three-dimensionally printed/additive manufactured antennas · been incorporated in various...

Three-Dimensionally Printed/Additive Manufactured Antennas

Min Liang* and Hao XinDepartment of Electrical and Computer Engineering, University of Arizona, Tucson, AZ, USA

Abstract

Additive manufacturing (AM), or often referred to as three-dimensional (3D) printing, is an importantemerging research area which has received much attention recently. It allows 3D objects with arbitrarygeometry to be printed automatically layer by layer from bottom to top. This technology offers severaladvantages compared to conventional manufacturing techniques including the capability of more flexibledesign, prototyping time and cost reduction, less human interaction, and faster product developmentcycle. 3D printing techniques have been applied in many different sectors including mechanical engi-neering, electrical engineering, biomedical engineering, art, architecture, and landscaping. This chapterreviews state-of-the-art 3D printed antennas from microwave to THz frequencies and offers practical andfuturistic perspectives on the challenges and potentials of 3D printed antennas. An overview of various 3Dprinting techniques relevant to antenna applications is presented first. A number of 3D printed antennaexamples categorized by different AM methods are then described. Finally, technical challenges andpossible solutions of 3D printing technology specific to antenna application, as well as new andrevolutionary antenna design/realization concepts enabled by 3D printing technology, are discussed.

Keywords

Additive manufacturing; 3D printing; Computer-aided design; Automatic fabrication

Introduction

Additive manufacturing (AM), often called “3D printing” or “rapid prototyping”, is an automatedfabrication technology to make a three-dimensional physical object directly from digital data. Contraryto the subtractive manufacturing which realizes a product by subtracting a material from a larger piece ofmaterial such as cutting out a screw from a piece of metal, it makes a product layer by layer additively.

AM was originated in the United States and was first commercialized in the late 1980s. At that time, itwas called “rapid prototyping (RP)” or “generative manufacturing” (Gebhardt 2012), and these terms arestill occasionally in use presently. In the early 1990s, several different AM processes including lasersintering (LS) (Agarwala et al. 1995) and fused deposition modeling (FDM) (Griffin and McMillin 1995)were developed and became available commercially. In the mid-1990s, another 3D printing process whichcreates an object by jetting a liquid binder onto a bed of powder and doing post-processing to solidify thewhole structure was invented (Bak 2003). After that, through the rest of the 1990s, further research anddevelopment were mainly focused on materials such as various thermoplastics (Kambour 1973) andelastomeric polymers (Kornbluh et al. 2000) in different forms to enable AM techniques to be used inmore applications. As the new century begins, the focus was shifted back to improving the AMtechnology by developing new printing processes. New techniques such as the laser melting (LM) and

*Email: [email protected]

Handbook of Antenna TechnologiesDOI 10.1007/978-981-4560-75-7_109-1# Springer Science+Business Media Singapore 2015

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electron beam melting (EBM) processes were developed. These techniques allow various alloyedmaterials to become available in the AM process. Over subsequent years, more and more AM companiesare founded from all over the world and starting to develop their own printable materials and AM systems.Many new types of materials and systems become available as the demand for AM increases. It is realizedthat these techniques are not just for rapid prototyping; instead, they can be developed and applied as anew form of manufacturing technology. Therefore, from then on, the name “additive manufacturing” hasbeen coined. Recently, AM has received much attention with impressive demonstrations ranging frommusical instruments, to vehicles, to housing components or even entire buildings. Many differentstructural materials such as metal, polymer, ceramics, concrete, and even biocompatible materials havebeen incorporated in various 3D printing technologies. Due to its ability to realize desired structures witharbitrarily designed spatial distribution, 3D printing technology has been argued to be the future ofmanufacturing as it offers huge potentials to revolutionize both the design and manufacturing procedures.

The technical realization of AM is based on layer-by-layer processes, and therefore it is called “layer-based technology” or “layer-oriented technology.” The working principle of the layer-based techniques isto create a 3D physical structure from many slices with the same thickness. Each slice is fabricatedaccording to the information from the corresponding 3D model and placed on top of the pervious layer.A schematic illustration of a typical AM procedure is shown in Fig. 1. The process starts with a 3Dcomputer-aided design (CAD) model which represents the 3D object to be printed. This CAD model canbe created directly from CAD software or by digital 3D scanning of a real structure. After the CADmodelis obtained, specialized software is used to slice the model into layer-by-layer cross sections. As a result, aseries of layered slices with equal thickness are generated. The information of these slices includingposition, layer thickness, and layer number is sent to a machine that could print each layer and bond it tothe previous one. The printing and bonding of the layers can be done in many ways based on differentphysical phenomena. By printing the object layer by layer, the entire structure is built from bottom up.

These basic steps are the same for almost all varieties of AM equipment available today. Thedifferences of different equipment are how they generate the layers, how the adjacent layers are joinedtogether to form the final part, and the corresponding built materials.

Compared to conventional manufacturing methods (such as injection molding, casting, stamping, andmachining), the AM approach has the following advantages.

Fig. 1 Schematic illustration of a typical AM process


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Arbitrary ComplexityAM approach has the ability to create 3D objects with arbitrary shape and complexity. The cost of the 3Dprinted components is only related to the volume of the parts; there is no additional cost or lead time formaking the structure more complex. Also, with multiple printing heads, it is possible to cohesivelyintegrate different materials at the same location simultaneously. Therefore, AM may revolutionizeproduct designs because of the much more flexible object geometry and material property distributionit offers. For example, 3D structures with arbitrary EM property distribution can be printed relativelyeasily.

Digital ManufacturingAfter an object is designed, the whole 3D printing process is accurately controlled by a computer withvery little human interaction needed to realize the design. This automatic 3D printing process means thatthe time between design iterations can be dramatically reduced compared to conventional manufacturingmethods.

Waste ReductionThe 3D printed component is created bottom up via layer-by-layer processes so that only materials neededfor the design are used. Therefore, material waste in AM process will be much less than conventionalsubtractive manufacturing techniques.

