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A compact branch-line coupler using folded microstrip lines

Authors:
Ashmi Chakraborty Das
Ashmi Chakraborty Das
Ashmi Chakraborty Das
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Lakhindar Murmu at Indian Institute of Technology (ISM) Dhanbad
Lakhindar Murmu
Santanu Dwari
Santanu Dwari
Santanu Dwari
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Abstract

This paper presents a compact 3 dB Quadrature branch-line coupler using folded microstrip line geometry. Commercially available IE3D software has been used to design and simulate the structure. The proposed structure is simple and takes lesser optimization time. It has been shown that the structure provides about 43.63% compactness compared to the conventional branch line coupler. The structure, therefore, can be incorporated in the microwave circuit design where medium level compactness and faster design are preferred.
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A Compact Branch-Line Coupler using Folded
Microstrip Lines
Ashmi Chakraborty Das
Electronics Engineering
Indian School of Mines
Dhanbad, India
ashmi.chakraborty@gmail.com
Lakhindar Murmu
Electronics Engineering
Indian School of Mines
Dhanbad, India
lakhindar.kgec25@gmail.com
Santanu Dwari
Electronics Engineering
Indian School of Mines
Dhanbad, India
santanu_dwari@rediffmail.com
Abstract—This paper presents a compact 3 dB Quadrature
branch-line coupler using folded microstrip line geometry.
Commercially available IE3D software has been used to design
and simulate the structure. The proposed structure is simple and
takes lesser optimization time. It has been shown that the
structure provides about 43.63% compactness compared to the
conventional branch line coupler. The structure, therefore, can
be incorporated in the microwave circuit design where medium
level compactness and faster design are preferred.
Keywords—branch-line coupler, microstrip bend, scattering
parameters.
I. INTRODUCTION
A branch line coupler is a four port circuit consisting to two
quarter wave lines of characteristic impedance Z0 coupled
together by means of other two quarter wave branch lines of
characteristic impedance Z02 connected at the ends. Such
circuits are widely used in different microwave circuits since
its interception. A brief description and even odd mode
analysis of Quadrature branch line coupler has been given in
[1].
In 2005 Sun, Yen and van der Weide [2] presented a
compact branch-line coupler using discontinuous microstrip
lines. Keshavarz, Dannaeian, Movahhdi, and Hakimi [3]
presented a compact dual-band branch-line coupler based on
the inter-digital transmission line. The design of a compact
broadband branch-line hybrid was presented by Chun and
Hong [4]. In the next year Chun and Hong [5] designed and
tested compact wide-band branch-line hybrids. Later, in the
same year, Chang, Nam and Kim [6] presented the design of
various compact branch-line couplers using artificial
transmission lines. In 2007 Tang, Chen and Wu [7] proposed
ultra compact microstrip coupler using high impedance open
stubs. A new method for designing the microstrip branch line
couplers with predetermined compact size and bandwidth was
proposed by Tang and Chen [8] in the same year.
__________________________________________________
978-1-4799-2174-4/13/$31.00 ©2013 IEEE
During this year few more works on branch line coupler
was carried out, out of which design of compact high
performance slow-wave microstrip branch-line coupler by
Wang and Wang [9] and design of novel miniaturized fractal-
shaped branch-line couplers using phase-equalising method by
Chen and Wang [10] worth to be mentioned separately.
Tang, Guang and Tsai [11] in 2008 presented
miniaturization of microstrip branch-line coupler with dual
transmission lines. Next year a novel compact branch-line
coupler using non-uniform transmission line was presented by
Hosseini, Hosseini and Yazdini [12]. Two years later, in 2011,
Zou, Wu and Ma [13] presented miniaturized branch-line
coupler with wide upper stop band using novel synthesized
microstrip lines with series LC tanks. Later of this year Yeung
[14] presented a compact dual-band 90o coupler with coupled-
line sections and Tsai, Yang, Chen and Chen [15] presented a
miniaturized 3 db branch-line hybrid coupler with harmonics
suppression. Other works that were done in this year include
design of fractal shaped branch-line couplers by Singhania,
Kumar, Dash and pal [16] and design of compact coupled line
quadrature hybrid coupler with enhanced balance bandwidth by
Velidi, Shankar, Divyabramham and Sanyal [17]. A case study
of slow-wave resonant structures in branch-line coupler
miniaturization was presented by Kurgan, Bekasiewicz and
Kitlinski [18] in 2012. The proposed structure in the above
work provides 88% surface area reduction compared to a
conventional branch line coupler.
In this paper we have presented a compact 3dB Quadrature
branch-line coupler using folded microstrip line geometry and
IE3D electromagnetic simulator software.
