New design of wideband microstrip branch line coupler using T-shape and open stub for 5G application

Ali Abdulateef Abdulbari, Sharul Kamal Abdul Rahim, Mohamad Zoinol Abidin Abd Aziz, K. G. Tan, N. K. Noordin, M. Z. M. Nor Wireless Communication Centre (WCC), School of Electrical Engineering, Universiti Teknologi Malaysia (UTM), Skudai, Malaysia Centre for Telecommunication Research and Innovation (CeTRI), Faculty of Electronic and Computer Engineering, Universiti Teknikal Malaysia Melaka (UTeM), Melaka, Malaysia Faculty of Engineering and Technology, Multimedia University, Melaka, Malaysia Faculty of Engineering, University Putra Malaysia, Selangor, Malaysia Faculty of Electrical Engineering, UiTM Cawangan Johor, Kampus Pasir Gudang, Johor, Malaysia


INTRODUCTION
The branch-line coupler (BLC) is an important component in the microwave integrated circuit such as power divider, modulators, phase shifter and feeding network to beamforming network. It has the ability to divide the power equally/unequally with a 90° phase difference between the output through and the coupled port. The traditional BLC is easily implemented using a quarter-wave transmission line (QWTLs) that includes the impedance of Zo/√2 = 35.35 Ω and Zo = 50 Ω [1]. The conventional BLC single section is realized by the λ/4 microstrip line (QWLMLs) by three lines as 90° transmission lines, one 180° transmission line, one -90° transmission line with specified characteristic impedance [2,3]. The major issues in conventional BLCs are the large size of λ/4 and the narrow bandwidth of 10%-20% [4] which limits their usage. Many techniques have been proposed to overcome these issues such as using cascading [5] and a T-shaped transmission line, which reduces the size by 55% [6]. The regular microstrip BLC is replaced with a lumped circuit, made from two open stubs by a series of transmission line (TL) [7], and it reduces the size of BLC to 55.2% with a fractional bandwidth of 56% based on the lumped element method, fractal geometry [8], three-section branch-line hybrid [9], a multilayer [10], a meander line [11], and a dual-band branch-line coupler [12]. Various techniques have been employed to miniaturize the size of the microwave component (s), to reduce the BLC size and to increase the bandwidth. Therefore, the size reduction is very important for developing the high-performance radio frequency, and stringent requirements should be in place for the future microwave communication systems [13].
In this study, a compact new design for a wideband BLC using a T-shape and an open stub on the TL is proposed. The transmission line consists of a horizontal open-stub with a slot of T-shape and a vertical T-shape connected to the center of the TL. The symmetrical T-shape transmission line is one of the best methods to realized microwave as in [14]. The proposed BLC wideband (QWTL) structure hasthe capability to achieve a wideband frequency ratio operation. The overall proposed structure is simulated by using CST Microwave Studio and fabricated using a cheap substrate material of FR-4. A good agreement between the simulation and measurement results was obtained throughout the 2.9 to 4 GHz frequency band.

THEORETICAL ANALYSIS TRANSMISSION LINE 2.1. Mixed-mode S-parameters of the BLC
In this part, scattering parameters integrated with the ABCD matrix of the circuit proposal were employed to obtain the analysis solution as SSB, SSE and SSD are to describe the S-parameters as in [15].
The relationship between the mixed-mode [S MD ] and the scattering matrix [S BLC ] of the proposed branch-line coupler (BLC) has been reported as in [15].
The matrix equation [MT] is a standard of S-parameters used in the proposed BLC and can be expressed as follows:

