Multi-Layer/Multi-Material 3D Printing Yields “Nearly Impossible” Antennas

What you’ll learn:

The benefits and limitations of antennas fabricated using standard 3D printing (additive manufacturing).
How a radically different approach to 3D printing offers new options and benefits.
Details of the two test units built by the researchers, along with test results.

Additive manufacturing (AM, more specifically known as “3D printing”) is now a well-established technique used to create unique and custom antennas and related RF devices, among other applications. Many of these are difficult or impossible to fabricate via conventional metal-forming techniques or clad printed circuit boards.

A team led by the University of California at Berkeley has developed a new 3D-printing/AM platform that they claim offers “unparalleled flexibility in antenna design and the capability for rapid printing of intricate antenna structures.” The new platform, dubbed charge programmed deposition multi-material 3D printing (CPD), is a universal system for rapid production of nearly all 3D antenna systems.

The process can pattern highly conductive metals onto a wide range of dielectric materials onto a 3D structure. Furthermore, just as 3D printing enables fabrication of structures that aren’t possible with conventional machining, this 3D-printing process can produce RF devices that can’t be fabricated otherwise.

Desktop Digital-Light Printer Plus Catalyst-Based Tech

The CPD method combines a desktop digital-light 3D printer and a catalyst-based technology that can pattern different polymers at different locations where they will attract metal plating. Its auto-catalytic or selective plating technology enables the polymers to selectively absorb metal ions into prescribed locations that are defined by the desired antenna design outcome.

Note that platform isn’t a custom or high-end 3D printer for metals using expensive metal powders and high-energy lasers. Instead, the technology can be applied to desktop-friendly light-based printers. In the published paper, the authors clearly describe the capabilities as well as the limitations of existing 3D-printing systems for antenna and RF-component applications. Relevant factors include limited practicality of multiple materials and layers, dimensional accuracy and resolution, and even toolpath restrictions.

CPD can broadly integrate with a variety of multi-material 3D printing methods, as it allows for the development of essentially any complex 3D structure, including complex lattices. Using it, they demonstrated deposition of copper with near-pristine conductivity, as well as magnetic materials, semiconductors, nanomaterial, and combinations of these materials.

It’s well-suited for antennas, because nearly all antennas need two components: the “metal phase” (the conductor), and the “dielectric phase,” which isn’t conductive. The researchers claim that until now, no technology has been capable of directly patterning or synthesizing the conductor and dielectric materials together.

How Does CPD Work?

In brief, the charge programmed deposition manufacturing program is based on patterning and controlling surface charge polarity via multi-material printing of photo monomers with varying pendant reactive groups. The charge-programmed 3D mosaic, combining positive, negative, and neutral charged areas, forms a patterned substrate upon which selective micro-fabrication of metallic and other functional materials can be carried out (Fig. 1). The figure also highlights some of the truly amazing configurations they have built.

1. (A) Charge programmed printing and deposition scheme. (B-F) Photos of charge programmed deposition additive-manufactured antennas, as follows: (B) gradient phase transmitarray with three layers of interpenetrating S-rings and dielectric materials; (C) Vivaldi antenna; (D) 3D-folded electrically small antenna featuring interpenetrating Archimedean spirals and Hilbert curves; (E) tree fractal antenna; (F) horn antenna with septum polarizer.

When the sub-domains within the 3D substrate and deposition material have opposite charge polarities, there’s attraction and deposition; like polarity or no polarity (neutral) repels or gives no plating. Surface charge is achieved by blending inherently charged photo monomers into the printing ink. The entire fabrication process has minimal steps without reliance on toolpath, post-sintering, or a substrate on which to write.

There’s much more technology involved, including choice of materials and the flexibility that having such a selection affords when applying the conductive and dielectric layers. However, it’s especially impressive to look at some examples of components they have created.

Two Eye-Opening Results of the CPD Technique

Two examples demonstrate the capabilities of their technique:

Example #1

They designed and printed an ultra-light, circularly polarized (CP), 19-GHz transmitarray antenna. Taking advantage of the CPD process, they developed the transmitarray unit cell topology with an S-ring unit cell that’s structurally optimized to minimize the use and weight of dielectric material (Fig. 2).

2. Schematic comparison of (A) conventional lithographic transmitarray unit cell with (B) ultra-light transmitarray unit cell printed with CPD: (C) Weight comparison between the ultra-light transmitarray and a traditional PCB-process-manufactured transmitarray of a similar design at the same frequency. (D, E) Photos showing the complex metal-dielectric structure of copper and acrylate polymer. (F) The transmission coefficient (|T_LR|: left-hand; |T_RR|: right-hand) of the unit cell under right-hand circularly polarized incidence with different incident angles (θ_inc). (G) Transmitarray simulation (Simu.) and measured (Meas.) results at 19 GHz for the co-polarized (Co-pol) left-hand circularly polarized (LHCP, solid lines) and cross-polarized (X-pol) right-hand circularly polarized (RHCP, dashed lines) components. (H) Horizontal tiling scheme. (I, J) The assembly of the 12- and 20-cm diameter transmitarray antenna. (K) LHCP (co-polarized) and RHCP (cross-polarized) experimental data in 0°-cut of all-in-one printing (AIOP) and tiled 12-cm transmitarray at 19 GHz.

