Design and implementation of a six-port junction based on substrate integrated waveguide

Turkish Journal of Electrical Engineering & Computer Sciences http://journals.tubitak.gov.tr/elektrik/ Turk J Elec Eng & Comp Sci (2017) 25: 2547 – 2553 c TÜBİTAK ⃝ doi:10.3906/elk-1605-86 Research Article Design and implementation of a six-port junction based on substrate integrated waveguide Masoud JAFARI∗, Gholamreza MORADI, Reza SARRAF SHIRAZI, Rashid MIRZAVAND Department of Electrical Engineering, Amirkabir University of Technology, Tehran, Iran Received: 08.05.2016 • Accepted/Published Online: 20.09.2016 • Final Version: 29.05.2017 Abstract: A six-port junction based on substrate integrated waveguide technology with high reliability and efficient performance is proposed in a sub-K-band of 21–23.5 GHz. This substrate integrated waveguide (SIW) six-port junction is constructed from three directional couplers and one T-junction power divider as fundamental components in SIW form, where using appropriate via techniques led to –6 dB coupling coefficients and –15 dB return loss. There is good agreement between the simulation and the measurement results. Key words: Six-port, substrate integrated waveguide 1. Introduction Six-port junctions have various applications in microwave engineering. They can be utilized in measuring the reflection coefficient of an unknown load. For this purpose, one of the six ports is considered as an incident port and another is connected to a load. By detecting the power that appears on the remaining four ports and tracing the q-circles, the reflection coefficient is obtained. It can also be applied to direct conversion receivers, where two input-isolated ports act as local oscillator (LO) and radio frequency (RF) inputs and the remaining four ports are the output I/Q signals of quadrature phase shift keying (QPSK) demodulation. In this case, the optimum state occurs when the phase differences of the q-circles are 90 ◦ and their amplitudes are equal. Additionally, they have many other applications, e.g., displacement detection, vibration measurement, mechanical stress diagnosis, and automotive radar calibration. On the other hand, substrate integrated waveguides (SIWs) have high-power applications, especially in millimeter wave ranges. These waveguides are the planar versions of conventional rectangular waveguides, which are composed of two rows of metallic vias with predetermined diameter and distance period limited in top and bottom metallic layers. Many empirical formulas have been presented for the equivalent rectangular waveguide width of SIWs. In [1], the calculation of the propagation constant in SIW transmission lines, based on the concept of surface impedance with the method of moments (MOM), was performed. An empirical formula based on the calculation of eigenvalues of the admittance matrix was obtained with the least square approach [2]. A more accurate formulation was presented for special conditions and various dimension ratios by considering small error effects due to increasing period of vias [3]. Additionally, an empirical equation for the equivalent width of SIW, based on the method of lines, was proposed [4]. In this paper, a SIW six-port junction ∗ Correspondence: 2547 JAFARI et al./Turk J Elec Eng & Comp Sci is implemented in the K-band using empirical Eq. (1) for determining the equivalent width of the SIW [5]: wef f = w − d2 , 0.95s (1) where w is the distance of the center of two rows of the via, s is the period of vias, and d is the diameter of the vias, as shown in Figure 1. The parameter w ef f in this formulation is the most essential design parameter of this SIW six-port junction. Consequently, the cut-off frequency of the TE 10 mode is obtained from Eq. (2) [6]: Table 1. S-parameter magnitudes. Magnitude (dB) –15 –15 –6 –6 –6 –6 –15 –6 –6 –6 –6 fc10 = S-parameter S11 S12 S13 S14 S15 S16 S22 S23 S24 S25 S26 1 √ , 2wef f µε (2) where ε is the permittivity and µ the permeability of the dielectric substrate. 2. Analysis and design The block diagram of the proposed six-port junction is shown in Figure 2. It consists of three 90 ◦ hybrid couplers and a T-junction power divider. A simulation is performed by using full wave analysis software on a Rogers 5880 substrate with a thickness of 0.787 mm. Table 2. Out-port phase differences. Phase (degrees) 90 90 90 90 Out-port phase differences Phase (S13) – Phase (S14) Phase (S15) – Phase (S16) Phase (S23) – Phase (S24) Phase (S25) – Phase (S26) The design goals of S-parameter amplitudes and out-port phase differences are listed in Tables 1 and 2. The initial application of this junction is in QPSK demodulators; however, it also can be used as a reflectometer. Several matching via techniques are applied in order to improve the scattering parameters such as return losses, coupling coefficients, and phase differences. 2548 Output Port 3 Output Port 4 JAFARI et al./Turk J Elec Eng & Comp Sci Input Port 1 Input Port 2 Power divider s 50 Ω w eff d Figure 1. SIW transmission line. Output Port 5 Output Port 6 w Figure 2. Block diagram of the proposed six-port junction. 2.1. Power divider The designed power divider in T-form is shown in Figure 3. Without matching vias, this junction will have high return loss due to the reflection of the inputs. The two adjacent vias at the input port have capacitive effects and create symmetric resonance with minimum return loss. The diameters and positions of these vias greatly affect the splitter’s input return loss. These posts act as two parallel diaphragms that compensate for the discontinuity (capacitive) effects of the bends, thus improving the coupling between the input and output ports as well as its input return loss. Table 3. Dimensions of tapered microstrip transition. W 50Ω L TAPER W AP Port 1 2.6 mm 5.85 mm 5 mm Other ports 2.6 mm 5.85 mm 3.7 mm Furthermore, the other inductive vias are forming a parallel inductive diaphragm to creep the waves to outputs. From Eqs. (1) and (2), w is obtained equal to 6.7 mm for output ports at a cut-off frequency of 15.5 GHz. Because of the operating frequency of the junction, this cut-off frequency is an appropriate choice due to a relatively wide frequency range between TE 10 and TE 20 modes. The design parameters of this power divider are as follows:s = 0.6 mm, S1 = 3.3 mm, S2 = 3.2 mm, S3 = 0.725 mm, S4 = 2.3 mm, S5 = 0.4 mm , d =0.3 mm, and D = 0.4 mm. Figure 4 shows simulation results in which the return loss is less than –15 dB and the coupling coefficients are equal to –3 dB in a wide bandwidth with respect to the center frequency of 22.25 GHz. 2.2. The 90 ◦ hybrid coupler There are three 90 ◦ hybrid couplers in this design, which play an essential role in the overall performance of the structure. The designed 90 ◦ hybrid coupler is shown in Figure 5. The array of via holes is acting as a boundary 2549 JAFARI et al./Turk J Elec Eng & Comp Sci 0 Output S5 -3 d -6 S3 s S1 D Output S2 S -Parameters (dB ) S4 -9 -12 -15 -18 -21 -24 -27 -30 19 Input 20 21 2 (...truncated)