# On the Use of Focused Incident Near-Field Beams in Microwave Imaging
## NF Beams and Results
### Interrogation of Objects by the NF Plate  
The NF plate test setup, shown in Figures 5 and 6, includes a thin passive plate with the length of about \(\lambda\) (orange plane) in the \(y\)-direction that consists of multiple capacitive elements and is placed within a parallel plate waveguide with the height of \(\lambda/20\); see [47] for the details regarding this NF plate. At the frequency of 1 GHz, this passive NF plate is excited by the cylindrical waves emanating from the inner conductor of a coaxial cable that has been extended into the parallel plate waveguide. The excitation source is shown by an orange cylinder on the excitation plane (light blue plane) in Figures 5 and 6. (The NF plate has the same separation of \(\lambda/15\) from the excitation and focal planes.) Once illuminated by the impinging cylindrical waves, the NF plate creates a highly oscillating electromagnetic fields at the plate that subsequently results in a focused NF beam at the focal plane. As noted in [47], the NF plate is designed by back-propagating the desired focused NF distribution at the focal plane toward the NF plate to find the required field on the NF plate, then calculating the required surface impedance profile needed to support the existence of this field, and finally implementing this surface. Our simulated NF plate resulted in about \(\lambda/12\) beamwidth on the focal plane, and was not a perfect _sinc_ function compared to the one reported in [47], which also had a narrower beamwidth of \(\lambda/18\). We also note that this NF plate can be regarded as an electromagnetic metasurface which transforms a given non-focused field into a desired focused one. All of the simulation case studies for the NF plate setup are performed under one of the following two configurations: (i) _Configuration I_ is the configuration in which the NF plate is present within the parallel plate waveguide to create a focused NF distribution, and (ii) _Configuration II_ is the configuration that the NF plate is removed from the imaging setup in order to reduce the focusing of the NF distribution. (That is, Configuration II is obtained by removing the orange NF plate in Figures 5 and 6.) For each of these two configurations, four different case studies are considered. The utilized objects (OI and the undesired scatterer) in all four case studies have the same relative permittivity of 1.50. In the first three case studies, the objects are dielectric boxes with the same dimensions of \(\lambda/20\times\lambda/10\times\lambda/10\) along \(x\)-, \(y\)-, and \(z\)-directions, respectively, while in the fourth case study we elongate our dielectric box in the \(y\)-direction, thus creating a dielectric slab with the dimension of \(\lambda/20\times 2\lambda/3\times\lambda/10\) with the same relative permittivity of 1.50. In all these four cases, the separation between the closest facet of the dielectric objects to the NF plate is about \(\lambda/60\). In addition, although our simulated configurationssuffer from large \(|S_{11}|\) values, these values are close to each other, thus making the comparison of the two configurations reasonable. The four case studies are illustrated with figures in Appendix A. In Case I, as can be seen in Figure 10 in Appendix A, the only object present in the system is the OI that is placed exactly at  
Figure 5: The simulated NF plate setup. The NF plate (orange plane), introduced in [47], is excited from the back by the inner conductor (orange cylinder) of a coaxial cable which lies on the excitation plane (light blue plane). This structure then illuminates the OI (light green dielectric box) and the undesired scatterer (dark green dielectric box). The dark blue and yellow planes show the focal plane and the receivers’ plane respectively. The two dielectric boxes have the same size of \(\lambda/20\times\lambda/10\times\lambda/10\) along \(x\)-, \(y\)-, and \(z\)-directions, and have the same relative permittivity. Figure 6: The simulated NF plate (orange plane) setup with a dielectric slab in front of it.