Various 3D printed antennas have been reported taking advantages of the AM technology. Antennas ofdifferent structures such as horn antennas (Huang et al. 2005), patch antennas (Liang et al. 2014a),meander line antennas (Adams et al. 2011), gradient index (GRIN) lens antennas (Liang et al. 2014a), andreflect-array antennas (Nayeri et al. 2014), made of different materials such as all-dielectric antenna(Wu et al. 2012), all-metal antenna (Garcia et al. 2013), and dielectric-metal combined antenna (Lianget al. 2014; Adams et al. 2011; Nayeri et al. 2014), working at different frequencies from GHz to THz,have been realized using different 3D printing techniques. In the next section, an overview of various AMtechniques relevant to antenna application is provided, and the pros and cons for each are discussed.

Overview of 3D Printing Techniques

At the present time, there are many kinds of 3D printing techniques, all of which follow the basic steps ofAM discussed in the previous section, for example, generating individual physical layers and combiningthem together. Various materials such as metal, plastic, ceramics, or even biocompatible materials can beused in the generation of the physical layers. According to the methods of generating physical layers andbonding adjacent layers together to form an object, five basic categories of AM processes are commer-cially available (Gebhardt 2012), including selective sintering and melting, powder binder bonding,polymerization, extrusion, and layer laminate manufacturing (LLM). Key aspects of these five processesare discussed and some commercially available 3D printers as well as printed examples are reviewed.

Selective Sintering and MeltingThe 3D printing technique using a laser to selectively sinter or melt powdered material is called selectivelaser sintering (SLS) (Agarwala et al. 1995) or selective laser melting (SLM) (Kruth et al. 2004). If anelectron beam is used instead of laser, the process is called electron beam melting (EBM) (Cline andAnthony 1977).

An SLS printer usually includes a building chamber to be filled with powdered built material and a laserbeam on top that can be scanned precisely in the XY (horizontal) plane. The bottom of the chamber is


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moveable in the Z (vertical) direction. During the printing process, the entire chamber is heated to a hightemperature close to the melting point of powder so that they are at an optimal temperature for melting. Toprevent oxidation, the chamber is often filled with shielding gas (e.g., nitrogen). The scanning laser beamis then used to fuse the powders at designated locations. As the laser beam travels in the XY plane, themelted powders cool down and solidify. After the scanning of an entire layer at designated positions, asolid layer with designed pattern is achieved. After one layer is printed, the powder bed is lowered by theamount of one layer thickness and an automated roller adds a new layer of powdered built material onthe top of the previous layer. Then the selective melting process repeats until the entire object is printed.The remaining unsolidified powders are then removed after printing. The SLS technique is quite versatilesince it can be used to print several classes of materials, including plastics, metals, and ceramics.

Typically, SLS-fabricated metal parts such as steel and titanium are dense. They may be post-processedby cutting or welding, depending on specific materials involved. Plastic parts such as nylon andpolystyrene fabricated using SLS have properties similar to those made by plastic injection molding.As an example, Fig. 2 shows an SLS printer (EOS P800) and a metallic part made by using SLS.

Selective laser melting (SLM) is developed in particular to process metal parts that need to be verydense (>99 %). In this case, the laser beam melts the metal powders completely into liquid phase whichresults in a close to 100 % density part after resolidification. SLM can be used to print many metalsincluding stainless steel, carbon steel, CoCr, titanium, aluminum, gold, and a large variety of alloys.

EBM is a similar 3D printing process in which metal powders are melted or fused by applying anelectron beam under a high voltage (typically in the range of 30 � 60 KV) instead of a laser beam. Toavoid oxidation, the process is performed in a high-vacuum chamber. Because the electron beampenetrates much deeper than a laser beam, EBM allows a higher scanning speed. In addition, deeperpenetration can be used for powder preheating so that the printing process works at elevated temperaturescompared to the laser case. As a result, the mechanical stress and distortion of printed objects are reducedand greater strength can be achieved. Figure 3 shows an example of an EBM 3D printer and a 3D printedobject using EBM technique.

Sintering and melting processes are very suitable for applications requiring high strength and/or hightemperature. Antennas printed by SLS or EBM can be very dense, void-free, and very strong. Thedisadvantages of selective sintering and melting techniques are that the printing resolution is limited bythe size of the powders (i.e., tens of microns) and a high-vacuum chamber or shielding gas is needed toavoid oxidation (Kruth et al. 2003).

Fig. 2 (a) Photo of a selective laser sintering printer (SLS) (model EOS P800; size: 2.25 m � 1.55 m � 2.1 m). (b) An SLSprinted metallic object


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Powder Binder BondingPowder binder bonding is another 3D printing technique that implements layer-by-layer bonding ofpowdered materials by selectively injecting a liquid binder onto the powder bed. This process was firstdeveloped in the mid-1990s. Currently, various materials such as plastics, metals, and ceramics can beprinted using this technique.

A typical powder binder bonding printer is very similar to a selective laser sintering printer with apiston at the bottom of chamber to adjust the height and a roller to recoat the powders. The printingprocess starts with depositing small drops of liquid binder onto a layer of built material powders atdesignated locations. The powders forming the designed structure are bounded together while thesurrounding loose powders support the next layer of the structure to be printed. The printing process isthen repeated for each layer until the entire structure is completed. Compared to the sintering or meltingprocess, this process is performed at much lower temperature. Therefore, no preheating, shielding gas, orvacuum chamber is needed.

