Proceedings of International Conference on Computational Intelligence & IoT (ICCIIoT) 2018
Design of a Dual-band Microstrip Balun from the concept of Rat-Race
Coupler
Priyansha Bhowmika,*, Tamasi Moyrab
a, b
Department of Electronics and Communication Engineering, National Institute of Technology, Agartala- 799046, West Tripura, India
Abstract: In this work a dual-band microstrip Balun is designed from the concept of Rat-Race Coupler (RRC). The quarter-wavelength Transmission Line
(TL) is constructed by cascading three Schiffman section. The difference in even- and odd-mode velocity is mitigated by introducing wiggle lines at the
inner edge of the coupled lines. The RRC provides equal power splitting at 2.1 GHz and 5.3 GHz. Terminating the isolation port of RRC with a 50 Ohm
resistor results in a Balun. The Balun occupies a size of 0.37×0.22 𝜆𝑔2 . The fractional bandwidth is 27.3% at first band and 5.5% at second band. The isolation
between the balanced port is below -20 dB.
1. Introduction
2. Design of Transmission Line (TL)
In recent technology, balanced circuit has gained importance owing to its
immunity towards environmental noise. Balun is a three-port device that
transforms an unbalanced signal to balanced signal (Zhang, Peng, and Xin,
2008). It is widely used in power divider, filter etc. to convert into balanced
circuit (Feng, Wang, Gomez-Garcia, & Che, 2018). Dual-band technology
is advantageous since it makes the circuit compact by reducing the number
of components.
In (Zhang, Peng, and Xin, 2008), balun is designed using the concept of
Branch Line Coupler (BLC). The dual-band balun is realized by using
stubs, which adjust the phase and stepped impedance structure, which
creates the dual-band. But the structure occupies larger area and bandwidth
is narrow. In (Shao, Zhang, Chen, Tan, & Chen, 2011), dual-band balun is
designed by introducing open stub at the balanced ports. In (Barik, Phani
Kumar, & Karthikeyan, 2017), BLC concept is used to design a balun. The
TL is realized using T-shaped open stub and shorted coupled Line structure.
The bandwidth is improved but the size was enlarged. In (Huang, Wang,
Zhu, Chen, and Wu, 2017), a marchand type balun is designed by placing
shorted edge coupled line at the balanced port. The performance is
improved at the cost of narrow bandwidth. In the above works performance
are enhanced but isolation between the balanced ports were not studied. In
(Kumari, Bhowmik, & Moyra, 2018), a single band Balun is designed from
the concept of Rat-Race Coupler and the isolation between the balanced
output port was improved.
In this work, a dual-band quarter-wavelength TL is designed by cascading
Schiffman section. The unequal even- and odd-mode velocity is improved
by introduction of wiggle lines at the inner edge. Placing the dual-band TL
in the conventional RRC, a dual-band RRC is formed with 180° phase
difference between two output ports. The fractional bandwidth (FBW) is
28.5% and 5.7% at 2.1 GHz and 5.3 GHz. A balun with FBW 27.3% and
5.5% at 2.1 GHz and 5.4 GHz respectively, is formed by terminating the
isolated port with a 50 Ohm resistor. The isolation between the balanced
ports are -31.42 dB and -22.08 dB at 2.1 GHz and 5.4 GHz respectively.
This section illustrates the design procedure of a dual band 70.7 Ohm
quarter-wavelength TL. The TL is designed by cascading three modified
Schiffman section. A conventional Schiffman section consists of a coupled
line connected at the end. The coupled lines have an unequal even- and oddmode velocity. Difference in the even- and odd-mode velocity is reduced
by placing wiggle lines at the inner edge of the coupled line. The layout of
the TL is shown in Fig. 1 (Bhowmik, & Moyra, 2018).
Fig. 1 Layout of the 70.7 Ohm Quarter-wavelength TL
Table 1 Dimensions of the TL (Fig. 1) in mm
L
L1
L2
LS LW W
WS
11.8 2.7 0.5 5
0.5 0.3 0.55
WW
0.5
S
0.2
The image impedance is obtained as (Bhowmik, & Moyra, 2018): 𝑍𝐼 =
√𝑍𝑜𝑜 𝑍𝑜𝑒
Where, even mode impedance is:
𝑍𝑜𝑒 = 2𝑍𝑐𝑚 = 𝑍11 − 𝑍12
And odd mode impedance is:
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𝑍𝑜𝑜 =
𝑍𝑑𝑚
= 𝑍11 + 𝑍12
2
conventional TL with the dual band TL, a dual band Rat-Race Coupler is
obtained.
