## 3 Results Analysis
### Measurement  
#### 3.2.1 Near-Field  
Considering the truncation level, the scanning scope was selected as 500 \(\times\) 500 mm. The transmitting antenna was a linearly polarized Bessel vortex EM wave generator operating at 20 GHz, and the receiving probe was a Bj-220 waveguide with matched polarization. The spacing in the \(x/y\) direction of the sampling plane met \(\Delta x=\Delta y=5\)mm. The measured near-field amplitude distributions operating at frequencies of 18/20/22/24 GHz for the plate (metal and FR4) and sphere are depicted in Figure 6. The reflection and transmission field upon the plate (metal and FR4) maintained the amplitude hollow characteristics like the Bessel antenna, and the reflected intensity of the metal plate was larger than that of the dielectric plate. The reflection distribution of the dielectric plate was slightly fuzzy, which is because the dielectric plate not only reflects the incident wave, but also has a transmission field. The metal aluminum plate can be approximately considered to be a total reflection case. In the reflection field of different operation frequencies, the reflected energy of 20 GHz was the largest, which relates to the antenna working at 20 GHz. Thereby, it had the strongest ability to transmit Bessel vortex waves. At 24 GHz, due to too much deviation from the operating frequency, the characteristics of the vortex wave were no longer visible. The probe, the center of the FR4 plate, and the center of the transmitting antenna were kept in the same horizontal line, and the FR4 plate and the transmitting antenna were installed on the same guide rail. The distance between the transmitting antenna and the FR4 plate was accurately controlled by the slide rail. ; we set the distance between the transmitting antenna and the probe to 965 mm, and the probe to 700 mm from the FR4 plate. It can be seen from Figure 6c that the transmitted field intensity of the vertically incident still had a concentric ring-shaped distribution. The transmitted field was 7 dB higher than the reflected field intensity, which indicates that the transmission coefficient was much larger than the reflection coefficient for the FR4 dielectric plate. For the forward scattering of the sphere, the diffracting degree was the most significant compared with the other cases. In order to investigate the off-axis scattering of the Bessel vortex wave upon the sphere, the corresponding near-field scattered data were measured by controlling the different distances away from the sphere center. Figure 8 shows the scattered near-field amplitude and phase distributions of the Bessel vortex waves on- and off-axis cases at 22 and 24 GHz, respectively. With the gradual increase in the off-axis distance from 0 to 100 mm, the  
Figure 6: Measured near-field amplitude distributions at different frequencies. (**a**) Metallic plate, (**b**) FR4 plate, (**c**) FR4 transmission, and (**d**) sphere. Figure 7: Measured near-field phase distributions at different frequencies. (**a**) Metallic plate, (**b**) FR4 plate, (**c**) FR4 transmission, and (**d**) sphere. intensity distribution ring gradually shifted to one side, and the symmetry of the annular intensity distribution seriously decreased. The corresponding helical phase distribution also gradually distorted, and the OAM spectrum purity decreased, which further indicates that the OAM characteristics of the scattered field are destroyed when the Bessel vortex EM wave irradiates the asymmetric target or the symmetrical target off-axis. In other words, the scattering of the off-axis sphere is the partial irradiation of the wave source, so the vortex characteristics of the scattered field are damaged. In order to investigate the scattering at a broader frequency, the simulation software FEKO 2019 was used to construct the scattering of the plate and sphere, as illustrated in Figure 9. The angle of reflection model was set to 45\({}^{\circ}\), and the incidence distance and scattering distance were expressed as \(d_{1}=d_{2}=400\) mm.