At the end of the printing process, the residual powders are removed and an infiltration process may beperformed for enhanced durability. For plastic parts, wax or epoxy resin can be used in the infiltrationprocess. If this technique is used to print a metallic antenna (Lopez et al. 2013), a subsequent high-temperature process is needed to provide strength and durability. For example, to print a bronze object, the

Fig. 3 (a) Photo of an electron beam melting (EBM) printer (model Arcam Q20; size: 2.3 m � 1.3 m � 2.6 m). (b) AnEBM-fabricated part


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printed part needs to be infused into bronze powder and heated up to more than 1,000 �C to replace thebinder with bronze (Lipkea et al. 2010). This process can also be used to print alloy materials by changingthe sintering temperature and time during the infiltration process (Kruth et al. 2005). Figure 4 shows apowder binder bonding 3D printer and an example printed using this technique. Similar to the sinteringand melting processes, the resolution of this technique is also limited by the size of the powders. For thecurrently available printer on the market, the minimum feature size is 0.1 mm.

PolymerizationPolymerization is a process that selectively solidifies liquid resin using ultraviolet radiation or other powersources. Typically, photosensitive polymers are used as built material. There are several kinds of AMmethods based on the polymerization process. Their differences are mainly in how the photon energy isapplied and how the layers are created.

Stereolithography is the most accurate polymerization process which employs an ultraviolet laser tosolidify a liquid ultraviolet curable polymer. To print each layer, a laser beam scans over the surface of aliquid polymer reservoir to cure the cross section according to the designed pattern. The curing thicknesscan be adjusted by the laser power and laser scanning speed. After one layer is printed, the building stagedescends a distance of one layer thickness. Then, a blade sweeps across the surface of the printed part,recoating it with fresh liquid polymer before the next layer is printed on top. It is possible to incorporatedifferent materials in the printing process, thus achieving multiple material stereolithography (Gebhardt2012). In this case, the resin needs to be drained and replaced by the new material. After an object is

Fig. 4 (a) Photo of a powder binder bonding 3D printer (model ProMetal S15; size: 3.1 m� 3.4 m� 2.2 m). (b) An exampleprinted using the powder binder bonding technique


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printed, it is cleaned and moved into a UV chamber for a final post-curing process to make it more stable.Figure 5 shows a stereolithography 3D printer example and an object realized by stereolithography.Compared to other AM techniques for 3D printing of antennas, the stereolithography process can achievea very good surface smoothness and finer resolution. In fact, a two-photon stereolithography process hasbeen reported to obtain a submicron printing resolution (http://www.nanoscribe.de/en/). However, thestrength of a 3D printed part by stereolithography is weaker than other techniques such as sintering,melting, or powder binder bonding.

If a photosensitive polymer is applied by printer heads, the AM process is called polymer jetting.During printing, the printer head deposits photosensitive polymers onto a stage with designed patterns.Upon jetting, the printed photosensitive polymers are immediately cured by an ultraviolet lamp on theprinter head and unlike stereolithography, no post-curing process is needed. The thickness of each layer ofthis process can be on the order of 20 mm, which provides a very smooth surface. Moreover, multiple typesof polymers can be printed simultaneously using multiple printer heads. A gel type of polymer can be usedas support material to print overhanging structures and released (e.g., water-soluble support materials canbe washed away) after the printing process. A schematic drawing of the polymer jetting procedure isshown in Fig. 6. The polymer jetting method can only be applied to print polymers, limiting itsapplications to all-dielectric antennas. An additional metallization process would be required to incorpo-rate the conductor part. It has a better resolution than sintering and powder binder bonding techniques.However, similar to stereolithography, parts printed by polymer jetting are not as strong as some of theother AM techniques.

ExtrusionExtrusion, often called fused deposition modeling (FDM), is an AM process that prints an object byextruding thermoplastics through a heated nozzle. A FDM printer includes a feeding roll, a heatedextrusion head, and a building platform. The building materials are usually thin thermoplastic filamentswhich are wound up and stored in a cartridge. The thin thermoplastic filament is guided into the extrusionhead by the feeding roll. During the printing process, the heated extrusion head melts down the filament

Fig. 5 (a) Photo of a laser stereolithography 3D printer (model systems iPro™ 8000; size: 1.26 m � 2.2 m � 2.28 m).(b) A sample fabricated using stereolithography technique


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http://www.nanoscribe.de/en/

and extrudes it through the nozzle at designated locations on the building platform. When the extrudedthermoplastic reaches the building platform, it cools down and hardens. After one layer is completed, theplatform lowers down by one layer thickness and is ready for printing of the next layer. Figure 7 shows theschematic of an FDM printer and a printed example.

There are a number of available built materials for FDM including polycarbonate (PC), acrylonitrilebutadiene styrene (ABS), polyphenylsulfone (PPSF), etc. The advantages of using this technique to printantennas are the relatively simpler processing (i.e., no post-processing needed) and lower printer costcompared to other AM techniques. The disadvantage of FDM is lower resolution (about 0.25 mm (Wongand Hernandez 2012)).

Layer-by-Layer BondingLayer-by-layer bonding is an AM technique that creates a 3D structure by cutting a prefabricated sheet orfoil into a designed contour and subsequently bonding a number of layers together. It is often called

Jetting Head

a b

Fullcure M(Model Material)

X axis

Z axisThe Objet PolyJet Process

Y axis

UV Light

Fullcure S(SupportMaterial)

Build Tray

Fig. 6 (a) Schematic picture of the polymer jetting technique. (b) Photo of a polymer jetting 3D printer (model StratasysEden350V; size: 1.3 m � 1 m � 1.2 m)

Fig. 7 (a) Schematic of FDM. (b) An example printed using FDM


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laminate object manufacturing (LOM). A LOM printer consists of a building platform that can move inthe z-direction, a foil supply system to supply and position the foil, and a cutting device to create thecontour. The LOM procedure is as follows: First, the foil is positioned and adhered to the buildingplatform by a heated roller. Second, the cutting tool scans on the foil to create the designed contour andperform crosscutting on the non-model area to make it into small pieces for easier removal after printing.After one layer is printed, the platform moves down and the roller positions the next layer of foil on top ofthe previous layer. Then the platform moves up into position to receive the next layer and the processrepeats until the entire 3D object is printed completely. A photo of a LOM 3D printer using paper materialis shown in Fig. 8 together with a 3D printed example.