[Not familiar with a “transmitarray”? This is a low-profile high-gain antenna that’s a good fit for remote sensing and communications, such as for CubeSats and SmallSats. A typical transmitarray is fed by a low-gain antenna (feed source) and generates highly directive radiation by correcting the spherical phase front of the feed. As the building blocks of a transmitarray, the phase-shifting unit cells are a critical part of a transmitarray design. Most transmitarray unit cells require at least three layers of metallic elements spaced by relatively bulky and heavy dielectric to achieve the desired transmission efficiency and phase control.]

The CP transmitarray design is featured with discontinuously distributed conductive S-ring elements throughout the structurally optimized 3D layout, where electromagnetic wave phase control is realized through element rotation. This typically allows for a wider operational bandwidth than the conventional transmitarray designs that use the elements’ size variation to achieve different phase compensation.

Example #2

UCB developed a lightweight horn antenna that not only allows for weight reduction by incorporating lattice structures, but it does so by taking advantage of the thin-film and coating nature of the technique. Horn antennas (and waveguides) are traditionally built with solid metal using metal machining, injection molding, or AM techniques such as metal laser sintering. Even a 3D-printed horn antenna using binder jetting/sintering or selective laser melting typically has a metallic body that’s at least 1 mm thick, giving it considerable weight.

However, according to the skin effect, the alternating RF current is distributed predominantly within the thickness of just a few skin depths (which is several microns at K-band frequencies) inside the metal. The rest of the metallic material has minimum effect on the electromagnetic performance of the horn, but it’s needed for mechanical strength

UCB developed a 19-GHz CP horn antenna as the feed source for the previously discussed transmitarray (Fig. 3). Unlike an all-metal horn or a printed polymer horn entirely coated with conductive materials, their horn weighed only 12 grams with a selectively patterned thin layer of copper only on the interior surface where electromagnetic wave propagates. The same horn in brass would be 5X heavier.

3. (A) CP horn antenna design with complex internal features. Dielectric is shown in tan/gray, and copper in orange. (B) Comparison between the measured and simulated radiation patterns of the horn at 19 GHz. (C) Measured reflection coefficient S₁₁ at the input port of the horn (inset: photo of the printed horn being measured in the spherical near-field range at UCLA).

Their horn has a complex internal architecture. It includes a “meandered” waveguide transition, a square waveguide section with a septum polarizer used to generate a right-hand circularly polarized (RHCP) wave that’s eventually radiated to free space through the circular horn, a square-to-circular adapter, and a circular horn section. The horn antenna was designed with a standard WR-42 waveguide interface, making it compatible with a commercially available coax-to-waveguide adapter for excitation.

In addition to testing the transmitarray and horn antenna as individual RF elements, the team put the two together to form an all-3D-printed antenna system and then performed a range of RF tests. Performance was in close agreement with both simulated and anticipated results (Fig. 4).

All-3D-printed antenna system — 4. (A) Schematic for all-3D-printed antenna system consisted of a horn antenna and transmitarray. (B) Photo of the assembled 20-cm transmitarray being measured. (C) Comparison between the simulated pattern and the measured pattern of the 20-cm transmitarray, at 19 GHz (solid line: the co-polarized LHCP pattern; dashed line: the cross-polarized RHCP pattern). (D) The measured directivity and axial ratio of the 20-cm transmitarray over frequency. (E) Schematic for a beam-steerable Risley prism antenna (RPA) comprised of a gradient-phase transmitarray (GPTA) and gradient-phase feed array (GPFA). (F) Photo of the printed RPA being measured for its radiation pattern. (G) Representative measured RPA patterns showing beams at 0° and 60°, when using different panel orientations.

They showed that the charge-programmed 3D-printing method was a versatile and universal platform for fabricating truly ultra-light 3D antennas composed of dielectric phases, conducting phases, or their interpenetrating composites. This technology allows for printing of previously inaccessible electronic architectures and systems when done with current manufacturing approaches, with weight savings reaching 90%.

The fascinating story is detailed in their paper “Ultra-light antennas via charge programmed deposition additive manufacturing” published in Nature Communications. The highly readable paper doesn’t have equations, but it describes all steps in their approach from concept through analysis, simulation, implementation, fabrication of antenna shapes, test results, materials used, and tradeoffs. A Supporting Information file provides additional details.