**3. Waveguide components**

### **3.1 Filters**

Waveguide filters are devices used to select and/or reject signals in an RF chain. The implementation in waveguide technology ensures low losses and high-power handling, while its main disadvantages are the associated mass and the volume. In contrast to other waveguide devices listed in this chapter, filters are, in general, rather sensitive to the manufacturing tolerances. As a result, the use of the previous design guidelines is even more relevant in the present case.

In the domain of metallic 3D-printed filters, it is possible to find multiple articles where AM is used to enable the prototyping of complex geometries [25]. In such examples, the printing direction has not been specifically considered as part of the design process, which is the purpose here. In the following, two examples are illustrated that are based either on vertical and horizontal printing (associated respectively to waveguides whose propagation axis is either parallel or perpendicular to the printing direction) (**Figure 4**).

In the case of vertical printing, traditional filter topologies shall be adapted by tilting all their down facing parts so that they become self supporting. A first example is proposed in [15], where *λ=*4 rejection stubs are tilted. The authors of such paper present prototypes in Ku/K-band that have been fabricated using different metal alloys. All the filters present high rejection (50 dB) and low insertion losses (from 1 to 0.1 dB, depending on the type of surface finish). Another example can be seen in [11], where a bandpass filter with titled irises is presented. The design in the article is a 11-pole filter operating from 17.3 to 20.2 GHz. The two fabricated filters exhibit a bandwidth slightly narrower than the simulated one. An interesting result is the great similarity between the two prototypes, which have been produced using different processes with the same alloy. The latter is a confirmation of what was stated in Section 1: using guidelines for 3D-printing enables robust filter designs. A third example can be found in [16], where a corrugated filter with chamfered corrugations is presented. The filter also operates in the Ka-band (17.7–20.2 GHz) and provides all-mode rejection up to 43 GHz. This work also provides a comparison between raw

#### **Figure 4.**

*Different orientations of a filter (gray) on the building platform (black) and the required supports (magenta) for each of them. The orange arrow indicates the printing direction.*

(not metallized) and metallized finish, where the associated ohmic losses are 0.35 and 0.25 dB, respectively.

Interestingly, there are also filter examples that by default (without requiring dedicated design guidelines) are compatible with vertical printing such as the filters based on spherical or quasi-spherical resonators. In [26], a dual-mode filter at 8.25 GHz implemented with spherical cavities (which can support two modes) and compatible with vertical printing is presented. Dual-mode filters are known for being very sensitive and often require tuning screws; however, the 3D-printed filter in the article achieves a performance very close to simulation without the need of tuning screws, which is another demonstration of the suitability of vertical printing for the production of waveguide filters.

Ridge waveguide evanescent mode filters can also be adapted to vertical printing. These filters are based on the cascading of ridge and hollow waveguide sections of the same width and height. By chamfering the ridges as in **Figure 5**, the filter becomes vertically printable, as demonstrated in [12].

**Figure 6** shows the measured and simulated performance of one filter of this kind. Two identical samples of the filter have been manufactured in order to verify its repeatability. As it can be appreciated, the results of both prototypes show excellent agreement between them and also with the simulation. The insertion losses (0.3 dB) are in agreement with the expected value for a nonmetallized component.

#### **Figure 5.**

*Representation of an evanescent mode ridge waveguide filter designed to be vertically printable. The ridges have been chamfered with a* 45<sup>∘</sup> *angle so that they become self-supporting.*

#### **Figure 6.**

*Simulated and measured scattering parameters of an evanescent mode filter with chamfered ridges. Two copies of the prototype have been manufactured in order to assess the repeatability. These figures are shown here for the first time.*

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

An example of horizontal printing can be found in [17]. In that work, the authors report a combline filter where the cavities have triangular cross section. When 3Dprinting this filter, the combline post grows from the base of the triangle, while the other two sides of the triangle (with an angle with respect to the base larger than 45<sup>∘</sup> ) converge vertically until they find each other, thus closing the cavity.

Another common practice in 3D-printed filters is the introduction of features in order to improve the performance (quality factor enhancement, extension of spurious-free band, etc). One example can be seen in [11], where a dimple has been created at the center of each cavity. Such feature has negligible impact on the insertion loss but pushes up the repeat band.

