3-D Printing Boresights Antenna Manufacturing
By Barry Manz Editor
Additive 3-D manufacturing, that is, making a three-dimensional solid object of virtually any shape based on a model created in a 3-D design tool, is changing the way many products are made and who can make them.
In only a few years it has risen from a way to produce prototypes using large equipment more akin to a laboratory environment than a manufacturing floor to systems that can make reasonably large quantities using “green” materials in less time and at lower cost than by traditional methods. If 3-D printers can make medical implants, customized shoes, lampshades, parts for vehicles, aircraft, and ships, batteries, and hundreds of other things, why not antennas? This is precisely what companies ranging from tiny enterprises to defense prime contractors are exploring.
Before launching into a discussion of additive manufacturing (3-D printing), it’s important to distinguish the two basic types. Additive manufacturing is designed to make complex three-dimensional items and is the sector of the industry that gets the most attention. The other sector is 3-D electronic printing, in which shapes such as antennas are printed on almost any material, in the case of electronic circuits using conductive ink containing silver nanoparticles.
Mike O’Reilly, product manager for Optomec’s Aerosol Jet product line, illustrates the latter sector. “Printed electronics is very different from additive manufacturing as it is designed to add material onto an existing surface. Two-dimensional electronic printing been around for decades but what’s new is the ability to print three-dimensional structures. Companies like ours can print onto a smartphone case, for example, and we can print antennas as fast as if not faster than the current technology used to do that in mass production. Printed electronics can serve mass production while additive manufacturing printing physical parts, for now at least, is for R&D and low-volume manufacturing.”
Additive printing of electronic circuits such as antennas is already well underway and not surprisingly the first big customers are manufacturers of small battery-operated devices such as smartphones, tablets, and laptops. The ability to print all of the antennas on the plastic or metal cases of these products is a welcome technology when they must accommodate up to 44 frequency bands for wireless carriers, as well as Wi-Fi, GPS, NFS, and sometimes even FM radio.
3-D additive printing is actually not a new technology, and some processes date back into the late 1970s when printers were large, expensive, and very limited in what they could produce. The first modern 3-D printer is credited to Chuck Hall of 3D Systems in 1984 but it took until about 2010 before the 3-D printing industry really took off. In the context of antenna design, the ability to produce finished products using a growing array of materials means that anyone with significant expertise in antenna theory and the use of 3-D design tools as well as the money to buy the printer, software, and ancillary components can become an antenna manufacturer.
This has the potential to completely change the way many types of antennas are designed and built, on what types of platforms they can be deployed, how electrically large (or small) they can be, how much they weigh and cost – and who can build them.
In the beginning
Additive 3-D printing gets its name from the fact that it adds material one layer at a time to produce the end result rather than subtracting material by either cutting, drilling, or gouging out from a solid block of material (with up to 90% of it being removed and usually discarded). 3-D additive printing requires far less material and creates much less waste as what is left over can generally be reused. Many passes of a prototype model can be made at far less cost and in much less time than traditional methods and it typically uses no hazardous substances unlike the chemicals employed in mechanical and electronic manufacturing. Additive printing also can be extraordinarily precise, creating intricate structures that would be difficult and time-consuming and sometimes even impossible to create by other means. The prototype for the mid-engined Audi RSQ concept car (Figure 1) for the sci-fi film “I, Robot”, was in part created by 3-D printers–and that was in 2004.
Superficially speaking, 3-D printing is no different than simply pressing “print” on a computer and watching an inkjet printer produce the result, the difference being that instead of traditional inks, 3-D printing uses many types of specialized inks, in the case of RF and microwave products a conductive type. The machines create the layers by making multiple slices through a design created in software, which like traditional machining techniques, are sent to the printer, where the design is realized layer by layer.
There are currently at least a half dozen additive printing technologies today and each one differentiates itself by the way in which layers are deposited and in the number and types of materials it can use. Selective laser melting (SLM), direct metal laser sintering (DMLS), selective layer sintering (SLS), and fused deposition modeling (FDM), melt material to produce the layers. In stereolithography (SLA) liquid materials are cured and laminated object manufacturing (LOM) cuts thin layers to a specific shape and connects them.