II. DESIGN AND RESULT
To design the folded microstrip line branch-line coupler we
have started with a conventional branch-line coupler. The
operating frequency is chosen to be 3 GHz whereas Rogers RT/
Duroid 5880 substrate with dielectric constant 2.2, thickness
0.381mm and loss tangent 0.0009 has been chosen as substrate
material. For these parameters we get strip thickness 1.167mm
and length 18.245mm for the quarter wave 50 ohm
()
Z0
transmission line and strip thickness 1.906mm and length
17.972mm for the quarter wave 35.36 ohm
()
Z02
transmission line. Next we have introduced four right angle
bends in each arm as shown in Fig.1. The inner radius of the
bend has been taken as 0.1mm. Therefore the outer radius of
the bends for 50 ohm and 35.36 ohm line will be 1.267mm and
2.006mm respectively. The mean radius of these two lines will
be 0.634mm and 1mm respectively. Since one right angle bend
is equivalent to a path length of R2π mm where “R” is the
mean radius of the bend and there are four bends in a particular
arm, total R2π mm arm length is covered by the bends. For
50 ohm and 35.36 ohm lines these lengths are 3.984mm and
6.283mm respectively. The remaining arm lengths which are
14.261mm and 11.689 mm are divided in between the bends,
as shown in Fig. 2. The respective parameters of the folded
microstrip line branch-line coupler are tabulated in table 1.
Table 1: Geometrical dimensions of folded microstrip line
branch-line coupler
()
Z0 a1
b
1 c
1
5.1305m
m
1m
m
2m
m
()
Z02 a2
b
2 c
2
2.8445m
m
2m
m
2m
m
The proposed design shows that the structure occupies an
area 2
14.75 mm 14.625 mm 215.72 mm×= . The conventional
branch line coupler, in comparison, occupies an
area 2
20.16 mm 19.14 mm 385.86 mm×= . This is equivalent
to compactness ratio 0.4409 or 44.09% compactness. The
magnitude response of the scattering parameters and the phase
response at the coupled ports are shown in Fig. 3 and Fig. 4
respectively.
Fig. 1. Compression of coupler arm due to bending
Fig 2. Schematic diagram of compact, folded microstrip line branch line
coupler.
Fig. 3. Variation of the magnitude of scattering parameters of the proposed
branch line coupler with frequency.
Fig. 4. Variation of the phase of the coupled ports of the proposed branch line
coupler with frequency.
III. DISCUSSION AND CONCLUSION
Fig. 3 reveals that the structure operates at 3.25 GHz with
about -3.123 dB powers at the coupled ports and -30.76 dB
power at the isolated port. The return loss at the input port is -
29.53 dB. The difference in powers between the coupled ports
is 0.001 dB which is negligible. Fig. 4 reveals that the phase of
S21 and S31 is 0
147.7 and 0
57.02 respectively. Thus the phase
difference between the coupled ports is 0
90.68 . For an ideal
Quadrature branch line coupler the power at the coupled ports
should be – 3 dB and phase difference should be 0
90 . Thus the
amount of error introduced in magnitude is 4.1% and the
amount of error introduced in phase is 0.76%. For most of the
applications this error is tolerable.
Fig. 3 and Fig. 4 further reveals that the operating
frequency has been shifted to 3.25 GHz from the starting
design frequency 3GHz. This shift in frequency is due to the
introduction of the bends in the coupler arms. During the
introduction the bends we assumed that a bend of mean length
R2π provides the same phase shift as that of a transmission
line of path length R2π but this is not true. This change in
phase effectively decreases the total electrical length of the line
and hence the circuit operates at higher frequencies. To operate
at the desired frequency 3GHz optimization will be required.
Since the operating frequency will be shifted at a lower
frequency an increase in coupler line length will be required
which, in turn, will decrease the compactness of the coupler.
For a conventional coupler operating at frequency 3.25 GHz
we get strip thickness 1.167mm and length 16.841 mm for the
quarter wave 50 ohm
()
Z0 transmission line and strip
thickness 1.906mm and length 16.589 mm for the quarter wave
35.36 ohm
()
Z02 transmission line. Therefore the
conventional 3.25 GHz branch-line coupler occupies an area
2
18.75 mm 17.76 mm 333.00 mm×= . Thus the proposed
structure provides compactness ratio 0.3522 or 35.22%
compactness in comparison with a conventional 3.25 GHZ
branch line coupler.
