**5. Versatile periodic and quasi-periodic structures**

Screens based on the periodic and quasi-periodic arrangement of unit cells are widely used nowadays due to their capability to alleviate the complexity of RF feeds in terms of operation frequency, beam shape, polarization, impedance matching, and focusing of the beam, among others. The screens that are currently employed in RF systems are mostly implemented in printed-circuit board (PCB) technology [53]. In order to allow for the wide industrialization of full-metal solutions, a change of paradigm is needed, both in RF design and in manufacturing.

*Metal 3D-Printing of Waveguide Components and Antennas: Guidelines and New Perspectives DOI: http://dx.doi.org/10.5772/intechopen.106690*

**Figure 11.** *Radiation efficiency (%) for both the raw and posttreated SLM circularly-polarized SWA in [52].*

On the one hand, getting rid of the dielectric materials in the design of periodic surfaces is not evident, and it brings significant limitations to the RF designer. A monolithic architecture is needed that allows to be manufactured in metal without supports. A 3D topology is expected to be the future choice in this context, since it allows for greater design freedom. Additionally it is needed to count on a cell with sub-wavelength periodicity in order to avoid the appearance of grating lobes. Such an objective is easily attained in PCB solutions due to the miniaturization brought by the dielectric permittivity. However, when full-metal solutions are made sub-wavelength, they become very reactive and thus, tend to present a high reflection to the impinging waves. As a conclusion, a trade-off seems to appear in full-metal unit cells between the periodicity and the reflectivity when operating in transmission.

On the other hand, 3D-printing appears in this context as the technology for that enables greatest design freedom. However, the guidelines for metal 3D-printing have to be considered early in the phases of RF design.

Most of the metallic 3D screens that can be found in the literature are implemented through plastic 3D-printing and postprocessing of the piece with additional metal coating. This method can be effective, but it is very sensitive to such coating process. In addition, the effective conductivity of some commercial metallic inks or sprays can sometimes be lower than expected, contributing to a strong rise of ohmic losses. Some examples can be mentioned in this category. Negative-refractive index lenses have been reported in [54], metallic reflectarrays in [55, 56], and pass-band frequency selective surfaces in [57–59].

SLM has been applied for the design of periodic distributions of 3D helices, which would be very difficult to implement with other manufacturing techniques. Arrays of helices create artificial anysotropic panels that can be used for the design of polarization converters [60, 61]. Furthermore, additional dielectric supports were necessary to gain robustness. This fact dilutes the term "fully metallic" and strengthens the complexity of conceiving full-metal and self-supporting surfaces.

A metallic transmittarray has been reported in [62]. Its behavior is based on the excitation of a dispersive mode in a waveguide-shape unit cell. Solutions based on waveguide unit cells have also been proposed for the design of dual-band polarizers that provide orthogonal sense of polarization rotation in each band [63]. Such structure was based on highly subwavelength cells, in order to avoid excitation of grating lobes. Unfortunately, the previous designs were not driven by any co-design guideline. The previous geometries were conceived basing on classical RF guidelines, and they did not allow to be implemented in additive manufacturing.

**Figure 12.** *3D-printed polarizing screen: (a) photograph of the prototype, (b) insertion losses (measured and simulated).*

More recently, a topology of 3D unit cell has been proposed, which is specially suited to be 3D-printed [10, 64]. To the best knowledge of these authors, this is the first proof of concept in the literature of a periodic structure that is self-supporting and monolithically 3D-printed in metal. Such a screen is designed to achieve polarization conversion in a broad frequency band. The screen is low dispersive thanks to the excitation of transverse electromagnetic modes within the unit cell. A co-design approach based on the design guidelines detailed in Section 1 was followed in this case, thus enabling accurate prototyping. As an interesting example, bent metallic columns (inclined 45 with respect to the printing direction) were considered. The concept is illustrated in **Figure 12(a)**, which shows a Ku-band prototype printed in aluminum. The bent columns can be easily visualized in the photograph. The insertion losses are plotted in **Figure 12(b)**, they remain below 0.5 dB in the whole frequency band. These results are shown here for the first time. The axial-ratio level is quite similar to that reported in [10]. The architecture depicted in the picture can be adapted for other dual-polarization functionalities beyond polarization-conversion [64].