STRUCTURE OF PROPOSED BLC USING T-SHAPE AND OPEN STUB TL
The quarter wavelength of the transmission line BLC is known to have a narrow bandwidth and bulky size, as shown in Figure 1(a), the size of the branch-line coupler (BLC) considering the major issues. Figure 1(b) shows the Z0 as well as the θo characteristic impedance and the electrical length of the BLC. The layout of the symmetrical T-shape TL, consists of two series transmission lines (Za1, θa1) and the shuntopen stub (Zb2, θb2). The design equation of the T-shape model is given by [16] and can be expressed as follows: Z is the characteristic impedance Ω of the BLC. The T-shape model BLC achieves a compact size when θa1 is <90°. However, when the series arm impedance Za1 increases, the series arm length θa1 will decrease, while the shunt impedance Zb1 decreases for a constant stub length θb1. The present equivalent circuit of the conventional branch-line coupler is depicted in Figure 1 (c) and Figure 1 The scattering parameters of the four-BLC ports, S11, S21, S31, S41, are expressed in term of even and odd reflection coefficients Γee, Γeoo, Γooe and Γoo which can be expressed as follows: S11= Γ ee + Γ oee + Γ oo + Γ eoo 4 The characteristic impedance, for example Z0=50 Ω, Z1=139.7 Ω, Z2=54 Ω, Z3=58.3 Ω and electrical length θ1=θ2=θ3=90° [17]. The BLC single section is printed on the substrate from FR-4, ƹr=4.4, loss tangent, tan δ=0.025, and h=1.6 mm. The primary resistances of the main-line of the BLC are Z1, Z2, and Z3. The electrical length θ1, θ2, and θ3 is the main-line for BLC. Port 1 is an input, port 2 and port 3 are outputs, while port 4 is an isolation port. The width of the microstrip transmission line at 3.5 GHz at the center frequency was calculated from the (8) and can be expressed as follows: is the impedance characteristic of the microstrip line and the subscript N refers to the number of ports; 0, 1, 2, and 3. The length of the microstrip transmission line from the quarter wavelength λ/4 of the BLC has been reported in [1] and was calculated from the (11), which can be expressed as follows: The velocity of light in space is c=186, 282 miles per second, and the effective permittivity ƹeff, of the BLC microstrip line, was obtained as in [18] and can be expressed as follows: Through the even-odd mode analysis, S-parameters of the reflection coefficient to the transmission line (TL) can be expressed in [19]. Γ is the reflection coefficient and T is the transmission line coefficient, meanwhile can be expressed as follows:

PROPOSED DESIGN BLC WITH T-SHAPE AND OPEN STUB
To overcome the major issues of the narrow bandwidth and the bulky size of the BLC, the proposed T-shape and an open stub were used to replace a conventional λ/4 of the transmission line. Figure 2 shows the simulated design of the proposed BLC where a compact design T-shape and an open stub were used. As shown in Figures 2(a) and (b), T-shaped slots were introduced to replace a conventional (QWTL) BLC design. In the proposed design layout, the author also introduced the equivalent open stub design with an open stub with a T-shape model and T-shape model structure to the proposed layout in order to give a better performance in the designas shown in Figure 2(c) and (d). The T-shape approach was adopted to reduce the size of the transmission line λ/4 and miniaturize the microstrip of the BLC. The impedances of the horizontal and vertical of the branch-line coupler (BLC) are 35.36 Ω and 50 Ω. Meanwhile, the output of the phase difference of the quadrature wavelength of the BLC is 90°, and the electrical length of the BLC is θ2=θ3=90°. All the impedances and optimized parameters for the BLC are presented in Figure 2.
The Agilent vector network analyzer was used to test the performance of the proposed BLC while as to ensure that the proposed BLC design achieved good performance and was compared with the simulated result. Figure 3 shows the experimental setup for the S-parameters measurement setting of the proposed BLC. The network analyzer was used to measure S11, S21, S31, and S41 with a frequency ranging from 2 GHz to 5 GHz. Table 1 presents all dimensions of the proposed BLC.

