**2. Role of additive manufacturing for antennas and radio frequency components**

As governments, industries, and universities move toward multipurpose wireless platforms, engineers must integrate functionality of disparate frequency bands into single systems. This requires planar and vertical integration of apertures, substrates, and feed networks to enable multiple modes of operation. Now RF front ends must integrate several different antennas and their feed networks consisting of transmission lines, amplifiers, filters, and switches across increasing bandwidth. Integrated designs require both hybrid material and fully volumetric, as opposed to planar, approaches. The ability of AM to achieve geometries not possible by today's manufacturing processes makes AM a critical enabler of future of RF systems.

In the recent past, designers achieved robust mechanical and structural components through AM processes, but companies have not yet developed AM materials and feedstocks suitable for the design of antennas and other RF components. Engineers must also conduct research in the area of conductive inks for AM. Current silver inks yield metal layers with lower conductivity compared to their bulk metal counterparts. Increased conductivity of printable inks enhances the power efficiency of RF components.

Currently, low dielectric constants of commercially available AM feedstocks are limiting antenna designs. However, through the development of high-dielectric constant and low-loss electromagnetic materials, AM opens the RF design space to complex geometries and material gradients not currently achievable. One example is the Luneberg lens [1], which relies on a graded dielectric constant. By controlling the fill density of printed substrates, AM achieves a continuously graded slope in dielectric constant that is the enabling feature of the Luneberg lens design [2].

Increasing the RF versatility of AM requires research into feedstocks that achieve high dielectric constants. Recent research shows AM filaments with dielectric constants 0f 4 or greater can be extruded from polymer/ceramic

#### *Additive Manufacturing for Antenna Applications DOI: http://dx.doi.org/10.5772/intechopen.92363*

previously mentioned. This technique could open the doors for even higher dielectric AM filaments because designers will not need to spool or flex the highly loaded

*Advanced Radio Frequency Antennas for Modern Communication and Medical Systems*

Currently, AM produces robust structural and mechanical parts, but designers have yet to fully develop and characterize electromagnetic properties of AM feedstocks for printing antennas and other RF devices. Recent research into the composition of high-permittivity feedstocks for AM opens the design space for hybrid material antennas, and necessitates an emphasis on the measurement of electro-

Novel high dielectrics and conductive inks for AM enable complex and integrated antenna designs in all three dimensions. This leads to our integration of our S/X-band antenna into a single aperture allowing for simultaneous multiband capabilities. The 3D and hybrid material approaches needed to achieve this scalable, agile, and multimode antenna demonstrate the necessity of AM for future of RF systems. Additionally, AM enables on demand supply closer to the point of need. This reduces the logistical burden, cost, and upgradeability of RF system maintenance in the field. These benefits result rapid development of new technologies, and

increases agility to address new RF needs as they emerge in near real time.

**2. Role of additive manufacturing for antennas and radio frequency**

As governments, industries, and universities move toward multipurpose wireless platforms, engineers must integrate functionality of disparate frequency bands into single systems. This requires planar and vertical integration of apertures, substrates, and feed networks to enable multiple modes of operation. Now RF front ends must integrate several different antennas and their feed networks consisting of transmission lines, amplifiers, filters, and switches across increasing bandwidth. Integrated designs require both hybrid material and fully volumetric, as opposed to planar, approaches. The ability of AM to achieve geometries not possible by today's manufacturing processes makes AM a critical enabler of future of RF systems.

In the recent past, designers achieved robust mechanical and structural components through AM processes, but companies have not yet developed AM materials and feedstocks suitable for the design of antennas and other RF components. Engineers must also conduct research in the area of conductive inks for AM. Current silver inks yield metal layers with lower conductivity compared to their bulk metal counterparts. Increased conductivity of printable inks enhances the power effi-

Currently, low dielectric constants of commercially available AM feedstocks are limiting antenna designs. However, through the development of high-dielectric constant and low-loss electromagnetic materials, AM opens the RF design space to complex geometries and material gradients not currently achievable. One example is the Luneberg lens [1], which relies on a graded dielectric constant. By controlling the fill density of printed substrates, AM achieves a continuously graded slope in dielectric constant that is the enabling feature of the Luneberg lens design [2]. Increasing the RF versatility of AM requires research into feedstocks that achieve high dielectric constants. Recent research shows AM filaments with dielectric constants 0f 4 or greater can be extruded from polymer/ceramic

antenna manufactured via legacy materials and techniques.

