## 6 Conclusions  
A total dielectric transmitarray was analyzed to generate a multi-focusing area in the near-field of the antenna in the Ka-band. The design process of the transit array is based on geometrical optics and considers a single feed, in central configuration, to focus the near-field on a given spot. Then, the number of feeds is increased to obtain a transmitarray fed by a cluster of horns placed along a circular distribution in the horizontal main plane of the transmitarray. The location of the feed is carefully chosen to keep the same distance from each phase center to the center of the transmitarray. This configuration guarantees obtaining a very similar amplitude in the incident field whilst the phase changes and determines the direction of the spot radiated. In order to verify this concept, a transmitarray is designed using dielectric-only cells. These cells are based on a dielectric and air inclusions that  
Figure 15: Measurements normalized to their maximum values of the amplitude (dB) of the electric field intensity along the propagation direction and \(x=y=0\) at different frequencies for the feed \(\theta=0^{\circ}\). \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Work**} & **Type of** & **Scanning (S) or** & **Far-Field (FF)** & & & & & \\  & **Antenna** & **Multi-Focusing** & **(MF)** & **or Near-Field** & **Technique** & & & & \\ \hline
[13] & Lens & S & FF & Multiport feeder & Dielectric & 50\({}^{\circ}\) & 24–28 GHz \\
[16] & TA & S & FF & In-plane feeder & Metal dielectric & 100\({}^{\circ}\) & 20 and 30 GHz \\
[17] & TA & S & FF & Transmitarray & Dielectric & 54\({}^{\circ}\) & 30 GHz \\
[12] & FZP & S & NF & In-plane feeder & Metallic & N.A. & 32 GHz \\  & & & displacement & rings & & & \\
[18] & TA & MF & NF & Aperture division with & Dielectric & Only & 35 GHz \\  & & & different focus points & & & 4 spots & \\ This work & TA & MF/S & NF & Cluster of feeders & Dielectric & 40\({}^{\circ}\) & 26–30 GHz \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of the proposed antenna with other published works. TA, transmitarray; FZP, Fresnel zone plate. control the filling of the cell and, therefore, change the phase shift of the cell. The design is full-wave simulated, giving successful results as the transmitarray not only focuses the central spot, but also the other incoming fields provided by the cluster. In light of these results, the transmitarray was manufactured using a 3D printing technique, and an easily manufactured and low-cost prototype was obtained. The prototype was measured in a planar acquisition range at three different feed locations. The measurements showed a high agreement with simulations in terms of the focused spots performances (\(DoF\) and maximum location). Additionally, it was verified that the proposed feed distribution generates one spot by each feed, which should use different shifted carriers or time slots in order to avoid interferences. The prototype radiates similar near-field focused spots within an angle of \(\pm 20^{\circ}\), where only at the widest positions, the spot starts to be defocused. Additionally, the in-band response of the transmitarray shows a very stable behavior in the whole Ka-band, especially from 27.5 to 29 GHz. In light of these results, only-dielectric transmitarrays fed by a cluster can be a potential alternative for the generation of multi-focusing near-field spots at the millimeter band. All the authors contributed to ensuring the quality of this work; conceptualization, A.F.V., M.A., J.R.C., and C.A.F. ; methodology, A.F.V. and M.R.P. ; writing, original draft preparation, A.F.V. ; writing, review and editing, M.A., C.A.F., J.R.C., S.A.M., and M.R.P. ; and funding acquisition, M.R.P. All authors read and agreed to the published version of the manuscript.