**7. SIW crossover**

As a microwave element, the crossover is not only able to make two signals crosstransmit and maintain high isolation, but also usually appears in the array antenna beamforming network, and is an important part of the Butler array. The use of SIW technology to design the crossover has become a research hotspot for many scholars. According to the circuit structure, the existing SIW crossovers can be divided into two categories, one is the traditional cascaded 3-dB directional coupler structure, and the other is based on the coupling theory, utilizing the orthogonality of the modes in the SIW resonators to achieve isolation of two signals.
