SIW Transition Structure of Double-layer Substrates and Its Application in 3dB Coupler

Multi-layer circuit technology has been widely applied in microwave circuits and antennas today, which can effectively reduce the dimensions of the circuit. Many achievements have been made in transition structure of SIW multilayer substrates, but the compact broadband transition structure is still an important research topic at present. The transition of micro-waves between different layers of substrates is achieved by etching a rectangular slot in the metal layer between two layers of substrates. There are two types of transition methods: direct-type, where the input and output ports are at both ends of the substrates, and folded-type, where the input and output ports are at the same side of the substrates. Simulated results show that regardless of the transition type, the smaller the width of the rectangular slot and the smaller the distance from the slot edge to the short-circuit, the better the transmission performance. An increase in substrate thickness will lead to an increase in the phase shift of the transition structure. The folded transition structure will cause phase inversion, but will not change the transmission mode of SIW. This transition structure is not only suitable for main mode propagation, but also for higher-order mode propagation. An X-band SIW 3dB directional coupler was designed using the folded transition structure. In ranging from 9GHz to 11.5GHz, the reflection coefficient and isolation coefficient are less than −10dB, the transmission coefficient is between −4.6dB and −3.4dB, and the phase difference at the outputs is 90° ± 2°. The overall performance is comparable to that of traditional 3dB coupler, but the circuit area is reduced by half, which reflects the effectiveness of the rectangular slot transition structure and the superiority of multilayer circuits.