I. Introduction
Slot antennas utilizing rectangular waveguides with Substrate Integrated Waveguide (SIW) technology have been the focus of extensive research. They are renowned for their compact design, minimal losses, high gain, and high radiation efficiency, making them highly suitable for integration into microwave and millimeter-wave circuits. Typically, a uniform arrangement of SIW slot antennas yields a sidelobe level (SLL) around -13.6 dB. Nevertheless, there is a general demand for SLL to be lower than -13.6 dB. Consequently, it becomes necessary to implement a technique for reducing the SLL in the slot array. Designing antenna array is a common approach to achieve higher gain and lower SLL. For example, arranging multiple designed antenna units horizontally and using power dividers for feeding [1]–[5]. Additionally, although some waveguide slot antennas based on SIW have low SLL, their working bandwidth are narrow [6]–[7]. For a single antenna unit, the main method to improve low sidelobe performance and working bandwidth include changing the feeding method and designing a new slot array.