The foil built materials for the LOM technique can be paper, plastic, or metal (Gebhardt 2012). Thecutting tool can be a scanning laser, a knife, or a milling machine. To bond adjacent layers, differentmethods such as gluing, soldering, and ultrasonic or diffusion welding can be used depending on thematerial properties. Compared to other AM techniques, the advantages for using the LOM in antennaprinting include lower material cost and faster building speed for large objects. The disadvantages are lessaccuracy (e.g., 0.3 mm for the Solidimension SD300 3D printer shown in Fig. 8) and some material wastedepending on the geometry.

AM Technique SummaryMost of the AM processes currently available can be classified by the abovementioned five basiccategories. Table 1 summarizes the key features of these techniques.

3D Printed Antennas

AM technology enables the flexible and rapid realization of structures with arbitrary shapes andcomplexity. It has been successfully applied in many scientific and industrial areas such as biomedical,aerospace, and toy industry, architecture, and landscaping (Gebhardt 2012). In the following sections,applications of AM techniques for realizing 3D printed antennas are reviewed. A number of antennaexamples printed by different AM techniques including electron beam melting, powder binder bonding,stereolithography, polymer jetting, conductive ink printing, and fused deposition modeling are presented.

Fig. 8 (a) Photo of a LOM 3D printer (model Solidimension SD300; size: 450 mm� 725 mm� 415mm). (b) An object madeof paper printed using the LOM method


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Antenna Printed Using Sintering and MeltingAs mentioned in section “Overview of 3D Printing Techniques,” sintering and melting is an AMtechnique which uses laser or electron beam to selectively melt powder material and build 3D structures.In Garcia (2013), two horn antennas operating at Ku-band are printed by employing the EBM technique.Figure 9a is a commercial standard gain horn antennamanufactured conventionally as the reference, whileFig. 9b, c are two 3D printed horns with different dimensions. The RMS (root mean square) surfaceroughness of the 3D printed horns is measured to be 25.9 mm and 39.7 mm for the antennas in Fig. 9b, c,respectively. The simulated antenna gains without considering surface roughness are compared withmeasured results at 15 GHz, as shown in Table 2. One can see that the measured antenna gains agree verywell with the simulated results for the reference horn antenna and the 3D printed horn antenna in Fig. 9b.For the 3D printed horn antenna in Fig. 9c, the measured antenna gain is about 0.6 dB lower than thesimulated result which is attributed to the relatively worse surface roughness.

Table 1 Summary of key characteristics of the five basic categories of AM processes

ClassificationAvailablematerial

Processtemperature Resolution Strength

Can printoverhangingstructure?

Print multiplematerials

Sintering andmelting

Polymer,metal andceramic

Hightemperature

Low Strong Yes No

Powder binderbonding

Polymer,metal andceramic

Dependingmaterial

Moderate Moderate Yes No

Polymerization Polymer Roomtemperature

High Moderate Need supportmaterial

Yes

Extrusion Polymer 200–300 �C Low Strong Need supportstructure

Yes

Layer byLayer bonding

Paper, plasticand metal

Dependingon material

Low Moderate Yes No

Fig. 9 Front view and side view of three Ku-band horn antennas. (a) A commercial standard gain horn as the reference.(b) EBM printed horn antenna with a dimension of 145.1 mm � 52.8 mm � 66.2 mm. (c) EBM printed horn antenna with adimension 131.9 mm � 47.3 mm � 55.2 mm (Garcia et al. 2013)


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In Kim (2013), an SLS technique is also applied to print an electrically small spherical helix antenna.The antenna is first printed with nylon using an SLS technique and then coated with several layers ofconductive paint to make it conductive. A photo of the antenna is shown in Fig. 10. This helix antenna isfed by a SMA connector from the bottom. Figure 11 plots the measured reflection coefficients at differentpainting steps. The measured resonance frequency is at 736.3 MHz, which is lower than the expected750 MHz. The reason is attributed to the gravity-induced deformation of the helix arms. The measured

Table 2 Comparison of simulated and measured gains at 15 GHz for the reference and EBM printed horn antennas shown inFig. 9 (Garcia et al. 2013)

Name Simulated gain (dBi) Measured gain (dBi)

Reference horn 19.56 20.00

3D printed horn 1 18.98 19.02

3D printed horn 2 18.42 17.78

Fig. 10 SLS printed spherical helix antenna with several layers of copper painting (Kim 2013)

brush1st spray

2nd spray

680 700 720 740 760 780

Frequency, MHz

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ion

coef

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Fig. 11 Measured reflection coefficients of the SLS printed spherical helix antenna during painting process (Kim 2013)


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radiation efficiency is 80 % compared to the simulated result of 97 % based on solid copper. Thediscrepancy is from the conductive loss of the copper paint. The measured radiation pattern of the antennain the elevation plane is plotted in Fig. 12 and it is close to omnidirectional with a null in the broadsidedirection.

In summary, microwave horn antennas printed by EBM and SLS methods have been successfullydemonstrated. It is observed that the surface roughness of 3D printing methods can influence the antennaperformance such as gain. This issue will be more severe for higher-frequency bands such as mmW andTHz. Moreover, the relatively coarse printing resolution that can be achieved by EBM, SLS, and SLMmay also limit their applications for those higher-frequency bands.

Antenna Printed Using Powder Binder BondingAnother 3D printing technique that is capable of printing pure metallic structures, the powder binderbonding technique, is utilized to realize a 3D volcano smoke antenna (Lopez et al. 2013) for ultrawide-band (UWB) applications. The antenna is built using steel material. However, due to the low conductivityof steel, two methods to improve the antenna performance are applied. One method is covering the 3Dprinted prototype with copper tape. The other method is electroplating the prototype with copper. Theantenna geometry and a photo of the 3D printed antenna prototype are shown in Fig. 13. The measuredantenna reflection coefficients for both methods are plotted in Fig. 14 together with the simulated result.For the copper tape-covered case, the discrepancy between the measured and simulated results is believedto be due to the glue on the copper tape which would introduce an air gap between the steel and copper.For the electroplating case, the difference between simulation and measurement may come from theinsufficient plating of copper layer at some part of the antenna. Nevertheless, the reflection coefficients ofthe 3D printed antennas show a broadband behavior for both cases.

