# A Circularly Polarized Non-Resonant Slotted Waveguide Antenna Array for Wide-Angle Scanning
## 1 Introduction
The cavity aperture can significantly reduce the mutual coupling between elements and enhance the isolation performance, being particularly beneficial in non-resonant and phased arrays with complex aperture distributions. Cavities also offer greater robustness compared to patches. Nevertheless, cavities pose challenges in miniaturization: the conventional CP cavity's aperture typically exceeds 0.6\(\lambda\), according to Equation (1); this dimension limits the widest scanning angle to \(\pm\)40\({}^{\circ}\). Figure 1 clearly illustrates these limitations. The study in [28] introduced a prospective approach to miniaturization employing a four-ridged cavity and a center-set slot to reduce the aperture of the element. However, this study lacked comprehensive details regarding the antenna and primarily concentrated on a small resonant array. Drawing from the insights of ref. [28], we analyzed its construction and operational principles, expanding its applications to non-resonant arrays and phased arrays. Meanwhile, we addressed several challenges in integrating this compact structure into complex arrays. Additionally, the cavity's CP frequency band is narrower than the bands of some CP patches [29]. The miniaturization of the waveguide slots poses another challenge in phased array antennas. As discussed above, the broad wall slots in phased arrays result in a very limited scanning angle. While reducing the width of the waveguide using ridge waveguides [8; 9] is an option, designing them with circular polarizers is not easy. The asymmetric ridge waveguide presents a potential solution [30]; however, existing analyses have primarily focused on resonant arrays. To our knowledge, there is a dearth of research addressing phase scanning slotted waveguide arrays generating circularly polarized waves. The non-resonant slotted waveguide array offers a low-cost solution for two-dimensional beam scan arrays, integrating phase and frequency scanning within the same array. This is particularly suitable for applications insensitive to frequency variations, such as meteorological radar. Excited by traveling waves, the non-resonant slotted waveguide has the capabilities for frequency scanning [31]. The frequency scanning direction is along the waveguide. With this capability, the non-resonant slotted waveguide array can offer a two-dimensional beam scan in both the phase domain and frequency domain [32]. Furthermore, compared to resonant arrays, the non-resonant array can accommodate a wider waveguide length range, enabling a wider bandwidth. To address the aforementioned problem, this article proposes an improved antenna structure aimed at achieving a wide beam scanning angle and integrating frequency domain and phase domain scanning array antennas. Our work introduced improved waveguide feeding slots, a circular polarizer, and staggered array design methods; a phase scanning angle of \(\pm 45^{\circ}\) and a frequency domain scanning range of \(+11^{\circ}\) to \(+13^{\circ}\) were achieved in this study. The key contributions of this study are as follows:  
1. A center-set waveguide slot element on the centerline of the broad wall is proposed to set up a waveguide circular polarizer with four ridges;
2. A compact circular polarized slotted waveguide planar array is presented that can realize a wide phase scanning range. The rest of this article is organized as follows. Section 2 introduces the design of the compact antenna element, including the circular polarizer and the center-set waveguide slot. Section 3 introduces the slotted waveguide array designed using this compact circularly polarized element, along with the technical details concerning beam scanning in the array design. Section 4 presents the simulation and the measured results. Finally, the conclusions are presented in Section 5.