Design of a Novel Structure SIW 90° Coupler

Corresponding Author: Abdelkhalek Nasri Department of Physics, Electronics Laboratory, Faculty of Science, Tunis El Manar 2092, Tunisia Email: abdelkhaleknasri@yahoo.fr Abstract: This paper focuses on the analysis of passive devices using a recent emerging technology named Substrate Integrated Waveguide (SIW). This technology has been used in the conception of planar compact components for the microwave and millimeter wave’s applications. Through using Ansoft HFSS and CST code a substrate integrated waveguide coupler has conceived and optimized in this study. The SIW 90° coupler design simulations show good performances with low return loss, high isolation better than -20 and -40 dB, respectively and broad operational bandwidth.


Introduction
The extensive use of the rectangular waveguide components in millimeter-wave and microwave communication systems, radar and other equipments are due to their significant features such as their high Quality factor (Q-factor), high power capability and low insertion loss (Labay and Rao, 2011;Labay and Bornemann, 2008;Ahmad et al., 2013). However, they are difficult to be integrated in modern microwave and millimeter-wave integrated circuits because of their big size, nonplanar structure and strict requirement of manufacturing precision (Hao et al., 2006).
A new novel planar circuit named Substrate Integrated Waveguide (SIW) is facing recently a growing interest as it has common advantages with printed circuits such as low cost, small size (Labay and Rao, 2011;Ahmad et al., 2013) and which is known as the most popular and developed technology until now. Moreover, the SIW components are characterized with low insertion loss, low radiation loss and insensitive to outside interference since its components are covered by metal surfaces on both sides of the substrate (Hao et al., 2006;Abdel-Wahab et al., 2011;Xinyu et al., 2005;Zhigang et al., 2011).
In this present paper, the design platform of the new SIW 90° coupler is presented and discussed. Besides, this letter presents 90° coupler prototypes which are optimized and simulated, in addition to the results which are presented and compared with two electromagnetic (3D) software.

Design of SIW 90° Coupler
SIW technology defines as a type of rectangular dielectric-filled waveguide which includes a planar substrate with arrays of metallic vias to realize bilateral edge walls and on the same substrate its transitions with planar structures for instance microstrip and Coplanar Waveguide (CPW) are designed and integrated (Murai et al., 2011;Patrovsky et al., 2008;Rahali and Fahem, 2013). Within the same planar platform the planar and nonplanar structures can be integrated, which leads in this case to the design and development of low-cost millimeter-wave Integrated Circuits (ICs) and systems (Ali et al., 2008;2009;. A 90° coupler with low cost and low loss Substrate Integrated Waveguide (SIW) has been designed for low profile and compact mm-wave applications.

Design of SIW
The SIW consists of two linear metallic connected via dielectric substrate with a height of b. The electromagnetic fields within the SIW are confined by these metallic via arrays (Ali et al., 2008). The width of the SIW is a, the diameter of the metallic vias is D while the space between the adjacent vias is s. The geometric parameters are primarily determined by the relationship between the conventional rectangular waveguide and the SIW (Guo et al., 2008;Djerafi and Wu, 2007;Djerafi et al., 2010;.
In Fig. 1, port 1 is the input port, port 2 is considered as the through port, while port 3 stands for the coupling port and finally port 4 is used as an isolation port.
In order to achieve a wide-band performance the coupler parameters are finely tuned using three-Dimensional (3D) Electromagnetic (EM) simulation with HFSS and CST software.   For the microstrip line which has the same substrate and metallization thickness as the SIW is selected and which leads to a 50 Ω line width of w1 = 1.3 mm. These parameters are identical for all microstrip-to-SIW couplers highlighted in this study and the remaining design-specific dimensions are presented in Table 1.

Parameter Studies for SIW 90° Coupler
As an example, SIW 90° coupler is designed and the extra metallic via Dvia is optimized with different diameters. This variation shows a good improvement in the return loss and isolation. The design of the SIW coupler with different parameters Dvia is optimized to improve the return loss and isolation of -17 to -23 dB and, -27 to -44 dB, respectively. These results are shown in Fig. 3 and 4.

Simulation Results
The electric field distribution of the TE10 mode (Abdel-Wahab et al., 2012), the reflection coefficients S11, the transmission coefficients S21, the coupling coefficient S31 as well as the isolation coefficient S41 are presented in Fig. 5 and 6, respectively. It is noticeable through the results of this analysis that the 90° coupler character in the band is [9.5-12.5] GHz, in which the levels of reflection and isolation are below -15dB in more than 24% of the bandwidth and the insertion loss S 21 and coupling S31 are between -3 and -6 dB.
The simulation phase difference between two outputs ports is shown in Fig. 7. It is obvious that the phase difference is distributed in the range 89~93° in which the frequency band fluctuates between 9.5 and 12.5 GHz.
So, it is clear that these simulation results demonstrate the good performance of this integrated structure.

Conclusion
This paper focuses on the analysis of 90° coupler using a recent emerging technology named Substrate Integrated Waveguide (SIW). Prototypes of these 90° couplers with different via diameters are designed and simulated by the HFSS and CST code. Therefore, the paper presents results of this modeling which are discussed and allow as well integrating these devices in planar circuits.