Most of the available compact branch line couplers,
available in the literatures, provide compactness typically in the
range 30% - 80%. However these circuit geometries are very
complicated in nature and hence take a lot of time to design for
any arbitrary frequency of operation. This time requirement
often overrides the cost reduction achieved due to their
compactness (as large computer time and man power is
involved in their design). In that respect the proposed circuit
provides a well balance between the compactness and time
required for designing at an arbitrary frequency. Therefore
such circuits are well applicable when medium level
compactness and faster design are of prime interest.
REFERENCES
[1] D. M. Pozar, Microwave Engineering, 3rd ed, John Wwiley & Sons:
New Delhi, 2008, pp. 333 - 336..
[2] K. Sun, S. Ho, C. Yen and D. Weide, “A Compact Branch-Line
coupler using discontinuous microstrip lines,” IEEE Microwave and
Wireless Components Letters, Vol. 15, No. 8, Pp. 519-520, August
2005.
[3] R. Keshavarz, M. Danaeian, M. Movahhedi, and A. Hakimi, “A
compact dual-band branch-line coupler based on the interdigital
transmission line,” IEEE Microwave and Wireless Compoents
Letters, Vol. 15, Issue 12, pp. 889 – 891 , 2005.
[4] Y. Chun and J. Hong, “Design of a compact broadband branch-line
hybrid,” IEEE MTT – s International Microwave Sysmposium
Digest, pp. 997-1000, 2005.
[5] Y. Chun, and J. Hong, “Compact wide-band branch-line hybrids,”
IEEE Transactions on Microwave Theory and Techniques, Vol. 54,
No. 2, pp. 704-709 February 2006.
[6] K. Chang , K. Nam, and J. Kim, “Design of various compact branch-
line couplers by using artificial transmission lines,” IEEE MTT – s
International Microwave Sysmposium Digest, pp. 1726-1729, June
2006.
[7] C. Tang, M. Chen and J. Wu, “Realization of Ultra-Compact Planar
Microstrip Branch-Line Couplers With High-Impedance Open
Stubs,” IEEE MTT – s International Microwave Sysmposium Digest,
pp. 995-998, 2007.
[8] C. Tang, and M. Chen, “Synthesizing Microstrip Branch-Line
Couplers with predetermined compact size and bandwidth,” IEEE
Transactions on Microwave Theory and Techniques, Vol. 55, No. 9,
pp. 1926-1934, September 2007.
[9] J. Wang, B. Wang, Y. Guo, “A compact Slow-Wave microstrip
branch-line coupler with high performance,” IEEE Microwave and
Wireless Components Letters, Vol. 17, No. 7, pp. 501-503, July 2007.
[10] W. Chen G. Wang, “Exact design of novel miniaturized fractal-
shaped branch-line couplers using phase-equalising method,” IET
Microwaves, Antennas & Propagation, pp. 773-780,2007.
[11] C.Tang, M. Chen, and C. Tsai, “Miniaturization of microstrip branch-
line coupler with dual transmission lines,” IEEE Microwave and
Wireless Components Letters, Vol. 18, No. 3, pp. 185-187, March
2008.
[12] F. Hosseini, M. Khalaj-Amir hosseini, M.Yazdani, “Novel compact
branch-line coupler using non-uniform transmission line”, Asia
Pacific Microwave Conference, pp. 1577 – 1580, 2009.
[13] J. Zou, C. Wu, and T. Ma, “Miniaturized branch-line coupler with
wide upper stopband using novel synthesized microstrip lines with
series LC tanks,” IEEE International Workshop on Electromagnetics,
Applications and Student Innovation, pp. 71-74, 2011.
[14] L.Yeung, “A compact dual-band 90o coupler with coupled-line
sections,” IEEE Transactions on Microwave Theory and Techniques,
Vol. 59, No. 9, pp. 2227-2232, September 2011.
[15] K.Tsai, H. Yang, J. Chen, and Y. Chen, “A miniaturized 3 dB
branch-line hybrid coupler with harmonics suppression,” IEEE
Microwave and Wireless Components Letters, vol. 21, No. 10, pp.
597-539, October 2011.
[16] A. singhania, R. kumar, A. Raut, S.K.Dash, S. Pal, “Design of fractal
shaped branch-line couplers,” Asia Pacific Microwave Conference,
pp. 1 – 3, 2011.
[17] V. Velidi, G. Shankar, K. Divyabramham, and S. Sanyal, “Compact
coupled line quadrature hybrid coupler with enhanced balance
bandwidth,” Applied Electromagnetics Conference, pp. 1 – 4, 2011.
[18] P. Kurgan, A. Bekasiewicz, M. Kitlinski, “Slow-wave resonant
structures in branch-line coupler miniaturization: a case study,”
International Conference on Microwaves, Radars and Wireless
Communication, pp. 695 – 698, May, 2012.
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