The TL is designed using FR4 substrate (dielectric constant - 4.4,
substrate height - 1.6 mm and loss tangent – 0.02) and the optimized
dimension of the layout is tabulated in Table 1. The dual-band criteria are
satisfied, when the input impedance is zero at both frequency and the image
impedance is 70.7 Ohm,. Fig 2 shows the characteristic required for dualband property.
Fig. 3 Layout of the dual-band Rat-Race Coupler
(a)
The dual-band RRC is simulated on MoM method Zeland IE3D-14.
Fig. 4 portrays the simulated response. It is observed that at 2.1 GHz and
5.3 GHz, the RRC provide equal power division with phase difference of
180°, where the amplitude imbalance is 0.2 dB and 0.8 dB at 2.1 GHz and
5.3 GHz respectively and the phase difference is 0.19° and 7° at 2.1 GHz
and 5.3 GHz respectively. The S-Parameter is tabulated in Table 2.
(a)
(b)
Fig. 2 (a) Image impedance and (b) Input impedance of the 70.7 Ohm
Quarter-wavelength TL
3. Design of Balun from Rat-Race Coupler
(b)
A Rat-Race Coupler (RRC) consist of three λg⁄4 and one 3λg⁄4. Port 1
splits the input signal equally at the output port (port 2 and port 3) with
phase difference of 180°, whereas port 4 remains isolated. Replacing the
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Fig. 4 Simulated (a) S-Parameter response and (b) Phase difference of
the proposed Rat-Race Coupler.
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Table 2. S-Parameter response of Rat-Race Coupler
Parameter
First Band Second Band
𝑆11 (in dB)
-19.66
-16.62
𝑆21 (in dB)
-3.2
-3.7
𝑆31 (in dB)
-3.08
-3.72
𝑆41 (in dB)
-35.88
-16.96
Phase difference
-184.9°
187°
FBW
28.5%
5.7%
A Balun is a 3-port device that splits the input signal equally with a
phase difference of 180°. A Balun is formed when the port 4 (isolated port)
of RRC is terminated with a 50 Ohm resistor. The layout of the Balun is
shown in Fig. 5. The S-Parameter is tabulated in Table 2.
(b)
Fig. 6 Simulated (a) S-Parameter response and (b) Phase difference of
the proposed dual-band Balun.
Table 3. S-Parameter response of proposed dual-band Balun
Parameter
First Band
Second Band
(at 2.2 GHz)
(at 5.4 GHz)
𝑆11 (in dB)
-25.75
-39.12
𝑆21 (in dB)
-3.15
-3.4
𝑆31 (in dB)
-3.11
-3.79
𝑆23 (in dB)
-31.42
-22.08
Phase difference
183.7°
181.8°
FBW
27.3%
5.5%
Fig. 5 Layout of the proposed dual-band Balun
Fig. 6 shows the simulated response. It is observed that the input is split
between two output ports and the isolation is greater than -10 dB. The ±1
dB bandwidth is 600 MHz at first band and 300 MHz at second band. Table
3 displays the S-Parameter.
A Comparison with existing work is presented in Table 4. The proposed
balun is compact compared to (Barik et. al. 2017) and the FBW is better
compared to (Zhang et. al., 2008), (Barik et. al., 2017) and (Huang et. al.,
2017) with additional advantage of isolation between the output ports in the
proposed balun.
Table 4. Comparison with existing work
Size (𝜆𝑔2 )
FBW
Isolation
(in %)
(in dB)
Zhang et. al.
0.15×0.17
5.6/1.5
NA
Barik et. al.
0.41×0.82
22.2/6.5
NA
Huang et. al.
0.2×0.2
5.1/1.9
NA
Our Work
0.37×0.22
27.3/ 5.5
-31.42/ -22.08
Work
4. Conclusion
(a)
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In this work, a dual-band microstrip Balun is designed by terminating the
isolated port of a dual-band RRC. Design methodology are described
precisely. The quarter-wavelength TL satisfies the criteria of dual-band.
The Balun has wide bandwidth, compact size and better isolation between
the balanced port, which makes it suitable for use in microwave
communication circuits.
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REFERENCES
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