# A Circularly Polarized Non-Resonant Slotted Waveguide Antenna Array for Wide-Angle Scanning
## 1 Introduction  
The slotted waveguide antenna offers notable advantages, such as a high power capacity, low loss, and low side-lobe levels; thus, it is extensively utilized in radar applications. A circularly-polarized antenna possesses the capability to receive multiple polarized waves, and the circularly polarized wave has smaller energy losses when penetrating rain or fog areas [1, 2]. The slotted waveguide antenna, when applied in phased array systems, has significant advantages in complex electromagnetic environments, and it can achieve many beneficial characteristics, such as fast beam tracking, beamforming, etc. [3, 4, 5, 6]. However, to date, there has been limited research attention directed toward the circularly polarized phased slotted waveguide antenna. Based on our research (1), the dimensions of the waveguide and polarizer are the primary limiting factors regarding the beam scanning range of the phased slotted waveguide antenna, as these large structures result in a notably narrow scanning range. Consequently, this article introduces a novel approach: a compact circularly polarized non-resonant slotted waveguide, aimed at achieving a broader scanning angle. Typically, a phased slotted waveguide array performs the phase dimensional beam scanning along the waveguide arrangement direction. This array commonly operates under resonant conditions, radiating linearly polarized electromagnetic waves [7, 8, 9, 10, 11]. The radiation elements typically consist of a series of slots positioned along the narrow wallof the waveguide. According to the basic phased array theory, the maximum achievable phase scanning angle satisfies the following:  
\[\theta_{max}=arcsin(\frac{\lambda}{d}-1) \tag{1}\]  
where \(d\) is the element distance. In the slotted waveguide array, \(d\) represents the distance between adjacent linear waveguides. The grating lobe will manifest in the visual space if the scanning angle exceeds \(\theta_{max}\). Due to the limitation of the element distance, it is challenging to design a phased scanning slotted waveguide array with multiple polarizations. However, the regular fixed-beam slotted waveguide antenna arrays offer numerous valuable electric characteristics, such as wide-band polarization [12], dual-band, dual polarization [13; 14; 15], and circular polarization. Circularly polarized slotted waveguide arrays can be categorized into three distinct types based on their circular polarizer structures: the compound slots, the circular polarization patch or dipole, and the circular polarization (CP) cavity. Compound slots typically consist of a pair of slots [16; 17] or some complex slots [18] that are carved into the wall of the waveguide. This type of CP element has the lowest profile and simplest configuration. However, achieving identical phase excitation requires the compound slots to be spaced at \(\lambda_{g}\), necessitating the use of dielectric-filled waveguides or SIW to reduce the waveguide's wavelength [19]. While dielectrics offer advantages in miniaturization, they also introduce additional electromagnetic attenuation losses, and many dielectric materials can compromise structural integrity. Another option is the ridge gap waveguide [20], but it also faces difficulties in miniaturization. The CP patch, dipole, and CP cavity are loaded on the waveguide slots, serving as polarizers. The CP patch or dipole has a flexible configuration, offering an excellent circular polarization performance (e.g., high efficiency [21], broadband CP [22]), and patches are easily combinable with metamaterials and other novel technologies [23; 24; 25]. However, the compact patches and dipoles loaded on the waveguides need dielectric layers, and compactly arranged patches may induce more mutual coupling effects, limiting the power capacity. On the other hand, CP cavities are typically open-ended waveguides with specific configurations [26; 27].