DESIGN I
As for the design (I) showed in Figure 4, the author had designed a conventional BLC by using all the calculated parameters obtained through the (11) for obtaining the length of TL and the (8) in obtaining the width of TL. All of the parameters were obtained by tuning the center frequency at 3.5 GHz and by using the characteristic of the material used which is FR-4. The final dimension obtained is as follows: A1=10 mm, A2=4.5 mm, B1=10.3 mm, B2=2.5 mm, C1= 0 mm, C2=2.5 mm. The occupied area of the proposed coupler is of the design (I) is 0.29 λg×0.17 λg (29×17 mm 2 ). The simulation result of design (I) is presented in Figure 5. The results show that the differential mode impedance matches, where the return loss S11 value is -26.4 dB at the 3.4 GHz operating frequency and their isolation S41 value are -28.68 dB at the 3.43 GHz. Additionally, the coupling factor S21 of design (I) and the insertion loss S31 is equal to -3.5 dB and -3.1 dB, respectively. Figure 5 also shows that the fractional bandwidth is 24.42% from 2.9 GHz to 3.7 GHz. Figure 6 shows the measured phase difference between the output ports through S21 and coupled S31. It indicates that design (I) is able to couple the signal diagonally from the input to the output at 3 GHz.  Figure 7, the value of the return loss S11 for the proposed design is -30.69 dB at the center frequency of 3.5 GHz. This figure also indicates that the isolation factors obtained at the operating frequency of 3.5 GHz band is equal to -29.28 dB. Additionally, the coupling factor S21 and the insertion loss S31 of the proposed design are at -2.97 and -3.65 dB, respectively. From the result shown in Figure 7, the introduction of the T-shape and the open stub in the proposed design wide the bandwidth value, wherein this design the fractional bandwidth is 2.9 GHz-4 GHz, which is equal to 32.29%. This data clearly indicates that the introduction of the slotting and the stub techniques within the BLC design can widen the bandwidth value. The bandwidth was calculated using (15) and can be expressed as follows: The simulated result of the proposed design (II) has a good agreement on the frequency of 3.5 GHz. Figure 8 shows the different phases of two output ports, S21 and S31 of the proposed BLC. Figure 9 shows the measurement result of the proposed design (II). The results signify that the range operating bandwidth is between 3 GHz to 4.1 GHz where it gives 31% bandwidth values, where the return loss S11 and isolation S41 are -27.47 GHz and -26.2 GHz respectively. Meanwhile, the through S21 and coupled S31 values are equal to -4.4dB and -4.1dB, respectively. Figure 10 shows the fabricated proposed BLC using the T-shape with the open stub.

. Comparison between design (I) and desin (II)
This section discusses the comparison structure analysis of the single section of design (I) and design (II) of the BLC. Figure 11 shows the difference between the conventional design (I) and the proposed design (II), using the techniques of the T-shape and the open stub on the horizontal and vertical arm transmission lines of design (II). Moreover, the results show that the fractional bandwidth has been improved from 24.42% to 32.29% and the shifting of the signal from the frequency 3.43 GHz to 3.5 GHz as a desired operating frequency. The results of the coupling factor between two ports, S21 and S31 for the conventional design (I) and the proposed design (II) increased from (-3.5 and -3.1) dB to (-2.97 and -3.65) dB respectively, as shown in Figure 12. As for Figure 13, it demonstrates a simulation for the conventional design (I) and the proposed design (II) of the phase difference between ports S21 and S31 respectively.

Comparison of the simulation and measurement result in the proposed design (II)
The comparison of the simulation and measurement result of design II is shown in Figure 14. According to the result shown, it was interpreted that the simulated and measured data is in good agreement. Figure 15 shows that the amplitude imbalance and the phase difference between two output ports for the simulation and measurement are approximately ±0.1 dB and 90°±3.54° and 90°±3° respectively.

Comparison of the simulation design (I) design (II) and measurement design (II)
This section shows a comparison of the simulation and measurement between design I and design (II) as shown in Figure 16. While Table 2 shows a comparison of the conventional design I and proposed design (II). All the data from Figure 16 is tabulated in Table 2. As can be seen, the proposed design gives a better return loss S11 and isolation S41 values where it gives -30.69 dB and -29.28 dB at center frequency 3.5 GHz. The table also clearly shows the effect of T-shape and stub techniques on miniaturizing the size of the proposed design. Table 3 shows a comparison of the proposed BLC design performance with previous researches in terms of their isolation, return loss, coupling factor, insertion loss, and phase shift values respectively. This table shows that the proposed BLC design is comparable with previous work although the proposed design implementing less complex structure.

CONCLUSION
This paper proposed a compact new design for a single-section wideband 3-dB BLC with a vertical slot T-shape on the TL connected with two open-stubs and a horizontal print T-shape quarter wavelength transmission line. The proposed BLC was designed using the right/left-handed transmission line. The design I of the BLC was a conventional design as validation with the enhanced structure in design (II). The proposed design (II) had the compact size of 27×16 mm (0.27 λg×0.16 λgmm 2 ) and improved the bandwidth from 24.42% to 32.29%. The results showed that the simulation and measurement of return loss, insertion loss and phase shift between ports had a good agreement with design (II) BLC λ/4 transmission line. The application of the proposed BLC design in this study can be used for future studies in the 5G wireless communication system.