This chapter focuses on the benefits and negatives of AM pertaining to antennas. AM dielectric substrates enable multiband approaches to AM phased arrays. We demonstrate our AM antenna by comparing experimental data to an identical

polymer in order to print with it.

**components**

ciency of RF components.

**190**

magnetic properties for printed dielectric substrates.

composites [3–6]. The process loads a low-loss host polymer with a given volume fraction 0f ceramic nanoparticles with high dielectric constants. The host polymer will tend to have a low dielectric constant and the volume fraction of the dispersed high-dielectric ceramic particles will determine the macroscopic dielectric constant of the extruded filament [5, 7]. Since we know the presence of voids during the 3D printing process can cause deviations in the dielectric constant, electromagnetic characterization of the final substrate becomes very important.

AM for hybrid material antennas has additional obstacles other than limitations in dielectric constant. The layered nature of 3D printing causes the potential for inconsistencies in the bonding of interfaces between printed layers. Similarly, 3D printing processes yield anisotropy in the electromagnetic properties of printed layers that can yield RF properties that differ depending on direction within the printed substrate. Porosity, surface roughness, and repetitiveness are also concerns relating to AM technologies for antennas and RF devices [8–11].

Beyond dielectric feedstocks for AM, research into increasing the conductivity of printable inks is also of interest. The best conductivity of conductive inks is currently 5–10 times less than that of bulk metals, and even these reduced conductivities require sintering processes in excess of 175°C [12]. Reduced conductivity will cause decreased radiation efficiency in antennas and increased transport inefficiencies in transmission lines. Whereas high sintering temperatures required for 3D printed inks would degrade and melt the thermoplastic-based dielectric compounds discussed previously. There are alternate methods for sintering currently under investigation such as laser sintering and flash annealing, but these methods still require further research to demonstrate their viability.

### **2.1 High-dielectric filaments compatible with additive manufacturing**

Current research investigates methods for extruding high-dielectric AM filaments through a robust and repeatable method that allows for the printing of AM substrates with a given value of dielectric constant. This will give engineers a continuum of achievable dielectric constants for AM substrates they can use in their models when designing RF components. A second challenge is developing a technique for sintering of conductive inks printed on an AM substrate without compromising the integrity of the dielectric substrate. A final hurdle is to produce a fully integrated AM antenna including ground plane, RF connectors, feed, multiple dielectric substrates, and aperture through a fully automated process.

Typically, composite materials are prepared using mixing techniques such as dissolving pellets of ABS using acetone [12, 13]. Afterward, mixers add plasticizers and surfactants in small concentrations. These additives are necessary, as the plasticizer acts as a lubricant between molecular chains in the polymer, enabling flexibility even with loading of ceramic powders; however, the addition of too much plasticizer negatively affects the composite by rendering it too elastic. Materials use surfactants to prevent aggregation between the ceramic particles [14].

Adding high-dielectric ceramic nanoparticles in in volume fractions between 15 and 30% to the mixtures allows to them to homogenize within the polymer. Once the acetone has fully evaporated, we cut the composite slab into pieces and then ground them into approximately 2-mm pellets. We can then extrude filaments at high temperature, but this process has its limitations. Adding more than 40% by volume of ceramic powder results in a filament too brittle for use, even with the addition of plasticizer. Furthermore, as the loading of ceramic inclusions increases, so too does the viscosity of the material. Designers must exercise care, as viscous materials are prone to printing defects, such as voids [14].
