**2. Prototyping**

having a global vision of the whole device itself. The engineer may also realize that some parts of the design could be improved or modified. Of course, computational 3D models are normally used, but with low-cost 3D printing, a full-scale model is available almost instantly. AM becomes

Besides, the availability of real models may be useful for another application full of potential: education [6, 7]. Models provide outstanding intuition without replacing theoretical results. Students have started to use computational tools to complement their academic training, and AM brings that to a whole new level. After studying the theory, the students are capable of handling state-of-the-art filters, antennas, orthogonal mode transducers (OMTs), or power

For these prototyping applications, the designed devices are not intended to work, that is, the model can remain a dummy with no further utility. However, in order to obtain a device with proper electrical response, it is mandatory to include metal in the process. When metal is the raw material used to 3D print the model, many advantages of AM appear more evident. For instance, unfeasible geometries by traditional means such as computer numerical control (CNC) milling may be achieved. Besides, there is a saving in consumed material, making the whole process more sustainable. Yet to combine those advantages with plastic 3D printing

Metallization of the plastic device may be done by different means, but the specific approach followed in this work is metallic paint. For some devices, the results show that it is acceptable to use the final devices in real communication systems, whereas others eventually require some modifications. In spite of that, they both open up another possibility within the educational field. If prototyping offers the possibility of *touching* microwave devices, manufacturing

In RF courses, the first step is usually to acquire a theoretical background. In the best-case scenario, students are later asked to design a device that must fulfill certain specifications and obtain a simulated response through software tools. However, experimental validation is sometimes avoided or limited due to the lack of resources. Consequently, students may measure or test specific devices intended for that use but very rarely do they test what they have done themselves. Low-cost 3D printing changes this paradigm and allows students to conduct a full design process: from the initial design stage to the measurement of their own models [6]. The physical structure may be printed in few hours once the design is completed. Therefore, it is possible to verify whether the real-measured response matches the expected one, causing a deeper engagement in the courses. Another advantage is that each student may design their own model with different specifications, as well as facing the real challenges of real-life manufacturing. Fabrication entails restrictions that must be borne in mind: students must design a physically achievable device, something that may be forgotten when the design

Finally, the common denominator in both cases is that the low-cost approach makes the technology accessible to all kinds of engineering environments: from individuals to small research groups instead of just large companies or high-level laboratories. The prototypes or functional devices are promptly manufactured within hours after being designed for less than 25 \$ each.

allows performing a full design process at a bachelor or master's level.

process ends with a simulated response in the computer.

a tool to gain a deeper understanding throughout the design process.

96 Emerging Microwave Technologies in Industrial, Agricultural, Medical and Food Processing

combiners in the laboratory.

represents a bigger challenge, that is, low-cost.

Prototyping is one of the applications of additive manufacturing in many diverse fields, including also microwave engineering. In recent years, there has been a notorious increase in the amount and quality of computer-aided design (CAD) tools for design and visualization purposes. These developments are of great help for designers and engineers, but, at the end, physical realization gives an irreplaceable insight: this is the context where AM plays a key role. Moreover, when the design process moves to the manufacturing stage, it is mandatory to pay special attention to mechanical and structural issues.

Since the printed prototypes are only relevant from a mechanical perspective, plastic is a suitable material with no further metallization needed. The manufactured geometries are feasible for real devices thanks to complex and expensive fabrication techniques such as CNC milling. However, FFF allows obtaining a model in few hours.

**Figures 1** and **2** represent two examples of prototypes of state-of-the-art waveguide devices. They are Ku-band power combiners with 8 and 16 ports, respectively. The top part in **Figure 2** is a circular TE01 mode converter which has already been presented in [8] together with its printed model to forecast any problems in the ongoing CNC milling fabrication process.

**Figure 3** shows a model of a diplexer intended to work in Ka-band made out of two band-pass filters with elliptic response. This kind of design is suitable for satellite applications due to its outstanding electric performance and robust and compact structure [9].

Finally, other manufacturing techniques may also benefit from low-cost additive manufacturing. One example appears in **Figure 4**, where the different pieces of an ortho-mode transducer (OMT) based on the turnstile junction, working in W-band are presented, allowing to anticipate and to discuss any issue in the final manufacturing. In this case, the final fabrication technology was layered SU-8 photoresist, and the plastic pieces allow seeing the different layers in which the structure is divided.

**Figure 1.** 3D–printed prototype of a Ku-band 8-port power combiner.

**3. Manufacturing of waveguide devices**

**3.1. Limitations of low-cost 3D printing process**

structural issues.

*3.1.1. Dimensions*

*3.1.2. Metallization*

total cost is very low.

the electrical responses of the devices presented later in this chapter.

After addressing the first application of low-cost 3D printing in the microwave field, another step is introduced. Now, the printed devices are meant to be measured, and therefore, they have to be metallized first. These two requirements set a frame of limitations, which confine the usability of this technology—at least the low-cost version—and are crucial to understand

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First, the three main restrictions are thoroughly explained. Then, a full set of waveguide devices and two horn antennas are presented. They cover from relatively simple straight sections to fully operational devices such as a waveguide diplexer with challenging specifications. They try to push low-cost 3D printing to its limits, also in terms of frequency and bandwidth.

The main drawbacks of low-cost 3D printing when manufacturing fully functional devices may be summarized as follows: the working frequency, the metallization process, and the

The first one is intrinsically related to the dimensions of the design and the printing accuracy, that is, the layer height. In a normal low-cost machine, the minimum value is ±0.1 mm. It marks the minimum variation of dimensions that can be built, and, therefore, it establishes an upper limit for the working frequency. It is unrealistic to print devices with working band above 15 GHz. Besides this limit, this issue becomes even more evident when dealing with sensitive devices, where a tiny variation of dimensions leads to a very different electrical behavior.

Printing accuracy is also related to the achievable matching losses in a waveguide device. In an experiment performed with printed 50-mm-long WR75 waveguide sections such as the one in **Figure 5**, the matching level is similar for the waveguides regardless of the paint they

All things considered, this limitation is probably the easiest to deal with. There is a quite clear limit regarding frequency, and computational design tools are of great help to account for precision. That means that before obtaining the final model of the device, all geometrical dimensions should have the number of decimal digits in agreement with the printing accuracy.

The second drawback includes all the aspects related to the metallization process. As it has been mentioned, it is imperative to cover the inner surface of the printed waveguide devices with metal paint, in order to confine the electromagnetic energy within the structures. Consequently, the devices cannot be printed as a single piece, as it would be desirable, but they have to be printed in separate parts instead. Their inner part must be accessible for painting and that may limit the feasible geometries in some cases. This may be seen as damaging one of the biggest advantages of 3D printing, yet it is a trade-off and from this perspective the

are covered with: around 20–25 dB in Ku band, as it appears in **Figure 6**.

**Figure 2.** 3D–printed prototype of a Ku-band 16-port power combiner.

**Figure 3.** Printed model of a Ka-band diplexer with two band-pass filters with elliptical response.

**Figure 4.** Printed layers of an ortho-mode transducer (OMT) based on turnstile junction, working in W-band.

In all these cases, very complex designs with reduced price are rapidly available. Some geometries involve many difficult details, even with the available visualization tools nowadays. These cases are therefore good examples of how 3D printing can help to reach a clearer communication between the different teams involved in an engineering product.