Antenna Printed Using Stereolithography (SL)Stereolithography is one of the most accurate AM techniques. It has also been applied in the realization ofmicrowave antennas. An example using stereolithography and electroplating approach to build hornantennas at Ku-band is reported in Huang et al. (2005). Two horn antenna prototypes are first printed usingpolymer. Then, the stereolithography printed parts are coated by conductive silver ink as a seed layer andelectroplated with copper. Figure 15 shows the schematic of the horn and a photo of the antenna after the

120

60 60

30305

EθEφ

0

−10

−20

150

180

150

120

θ=90θ=90

Fig. 12 Measured radiation pattern of the 3D printed spherical helix antenna at 737 MHz (Kim 2013)


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e

h

g

d

c

a

a

b

f

b

Fig. 13 (a) Schematic of a designed 3D volcano smoke UWB antenna. (b) A photo of the 3D printed volcano smoke UWBantenna prototype (without copper tape covering or electroplating) by the powder binder bonding technique. The parameters in(a) are a = 16.7 mm, b = 21 mm, c = 1.5 mm, d = 2 mm, e = 4.2 mm, f = 8 mm, g = 3.5 mm, and h = 45.6 mm (Lopezet al. 2013)

SimulationMeasurement (electroplating with copper)Measurement (covered with copper tape)

4 5 6 7 8 9 10 11 12Frequency (GHz)

S11

(dB

)

0

−5

−10

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−25

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−35

−40

−45

Fig. 14 Measured reflection coefficients of the powder binder printed volcano smoke antennas compared to the simulationresult. The solid line is the simulation result, the dotted line is the measurement result of the electroplated antenna, and thedashed line is the measurement result of the copper tape-covered antenna (Lopez et al. 2013)


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stereolithography printing process. Themeasured reflection coefficients of two printed horn antennas withdifferent dimensions are plotted in Fig. 16 together with the simulation results. It can be clearly seen thatthe measured reflection coefficients of both antennas are quite good and they agree reasonably with thesimulation results. The measured radiation patterns of the 3D printed horn antennas are plotted in Fig. 17,and the results are also very close to the simulated patterns. The measured and simulated antenna gains ofthe two printed horns are compared in Table 3.

In Chieh et al. (2014), a Ku-band corrugated conical horn antenna made via the stereolithographytechnique is also reported. This antenna is printed using acrylonitrile butadiene styrene (ABS) and thencoated with conductive aerosol paint. Figure 18 shows the photos of the 3D printed horn antenna before andafter the conductive aerosol paint process. The total time in printing the antenna is about 8 h. The surfacecoating of the antenna is Super Shield 841 which is a conductive spray typically used for EM interference(EMI) reduction. A total DC resistance of 2 O is measured across the length of the antenna after finalcoating. The measured and simulated reflection coefficients are compared in Fig. 19. It can be seen that themeasured results agree well with simulation from 11 to 18 GHz. The spike around 18 GHz is believed to becaused by the excitation of the TE01 mode from the rectangular-to-circular waveguide transition.The measured co- and cross-polarization patterns at 14 and 16 GHz are plotted in Fig. 20. It is observed

Fig. 15 (a) Schematic picture of a stereolithography printed horn antenna. (b) A photo of a 3D printed horn usingstereolithography technique (Huang et al. 2005)

0

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0ba

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−3513 14 15 16

Frequency (GHz)

S11

(dB

)

S11

(dB

)

17 18 1312 14 15 16

Frequency (GHz)

17 18

simulationmeasurement

simulationmeasurement

Fig. 16 Measured and simulated reflection coefficients of the stereolithography printed (a) horn antenna I and (b) hornantenna II


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that the measured main beam agrees with the simulated results well and the cross-polarization level is atleast 20 dB lower.

These works show successful examples of using stereolithography printing and metallic coating tofabricate microwave antennas at Ku-band. Because of the high resolution of the stereolithographytechnique, it can be applied in the realization of a finer structure than other AM techniques and thereforeachieved at a higher operating frequency. Nevertheless, the required separate metallization process couldbe cumbersome and nonideal.

Antenna Printed Using Polymer JettingPolymer jetting is another kind of AM technique based on polymerization. Nayeri et al. (2014) report anexample using polymer jetting technique to print dielectric reflect arrays as high-gain antennas operatingat W-band (75–110 GHz). A reflect-array antenna is a class of antennas that uses one or a number ofdriving elements in front of a designed flat reflecting surface to produce a high directional beam. In Nayeriet al. (2014), the phase control of the reflect-array elements is achieved by varying the thickness (height)of the dielectric slab in each unit cell, and the designed structure of the reflect-array antenna is printedusing a polymer jetting printer (Objet Eden350). There are 400 unit cells in the reflect arrays. After thepolymer structure is printed, a thin layer of gold with thickness of about 100 nm is sputtered on the back of

E-Plane0a b

c d

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−40

simulation

measurement

angle(degree)

−200 −100 0 100 200

Pat

tern

(dB

)

E-Plane0

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simulation

measurement

angle(degree)

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Pat

tern

(dB

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simulation

measurement

angle(degree)−200 −100 0 100 200

Pat

tern

(dB

)

H-Plane0

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simulation

measurement

angle(degree)

−200 −100 0 100 200

Pat

tern

(dB

)

Fig. 17 Simulated and measured radiation patterns of the stereolithography printed (a) horn antenna I in E-plane, (b) hornantenna I in H-plane, (c) horn antenna II in E-plane, and (d) horn antenna II in H-plane (Huang et al. 2005)

Table 3 Measured and simulated gain and efficiency of the stereolithography printed horn antennas

Size(a � b � h) Measured gain Simulated gain Efficiency

Horn antenna I 40.2 � 29.3 � 51.0 mm3 14.58 dB 14.52 dB 100 %

Horn antenna II 23.7 � 117.3 � 40.9 mm3 10.15 dB 10.35 dB 95.5 %


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the polymer structure as a seed layer. Then, electroplating process is applied to metalize the backside ofthe reflect array. Photos of the printed reflect-array antennas are shown in Fig. 21. AW-band pyramidalhorn placed 22.5 mm away from the reflect-array aperture with a tilted angle of 25� is used as the feed.Figure 22 shows the measured and simulated radiation patterns of the first design in Fig. 21. It can be seenthat the measured results agree well with simulation. The measured gain of these three designs at 100 GHzis 22.5 dB, 22.9 dB, and 18.9 dB, respectively. This work shows that the polymer jetting printed dielectricreflect array is a promising approach of realizing a high-gain antenna at a submillimeter frequency.

