# A Circularly Polarized Non-Resonant Slotted Waveguide Antenna Array for Wide-Angle Scanning
## 2 Configuration of the Antenna Element
### Slotted Waveguide with Metallic Cylinder Inside  
A linear center-set slotted waveguide array with a compact ridge is proposed. Longitudinal slots, serving as radiating elements, are positioned along the centerline of the waveguide's broad wall. Metallic cylinders are staggered on the upper inner wall of the waveguide, on alternating sides of the slots. The configuration of the single-slot radiator is illustrated in Figure 6.  
Conventional waveguide slots are typically carved at specific offset distances from the centerline of the broad wall. However, as previously discussed, the polarizer is loaded above the center of each slot at a 45\({}^{\circ}\) rotated angle. Consequently, conventional offset slots would lead to offset polarizers, resulting in wider antenna elements. With the proposed arrangement of the waveguide slots and polarizers, the element width can be significantly reduced. This approach allows all radiating slot elements to be arranged in a regular grid, eliminating the need for offsetting them as in traditional designs. The introduction of special structures into the waveguide can alter its electromagnetic performance [33; 34]. As depicted in Figure 6, a metallic cylinder is positioned on the inner upper wall of the waveguide at a specific offset distance from the centerline. The role of the cylinder is to disrupt the electromagnetic field distribution within the waveguide, resulting in a concentration of the electric field. In the conventional ridge waveguide, the waveguide's upper wall and the ridge's upper wall can be approximated to a plate capacitor. The capacitor's field distribution and voltage performance are influenced by the two plates' distance, which is the distance between the waveguide upper wall and the ridge upper wall in this situation. With a metallic cylinder introduced into the waveguide, the capacitor model is altered to three capacitors; the cylinder's bottom surface and the ridge's upper surface are approximated to a capacitor, and the remaining parts are approximated to other capacitors. According to capacitor theory, the closer plates have a higher voltage. Hence, the voltage at the cylinder is higher than that in other areas. Figure 7 illustrates the field distribution of the waveguide with the cylinder, showing the gathering of electric field and surface current at the cylinder position. This disruption leads to an asymmetric distribution of surface current, causing an electric potential difference at the centerline of the waveguide. Under these conditions, the center-set slot can be effectively excited by the asymmetric surface current, with the available slot positions indicated by red marks in Figure 7a. Through the preceding analysis, it is evident that both the location and scale of the cylinder influence the field distribution. Figure 6: The configuration of the proposed feed-slotted waveguide. (**a**) Perspective view. (**b**) Top view. (**c**) Front view. Basic slotted waveguide antenna theory indicates that the radiation efficiency is contingent upon the excitation voltage of the slot. Leveraging these theories, we adjusted the parameters of the cylinder to analyze the impact on the radiation performance of the waveguide slot. Figure 8 depicts the simulation model of the slotted waveguide element. Figure 9 depicts the S21 results for various cylinder heights under resonant conditions. The simulation results demonstrate that longer cylinders result in more radiated energy. Furthermore, while the offset distance of the cylinder does influence radiation efficiency, its effect is not particularly significant; rather, it primarily affects the gathering position of the surface current. The radius of the cylinder impacts the intensity of the reflected wave, with thicker cylinders potentially being intersected by the slot, resulting in irregular shapes and complex field distributions. Additionally, the slot length can be adjusted to tune the resonant frequency of the element, akin to conventional waveguide slots.