**4. Antennas**

#### **4.1 Horns**

Horn antennas are widely used in various microwave and millimeter-wave applications, from feeds for reflectors to phased arrays or in antenna measurements. Such *Metal 3D-Printing of Waveguide Components and Antennas: Guidelines and New Perspectives DOI: http://dx.doi.org/10.5772/intechopen.106690*

#### **Figure 8.**

*Second concept of a dual-Ka-band (18–22 GHz and 27–31 GHz) self-supporting turnstile-based OMT in vertical full-metal 3D-printing to avoid overhanging parts during the print process: (a) perspective view of the total OMT's RF layout (the blue part is vacuum), (b) vertically cut view of the total OMT's RF layout (the blue part is vacuum and the gray part is the metallic turnstile post), (c) photograph of the printed prototype, and (d) measurement results. These results are shown here for the first time and they are courtesy of SWISSto12.*

wide range of applicability is attributed to their robust RF performance such as medium or high gain, wide bandwidth, high XPD, and low losses.

As far as 3D-printing is concerned, horn antennas can be separated into two fundamental categories: (1) smooth-walled flared horns (this category includes different type of profiles such as pyramidal, spline, or Potter) [9, 31–36]; and (2) axially corrugated horns [37–39]. Regardless of the horn type, the printing direction shall be vertical in order to preserve the symmetries of the structure.

Stepped smooth-walled horns are discussed in [33], which presents designs operating in Ku-, Ku/K-, and Q/V-band. The three horns are printed vertically; consequently, the measured and simulated results of all antennas exhibit a good agreement. Particularly interesting is the results of the Q/V-band device, where a XPD better than 28 dB over the whole frequency band has been obtained experimentally.

Spline horns (smooth-wall horns whose flare follows a spline function) are presented in [32, 35]. These industrial works coincide in presenting Ku-band horns that are later integrated in a horn cluster (monolithic device including several horns as well as their corresponding feed devices). Moving on to horn clusters for the production of large horn arrays enables strong mass and cost reduction: on the one hand, the total number of parts (from bolts to mechanical brackets to the RF device) is drastically reduced, which has an impact on both the mass and the cost (dealing with all these parts requires an associated effort). On the other hand, the integration of a cluster is much simpler and faster, which reduces the overall program cost. This trend, which is enabled by 3D-printing, seems to be the future of Geostationary (GEO) telecommunication satellites [40].

In [34], a ridged horn antenna with multistep flare is presented. It is worth highlighting the high frequency achieved by this design (110 GHz), which is another proof of the very high performance that one could achieve when following the design guidelines. Moreover, extra features (corrugations) are added around the horn aperture in order for the horn to maintain high RF performance over a very wide bandwidth (45–110 GHz). This is another example of exploiting the design freedom of 3D-printing to improve the RF performance without increasing the manufacturing complexity.

Similarly, the work in [36] exploits the design freedom to consider perforations on the metallic walls of the horn to reduce mass without affecting the performance. The considered perforated gaps are smaller than *λ*0*=*15 and hence opaque for the electric field. The experimental results demonstrate the suitability of such practice, which leads to a mass reduction in the order of up to two-thirds with respect to a horn.

To complete the survey of smooth-walled horns, it is also worth highlighting the work in [31], which presents a very high efficiency horn operating in the downlink Kuband (10.7–12.75 GHz). The device, which is called quad-furcated profiled horn antenna, consists of four asymmetric small horns forming a 2 2 array that feeds a square waveguide aperture. The device also includes the feed network. All features in the component are compatible with vertical printing. The measured performance (Sparameters and radiation patterns) shows excellent agreement with the simulated one. This approach, which holds potential to use the power on board the GEO satellite more efficiently than other horn designs, can only be conceived with use of 3D-printing.

The design and fabrication of axially corrugated horns are presented in [37–39]. In particular, the accuracy and repeatability of 3D-printed choke horns at X/Ku band are investigated in [38]. Fifteen antennas were manufactured and tested, showing a coherent agreement between simulations and measurements in terms of matching, radiation efficiency, and radiation patterns. An axially corrugated horn covering the full Ka-band (26.5–40 GHz) is presented in [39], whose measured radiation patterns show good beam symmetry and XPD better than 29 dB. Reference [37] shows a choke horn that acts as feed in a transmitarray for cubesat applications. The antenna works in the range 23–26 GHz and is fabricated together with a septum polarizer.

Finally, to the best knowledge of these authors, there is no work reporting conventional corrugated horns where the corrugations are adapted to vertical printing. Nevertheless, the techniques described in [15] or [16] should be applicable to horns too.

### **4.2 Slotted antennas**

Slotted waveguide antennas (SWAs) are attractive solutions for millimeter-wave applications because they enable high gain with a simple and flat beamforming network architecture. SWAs consist of a waveguide where one of the walls is periodically