Fig. 18 A 3D printed Ku-band corrugated conical horn antenna using stereolithography before and after conductive aerosolpaint (Chieh et al. 2014). (a) Half of the printed horn antenna before painting. (b) Half of the printed horn antenna afterpainting. (c) The complete final antenna

Fig. 19 Measured reflection coefficients of the 3D printed corrugated conical horn antenna compared with simulation (Chiehet al. 2014). The discrepancy around 18 GHz is due to the excitation of the TE01 mode from the rectangular-to-circularwaveguide transition


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An all-dielectric antenna operating in the mmW/THz frequency range has also been realized by thepolymer jetting 3D printing technique. InWu et al. (2012), a 3D printed electromagnetic crystal (EMXT)-based all-dielectric THz horn antenna is reported. The EMXT structure exhibits electromagnetic bandgaps in certain frequency bands due to its periodicity. Within these band gaps, EM wave propagation isprohibited and therefore a hollow channel in the crystal structure will be able to confine and guide wavepropagation along the channel. Photo images of the 3D printed EMXT horn antenna are shown in Fig. 23.Because of the band gap structure, in certain frequency bands, the wave propagation will be confined inthe horn-shaped hollow core, and the antenna would work similarly like a regular horn antenna. Thesefrequency bands are termed as the passbands of this EMXT horn antenna.

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Fig. 21 (a) Top view and (b) bottom view of the dielectric reflect-array antennas printed using polymer jetting technique andelectroplating (Nayeri et al. 2014)


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This all-dielectric EMXT horn antenna is experimentally characterized by a THz time domainspectroscopy system. Figure 24 plots the measured and simulated radiation patterns of the antenna forthe first (105 GHz) and third (146 GHz) passbands. It can be seen that the measured results agree well withsimulation. This printed all-dielectric horn antenna demonstrates comparable or better radiation perfor-mance compared to a copper horn antenna with the same geometry (Wu et al. 2012). Moreover, as animportant free-space coupling component, the demonstration of the all-dielectric antenna may lead topotential integrated THz systems that can be manufactured by the polymer jetting technique.

Another example using polymer jetting technology to print an all-dielectric microwave lens antenna isreported in Liang et al. (2014) in which a broadband 3D Luneburg lens antenna (Luneburg 1964)operating from X to Ku-band is printed by employing the polymer jetting technique. The Luneburglens is an attractive gradient index device used as an antenna for wide-angle radiation scanning because ofits broadband behavior, high gain, and the ability to formmultiple beams. Every point on the surface of anideal Luneburg lens is the focal point of a plane wave incident from the opposite side. The refraction index

n distribution of a spherical Luneburg lens is given by equation n rð Þ2 ¼ er rð Þ ¼ 2� r=Rð Þ2, where er isthe relative permittivity, R is the radius of the lens, and r is the distance from the point to the center of the

Fig. 22 Simulated and measured radiation patterns of the first reflect array in Fig. 21 at 100 GHz (Nayeri et al. 2014)

Fig. 23 Photos of the polymer jetting printed THz EMXT horn antenna. (a) Side view. (b) Front view. The total size of theantenna is 85 mm for length and 34 mm for diameter and the flare angle of the horn is 12.4� (Wu et al. 2012)


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sphere. In Liang et al. (2014), the required gradient index distribution of Luneburg lens is realized bycontrolling the filling ratio of a polymer-/air-based unit cell and therefore changes the effective permit-tivity at different locations. For example, at the center part of lens, a larger required effective permittivitywill need a larger polymer filling ratio. At the edge of lens, a smaller required effective permittivity willneed a smaller polymer filling ratio. One important thing to note is that the unit cell size needs to be smallenough to guarantee the effective medium assumption of the design. Figure 25 shows three examples of3D printed Luneburg lens: the first one has a diameter of 12 cm, the second one is a larger version of thelens with a diameter of 24 cm, and the third one is a Luneburg lens working at Ka- and Q-band with muchsmaller unit cell size. In the measurement, the feed of the Luneburg lens antenna is either a rectangularwaveguide or a coaxial probe mounted on the surface of the lens. Figure 26 plots the simulated andmeasured radiation patterns of the 12-cm-diameter lens antenna at 10 GHz using a waveguide feed. All theradiation patterns from 8.2 GHz to 20 GHz show a highly directional beam and good agreement betweenexperiment and simulation is obtained. The gain of this lens antenna using a waveguide feed is measuredto be from 17.3 dB (at 8.2 GHz) to 24 dB (at 19.8 GHz). The side lobe is measured to be about 25 dB lowerthan the main beam in the H-plane and about 20 dB lower than the main beam in the E-plane. Thedifference is due to the asymmetry of the waveguide feed. Compared to traditional Luneburg lensfabrication techniques, the polymer jetting technique has a much lower cost, and the fabrication processis more precise, convenient, and faster.

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The above mentioned antenna examples show that the polymer jetting technique is a very goodcandidate in realizing 3D printed antennas, even up to THz frequency. Some of the complicated structuresof these antennas would be very difficult or even impossible to fabricate using conventional manufactur-ing process. Moreover, compared to the conventional method, the polymer jetting technique will have alower cost, and the fabrication process is more precise, convenient, and faster.