#### **3.2 Orthomode transducers and polarizers**

Orthomode transducers (OMTs) are commonly used to separate or combine two orthogonally polarized signals from or into the same waveguide. OMTs typically operate in linear polarization, and its key figures of merit relate to the isolation between the rectangular ports and cross-polarization discrimination (XPD) between the signals in the dual-polarized port. Septum polarizers are a type of OMT that also converts from linear to circular polarization and vice versa.

The isolation and XPD of an OMT depend on the structural symmetry of the component [27]. Potential asymmetries as consequence of fabrication will produce unwanted coupling between signals that are supposed to be orthogonal. As it has already been mentioned, the structural symmetry is better assured when the components are printed in vertical direction, hence the importance of the design guidelines for OMTs.

It is worth starting by the contribution in [9], which reports both side-arm OMT and septum polarizer in the Ka-band that are adapted to vertical printing. While the septum polarizer does not require much effort for 3D-printing, the side-arm OMT has been redesigned in a way that the "side" waveguide presents the top faces tilted in order to be self-supporting. Several E-plane matching steps are required to obtain optimal matching. Both devices are single-band and exhibit measured isolation and XPD in excess of 30 dB.

In [28], the authors present a Ka-band dual-band Bøifot OMT also adapted for vertical printing. The back face of the inner septum in charge of splitting/combining the polarizations has been chamfered according to the printing direction. It exhibits isolation and XPD greater than 40 and 30 dB, respectively, over 18.6–20.2 GHz and 28–30 GHz.

Interestingly, and despite being one of the most commonly used OMTs, there is no work in the literature with vertically printable turnstile OMTs. Two design concepts are discussed here for the first time. The first concept of such a design is depicted in **Figure 7(a)-(b)**, which shows the CAD model of the five-port turnstile junction. The main difference with respect to conventional designs is the four rectangular singlepolarized waveguides, which are tilted to the junction axis so that they do not show unsupported faces. As **Figure 7(c)** also proves, the performance of the modified junction (reflection coefficient level lower than 25 dB is achieved for the full Kuband SATCOM bandwidth, 10.7–14.8 GHz) does not show any limitation despite the modification.

The second design concept, in **Figure 8**, exploits the use of truncated cones for the realization of the turnstile post. One design approach when the bandwidth of OMT gets larger is the use of multistep posts [29]. As it has been discussed earlier, this type of

#### **Figure 7.**

*First concept of a Ku-band (10.7–14.8 GHz) self-supporting turnstile-based OMT in vertical full-metal 3Dprinting to avoid overhanging parts during the print process: (a) perspective view of the RF layout of the five-port junction (the transparent blue part is vacuum and the brown part is the metallic turnstile post), (b) transparent vertically cut view of the mechanical layout of the five-port junction (the blue part is vacuum and the gray part is metal), and (c) simulation results (port 1 refers to the common circular port).*

feature is not well suited for vertical printing, hence truncated cones are used to design components with increased bandwidth (17.7–31 GHZ, with matching better than 20 dB in the uplink and downlink SATCOM bands). In this case, the OMT is terminated using E-plane power combiners designed in hexagonal waveguide. This second design has been fabricated and measured, and the results are depicted in **Figure 8(c)**. The measured reflection coefficients for both polarizations remain below 20 dB over the bands of interest (18–21.2 GHz and 27.5–31 GHz), while they also show a good agreement with simulations (not shown for clarity). The cross-polarized transmission coefficients and the rectangular ports present coupling levels below 35 dB and 45 dB, respectively. The transmission coefficients are better than 0.5 dB (it must be noticed that the test setup requires extra waveguides in order to match the device nonstandard ports to standard ports and therefore the measured losses are higher than the ones of the component itself).

To complete the section, it is worth highlighting an interesting recent trend consisting in the combination of several OMTs in a single device to create a N-way OMT, possibility, which is enabled by the design freedom inherent to 3D-printing. The work in [30, 31] presents a four-way OMT-power divider, which is built as an array of 2 2 asymmetric side-arm OMTs in a tight square grid (around 1*:*25*λ*0) fed by two distributed 1 4 single polarized power dividers. The component was conceived without overhangs so it can be vertically printed. The measured prototypes feature isolation and XPD better than 40 dB.
