Design and development of wideband 3dB/90 couper using multiple-rows vias substrate integrated waveguide (SIW) technique

Abstract

Substrate Integrated Waveguide (SIW) technique is a good candidate when designing a 3dB/90° coupler at high frequency as this technique exhibits low loss compared to traditional microstrip. In microstrip technique, some of the electric and magnetic field is propagating in the air (lower permittivity compared to substrate), which contributes to the loss of the signal. The loss in microstrip is more significant when operating at high frequency. In contrast, a signal is travelling inside a substrate in SIW technique, guided by metallic vias at both side of the transmission lines. This propagation is similar to the waveguide propagation where no signal is propagating in the air. The possibility of signal loss can be happened through leakage between vias which lead to the performance degradation of the SIW coupler. The presented techniques to enhance the performance of SIW coupler in open literature either has limited bandwidth, larger dimension, or limited to simulated performance only. Therefore, the multiple-rows vias to enhance the operational bandwidth (>20%) of SIW coupler for high frequency applications is presented in this thesis. The proposed SIW coupler technique are designed, analysed and validated at two different frequency ranges, Ku-Band (11-17 GHz) and K-Band (20-26 GHz). Both designs are implemented on Roger RO4003c with thickness (h) of 0.508 and relative permittivity (ɛr) of 3.55. The proposed SIW coupler design optimization is investigated using CST Microwave Studio simulation tool. The investigation is started by sweeping different values of d (diameter of vias) and p (pitch between vias). Through the parametric study analysis, the optimum value of d and p to achieve the minimum insertion loss (S21, S31) is d=0.6 mm and p=0.92 mm, respectively. These optimized dimensions values are then applied in the next parametric study of the SIW couplers where at this stage, additional rows with different number of metallic vias are applied at the centre of both edge side of the coupler structure. It is revealed that the best performance of the SIW coupler can be achieved when 3 rows of metallic vias implemented at the centre of both edge side of the coupler structure. By considering return loss and isolation better than 10 dB, coupling coefficient of 3±1.5 dB and phase difference between output port of 90°±5°, the simulated results show an improved operational bandwidth of 43.88% at Ku-Band and 26.31% at K-Band. The experimental results of the proposed SIW coupler are accomplished using Agilent PNA-X 5245A PNA Network Analyzer where the measured results agree well with the simulated results. In overall, the proposed method for the SIW coupler features wide bandwidth with flat coupling response and better return loss and isolation. Accordingly, it has good potential to be implemented in high frequency application.