Antenna Printed Using Direct Printing of Conductive InkDirect printing refers to any technology that has the ability to deposit a variety of materials onto a surfacein a designed pattern (Lopes et al. 2012). The surface to be printed on can be either flat or curved. Silver

Fig. 25 Photos of the polymer jetting printed Luneburg lenses. (a) A lens with a 12 cm diameter. (b) The cross-section cutthrough the center of the 12-cm-diameter lens. (c) A larger lens with a 24 cm diameter. (d) Cross-section cut of the 24-cm-diameter lens. (e) A Luneburg lens working at Ka- and Q-band. (f) Cross-section cut of the Ka- and Q-band Luneburg lens(Liang et al. 2014; Gbele et al. 2014)


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conductive ink is a commonly used material in direct printing to realize the conductive surface ofmicrowave antennas. After a post-printing annealing process, the conductivity of the printed ink maybe on the same order of the conductivity of pure metal.

An example using direct printing technology to implement an electrically small antenna is reported inAdams et al. (2011) in which conductive meander lines are printed through a tapered cylindrical nozzleonto a hemispherical glass substrate to realize the antenna (Ahn et al. 2009). A post-annealing process at550 �C is applied for 3 h. The printed meander line shows a DC conductivity of 2 � 107 S/m, which iswithin a factor of 3 compared to the conductivity of pure silver. Photos of the direct printed antenna beforeand after connecting to the feed lines are shown in Fig. 27. Figure 28 plots the measured VSWR as afunction of frequency for one of the printed antennas together with the simulation results. The measuredcenter frequency is at 1.7 GHz with a bandwidth of 12.6 %, indicating a good performance approachingthe Chu limit of this electrically small antenna (Chu 1948).

Fig. 26 Simulated and measured H-plane radiation patterns of the polymer jetting printed 12-cm-diameter Luneburg lensantenna at 10 GHz (Liang et al. 2014)

Fig. 27 Photos of the meander line antenna printed using conductive ink before (inset) and after connecting to feed lines(Adams et al. 2011)


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Salonen et al. (2013) also report a printed antenna using the direct printing technique on a 3D surface.The antenna is realized using the nScrypt@ nozzle dispensing tool (http://www.nscrypt.com/). A photo ofthe handset antenna during the printing process is illustrated in Fig. 29a, b which shows the antenna after

Fig. 28 Measured and simulated VSWR versus frequency of the conductive ink printed meander line antenna (Adamset al. 2011)

Fig. 29 (a) A photo shows a handset antenna during the direct printing process. (b) The handset antenna after printing(Salonen et al. 2013)


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printing. In the printing process, the conductive silver paste is extruded through a nozzle and depositedonto the substrate. The nozzle position is controlled by a 3-axis motion system and the material flow isadjusted by a precision pump. After printing, the silver paste is heat treated at 120 �C for 10 min toincrease the conductivity. 120 �C is selected because a higher temperature causes some deformation of theplastic substrate. Figure 30 plots the measured reflection coefficient and the radiation efficiency of thedirect printed antenna compared to those of the standard laser direct structuring (LDS) technique-fabricated antenna. The results show that these two antennas have similar performance in terms ofreflection coefficient. The radiation efficiency of the direct printed antenna is 0.5 dB lower at the lowerband and nearly equal at the higher band, compared to the LDS antenna. The slightly lower efficiency isbelieved to be caused by the skin effect which is more dominant at the lower band due to the smallerconductivity of the ink.

Antenna Printed Using Fused Deposition Modeling (FDM)The FDM technique has the ability to print a large number of thermoplastic materials. In Ahmadloo andMousavi (2013), a 3D meander line dipole antenna is printed on a V-shaped substrate using the FDMtechnique. The conductive part of the antenna is realized using a printed conductive ink. The curing

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process of the printed conductive ink is at 85 �C for 15 min to reduce the resistivity. After curing, theconductivity of the silver ink is measured to be 1.5� 106 S/m. A higher curing temperature can achieve ahigher conductivity. However, to avoid the substrate deformation during the curing time, 85 �C isselected. A photo of the printed antenna is shown in Fig. 31. The meander line dipole is fed by a SMAconnector. Figures 32 and 33 compare the measured and simulated reflection coefficients and radiationpatterns of this antenna. It can be seen that the measured results agree well with the simulation results.

As a potential solution for integrating dielectric and conductor printing while maintaining high qualityfor each material, a microwave patch antenna is realized by FDM and an ultrasonic wire mesh embeddingprocess (Liang et al. 2014). The substrate of the patch antenna is created using an FDM 3D printer and theconductive part of the antenna is realized using an ultrasonic machine which has the ability to embedcopper wires on a 3D surface. Compared to the conductive ink approach, the ultrasonic wire embeddingtechnique is performed at room temperature and therefore will not influence the thermoplastic substrate.In addition, since pure metal wire is used, the conductivity of the material is much larger than that of theconductive ink. The schematic design of the patch antenna and a photo of the 3D printed patch antenna areshown in Fig. 34. The feed of this antenna is a SMA probe from the back of the patch. The measuredreflection coefficient and radiation pattern compared to simulation results using wire mesh and ideal

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conductor are illustrated in Fig. 35. It can be seen that the wire mesh structure works as well as a regularconductor sheet at microwave frequency and the measurement agrees well with simulation.

This example successfully demonstrated 3D printing of a microwave patch antenna by combining theFDM method with ultrasonic metal wire mesh embedding. No metal sintering or any other high-temperature conductor printing process is needed, and the printed metal wire mesh works as well as aregular metal sheet at a microwave frequency. The demonstrated 3D printing process of both dielectricand conductor can be applied to the other more sophisticated EM structures such as vertically integratedphased arrays. The disadvantage of this wire mesh embedding technique is the lower resolution comparedto other AM techniques due to the minimum diameter of the metal wire (50 mm). This will limit themaximum operating frequency of components printed using the wire mesh embedding technique.

