munication systems. Compact size and multi-band operation are
crucial in the development of advanced branch-line couplers. In
the past years, many researches have been aimed at the design
of dual-band branch-line couplers [1]–[6]. The transmission
line with one or two shunt stubs [1]–[3] and the cross couplingbranch [4] were presented to realize a dual-band branch-line
coupler. Furthermore, the composite right/left-handed trans-
mission line was adopted to design a dual-band branch-line
coupler by using surface mounted devices (SMDs) [5], [6].
However, the composite right/left-handed coupler operating
in the high frequency range will suffer significant loss caused
by the SMDs. Recently, a multi-band branch-line coupler with
open stubs has been proposed [7], but the passband frequencies
cannot be allocated arbitrarily.
In this letter, a new triple-band branch-line coupler with arbi-
trary operating frequencies is proposed, and its design method is
presented. Based on the compensation technology [8], two addi-tional capacitances are attached into the triple-band resonator to
improve the matching properties of the triple-band branch-line
coupler. The series connection of three shunt LC resonators can
be transferred to the parallel connection of one shunt LC res-
onator and two series LC resonators [9]; the folded microstrip
Manuscript received July 07, 2009; revised August 03, 2009. First publishedOctober 20, 2009; current version published November 06, 2009. This work was supported in part by the National Science Council of Taiwan, R.O.C., underGrant NSC 96-2221-E-027-022, and Grant NSC 96-2628-E-027-001-MY3.
The authors are with the Graduate Institute of Computer and Communica-tion Engineering, National Taipei University of Technology, Taipei 106, Taiwan(e-mail: [email protected]).
Digital Object Identifier 10.1109/LMWC.2009.2031998
Fig. 1. (a) Equivalent-circuit model of the triple-band branch-line coupler.(b) Equivalent single-band resonator and initial triple-band resonator. (c)Impedance inverter with two shunt capacitances for phase and impedancecompensation.
stubs are equivalent to it, which can be treated as a triple-band
resonator for the branch-line coupler design. Finally, the pro-posed coupler is implemented to validate the proposed design
method. Additionally, this coupler has the advantages of com-
pact size and deep rejection between each operating frequency,
and the dc grounded input and output ports.
II. DESIGN METHODOLOGY
The equivalent circuit of the proposed triple-band branch-line
coupler is depicted in Fig. 1(a). First, the single-band resonator
is shown in Fig. 1(b), and and can be determined by
using the 1st-order low-pass filter prototype [10]. Thus the ini-
tial triple-band resonator is obtained by cascading three res-
onators with difference operating bands [Fig. 1(b)]. Second,to simultaneously improve the phase and impedance matching
conditions of the impedance inverter within the operating bands,
two capacitances are shunted at each end of the quarter-
wavelength transmission line as shown in Fig. 1(c) [8]. Thechar-
acteristic impedance and capacitance are determined by
(1)
(2)
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LIOU et al.: NOVEL TRIPLE-BAND MICROSTRIP BRANCH-LINE COUPLER 685
Fig. 3. Geometry of the triple-band branch-line coupler. ( mm, mm, mm, mm, mm,and
mm.)
Fig.4. Equivalent-circuited modeled, full-wave simulated, and measured S-pa-rameters of the triple-band branch-line coupler.
the triple-band branch-line coupler are 170 MHz, 110 MHz, and
270 MHz.
IV. CONCLUSION
A novel triple-band branch-line coupler with arbitrary op-
erating frequencies has been presented and its design method-
ology based on a compensation technique is proposed. This cou-
pler is realized by using folded microstrip open-circuited and
short-circuited s tubs t o obtain a c ompact s ize at
4 GHz. The measured, full-wave simulated and equivalent-cir-
cuit modeled results agree closely with each other. The coupler
provides the advantage of deep rejection between each operating
frequency, and the dc grounded input and output ports. These re-
sults demonstrate the advantages and practical feasibility of the
triple-band coupler. Finally, the proposed design procedure and
resonator construction can be further applied to design various
multi-band microwave passive circuits, such as power dividers,
couplers and baluns.
Fig. 5. Amplitude imbalance and phase difference of the triple-band branch-line coupler. (a) first band, (b) second band, and (c) third band.
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