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

perforated with radiating slots. Generally speaking, SWAs can be grouped into two categories: resonant SWAs and non-resonant SWAs.

Resonant SWAs, which produce broadside radiation, use *λg=*2 inter-element spacing between the successive slots (where *λg=*2 is the wavelength in the waveguide) as well as they have a shorted termination. On the contrary, non-resonant SWAs do not produce broadside radiation and have inter-slot spacing that is different from *λg=*2 as well as a matched termination.

Regardless of the antenna type, SWAs exhibit a narrow bandwidth, which depends on several geometrical parameters, such as slot dimensions and shape, metal thickness, and inter-slot distance [41]. Nevertheless, this bandwidth can be widened using well-known design approaches such as separation of the array into several sub-arrays [42], use of ridge-waveguide [43], use of elliptical slots and direct coaxial feeding [44], as well as coupled-slot and differential feeding mechanisms [45].

Although the printing rules described in Section 1 apply also for slotted antennas, most of the works available in the literature report printing orientations of 45<sup>∘</sup> with respect to broadside direction. Such works are not sensitive to the symmetry of the piece, therefore such orientation gives satisfactory results.

In [3], an 8x8-element resonant SWA in Ku-band with a corporate beamforming network is presented. Interestingly, this work also reports a counterpart based on subtractive manufacturing of several parts that are later assembled. The comparison between the two antennas shows that the 3D-printed antenna has a gain that is 1–1.5 dB larger than the traditionally manufactured antenna. In the latter, gaps and alignment errors between metal layers cause leakages and reflections, which are critical enough to eventually degrade the antenna performance. The article also discusses the fabrication orientation and the fabrication supports, which are the main disadvantages of this manufacturing approach. The same authors expand their work on SWAs in [46], which also operates in Ku-band but is much larger (16 16) as well as implements a monopulse comparator. As in the previous work, and despite the higher complexity and size of this component, the measured results show great similarity with the simulation, which validates the printing direction orientation.

An assessment on planar and conformal 1D and 2D resonant SWAs is presented in [47]. Several orientations and machine settings were studied to obtain consistent parts with high detail resolution and quality. The study includes also investigation of manufacturing defects and surface roughness of the components.

Concerning non-resonant SWAs, it is worth highlighting the leaky-wave antennas [48] reported in [49–51], operating in different frequencies ranging from K- to V-band. Despite these publications involving plastic printing, the antenna in [50] was later used to build an array in metal. The novelty in these works is that additive manufacturing enables dual-polarization radiation from a single leaky-wave aperture and to consider an OMT printed together with the antenna. Moreover, inner ridges with modulated geometry have been considered inside the leaky waveguide, which enable low side-lobe level for the two linear polarizations. The array shown in **Figure 9** was conceived as an extension of the previous work and is here presented for the first time. The beamforming network is folded toward the back side of the antenna, and the whole is printed in a single piece. A summary of the measured performance is presented in **Figure 10**. A very good agreement with simulation is obtained for patterns and directivity over the operating bandwidth, demonstrating again the suitability of 3D-printing for SWA. The backward lobe in the pattern is produced by the short-circuit created at the end of the antenna. Such beam could be avoided by using a matched load.

**Figure 9.** *SLM printed compact ridged linearly polarized 2D leaky-wave array with folded beam-forming network.*

**Figure 10.** *E-plane pattern (dBi) of the SLM shorted array at 20 GHz.*

Finally, an evolution of the previous works is presented in [52], which reports a fully metallic leaky-wave antenna radiating in circular polarization. One of the main original contributions of such work is related to the further plating of the SLM antenna. Such posttreatment step allows to reduce surface roughness and to improve the antenna radiation efficiency (around 10% improvement), as it can be seen in **Figure 11**.