Challenges and Potential Solutions

It has been argued that 3D printing could be the future of manufacturing due to its ability to print structureswith more flexible design than conventional methods. Recently, rapid progress has been made in the 3Dprinting area. However, a number of challenges still need to be resolved before advanced functionalantennas can be printed in a 3D fashion robustly.

Surface RoughnessSurface roughness is an important parameter for manufacturing of antennas, especially for higher-frequency ranges such as mmW and THz. A rough surface can seriously degrade the EM performance

Fig. 34 Schematic of a microwave patch antenna made of FDM printed substrate and ultrasonically embedded wire mesh (left)and a photo of the patch antenna printed using FDM and wire mesh embedding technique (Liang et al. 2014)

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in terms of increased conductor loss. Currently, a drawback of many 3D printing techniques is the roughsurface finish. For example, the EBM technique has a surface roughness on the order of tens of microns(Garcia et al. 2013). The FDM method has a surface roughness from 10 to 50 mm (Ahn et al. 2008).A post-polishing step often needs to take place after the printing process if fine surface roughness isrequired. However, for some of the complicated parts, it is inconvenient or even impossible to polish theinner part of the structure. Therefore, improving the surface roughness is one of the challenges in AM forhigh-frequency antenna applications.

ResolutionThe printing resolution is also a challenge for AM technology, especially for those components working athigh frequencies (e.g., mmW or THz). A higher resolution means a more accurate structure which isnecessary for higher-frequency applications. Currently, most of the 3D printing techniques have aresolution no better than 20 mm with the exception of a two-photon polymerization stereolithographytechnique which has a sub-micrometer resolution. However, there is always a trade-off between printingresolution, printable size, and printing speed. For example, it is difficult to print large-size components(e.g., >1 cm) using the two-photon polymerization technique because of the printing speed. The long-term solution lies in the integration of multi-scale 3D printing techniques so that high-resolution or high-volume/high-speed techniques can be seamlessly integrated and applied when necessary.

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Fig. 35 Measured and simulated (a) reflection coefficients and (b) radiation patterns of the patch antenna printed using FDMand wire mesh embedding technique (Liang et al. 2014)


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Limited Electromagnetic (EM) Property RangeAnother issue for 3D AM of antennas is the lack of available printable materials with desired EMproperties (i.e., permittivity e and permeability m). At the present time, most of the commercially availablematerials used in 3D printing technologies are designed or selected with only the mechanical property inmind, and none of the material is designed for the EM applications. The lack of 3D printable materialswith desired EM properties restricts their applicability to microwave components. For example, most ofthe existing printable polymers have a permittivity from 2 to 3. However, a higher dielectric constant isoften desired for a microwave circuit and antenna for miniaturization purpose. Therefore, research anddevelopment in new printable materials with flexible EM properties would be necessary. A potentialsolution is using a polymer matrix composite method which mixes a 3D printable polymer withnanoparticles (Liang et al. 2014). By employing this method, novel materials with a wider range of EMproperties (i.e., e and m) may be realized.

Performance of Printed ConductorAs discussed previously, the incorporation of a high-quality conductor with 3D printed dielectrics is also achallenge. Printing conductive inks on a dielectric substrate is one of the most popular methods to realize3D spatial control of conducting material. However, to achieve high conductivity, high-temperaturecuring processes, which are usually not compatible to 3D printing techniques such as thermoplasticextrusion and photopolymerization, are necessary. With the limit of low curing temperature, the conduc-tive inks tend to have relatively poor conductivity which leads to higher loss and could be detrimental toantenna performance, especially at a high frequency. Using the ultrasonic wire embedding technique inLiang et al. (2014) to realize the conductive part of the microwave antenna is an attractive alternative sincethis method guarantees a high conductivity (s) at room temperature. Nevertheless, the resolution of thewire mesh embedding technique is lower than other AM techniques. Therefore, developing methods torealize the printing of a high-conductivity material at room temperature with arbitrary geometry will becritical to advance the state-of-the-art 3D printing of electronics including antennas.

Multiple Scales and Multiple MaterialsAnother desired capability in AM technology is multiple-scale printing. Currently, for most of thecommercially available 3D printers, their printer heads only have one resolution which means it wouldtake a long time to print a large object if a small part of the object needs a fine resolution. To address thisproblem, a multi-scale printer which has the ability to print some part of the object with fine resolution andprint other parts of the object with coarse resolution is necessary.

A 3D AM technology capable of printing multiple materials is also highly desirable. Being able toefficiently realize arbitrary 3D spatial distribution of EM (e, m, and s) is the holy grail of 3D printingtechnology for antenna applications. It will enable many new design possibilities which may lead torevolutionary antenna configurations having unprecedented advantages. An antenna structure withartificially controlled EM properties (e, m, and s) in arbitrary 3D spatial distribution, novel 3D gradientindex (GRIN) metamaterial-based lens antenna, and 3D printed vertically integrated phased array systemare just some of the examples. If printable materials with time-dependent and/or nonlinear EM properties(i.e., 3D printed semiconductors (Ahn et al. 2006)) become available, AM technology may be used toachieve advanced fully functional systems.


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Summary

In this chapter, the working principles and processes for various 3D AM techniques that are relevant toantenna design and implementation are described. Recent progresses and challenges for 3D printing ofantennas are reviewed and discussed. The reported experiments demonstrate a number of successfulantenna examples printed by various AM techniques. There are still substantial challenges that need to beovercome before complete and fully functional antennas and microwave systems can be truly realized viaAM. However, further investigation and development of 3D printing technology in the areas of mechan-ical engineering, material science and engineering, and electrical engineering will no doubt lead to a newparadigm of 3D printed antennas and other microwave components and systems.

Cross-References

▶Advanced Antenna Fabrication Process (MEMS/LTCC/LCP/printing)▶Lens Antennas▶Reflectarray Antennas▶RF Material Characterization

References

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