Even though they are not created by rows of CNC machines that suggest sturdiness, extremely durable 3-D printed products can be made that are often stronger and much lighter than the parts that they replace. This is an enormous benefit in applications such as aircraft in which even incremental reductions in weight can increase fuel economy and reduce emissions. For example, by reducing the weight of a commercial airliner by only a few pounds a carrier can save several thousand dollars a year in fuel. And as 3-D printers become capable of producing larger shapes, it is conceivable that entire airframe such as a UAV could be made in a single process by a single machine. This may soon be true for entire vehicle bodies and no doubt many other large items as well.
Another unique benefit of additive manufacturing is its ability to make products in small volumes that are designed specifically to the requirements of a single customer. It seems very likely that in the not-too-distant future it will be possible to design a product online from a template, customize it to become uniquely that of the customer, and then order it for delivery.
Obviously, as consumer products are designed to be made in the hundreds of thousands or millions, this would not displace traditional high-volume manufacturing methods but complement them.
It could in fact create an entirely new industry for budding entrepreneurs without the resources typically required to start up a manufacturing company. That is, the barrier of entry to manufacturing could be dramatically reduced, making the dream of starting one’s own manufacturing company possible for more people than ever before. For example, with an investment of several thousand to a few tens of thousands of dollars, a designer could set up a modest facility and make a few specific products and sample them on the market. If they were successful, the business could be ramped up to produce higher volumes of these items as well as new ones limited only by the designer’s imagination. So while milling machines, presses, foundries, and injection molding equipment are not likely to disappear anytime soon they surely will be complemented by 3-D printing equipment.
Some of the most widely publicized work in 3-D printing of electrically small antennas was the effort by a team at the Frederick Seitz Materials Research Laboratory at the University of Illinois at Urbana-Champaign. The researchers produced dipole and loop antennas by omnidirectional printing of metallic nanoparticle inks and claim to be the first to demonstrate 3-D-printed antennas on shaped surfaces that are typically one-twelfth wavelength or less with performance an order of magnitude better than can be realized by standard monopole antennas and a Q factor that closely approaches the fundamental limit dictated by physics.
To fabricate an antenna that can withstand mechanical handling, the silver nanoparticle ink is printed on the interior surface of glass hemispheres (Figure 2). Other non-spherical antennas can be designed and printed using a similar approach to enable integration of low Q antennas on, for example, the inside of a cell phone case or the wing of an unmanned aerial vehicle.
“From a purist point of view the antennas in smartphones are really not very good,” says Jennifer Bernhard, Professor, Department of Electrical and Computer Engineering and Associate Dean for Research at the University of Illinois at Urbana-Champaign College of Engineering. “But what saves them is the fact that there are so many base stations, because the smaller you make the antenna the more critical the material parameters become.”
Her team and in particular Professor Jennifer Lewis (who has since moved on to Harvard) developed a family of specialized inks that have near bulk properties at microwave frequencies. The inks use silver nanoparticles in a slurry that are either dried or cured. As higher conductivity results in lower losses, a relatively efficient small antenna can be created that will have better performance than its size suggests.
Since the team printed the antenna on the sphere, they have begun to focus on other areas such as wearable types in “interesting” form factors. Other projects include sensors embedded inside a biologically-compatible sample so that signals can be received outside the sample without disturbing the area by running wires from it.
Researchers at the University of Utah’s Department of Electrical and Computer Engineering have also demonstrated complex designs that could only be manufactured cost-effectively with a 3-D printer. The researchers used small cubical cells optimized for a frequency range of 2.4 to 3 GHz and built an antenna using 3-D printing of plastic covered by conductive paint. A dipole was also optimized to function from 510 to 910 MHz. Their results showed excellent performance even with random shapes.
Antennas and smartphones, tablets, and laptops have typically been discrete from their surroundings and not actually integrated into the system. Now that space is at a premium, manufacturers want to print all of the antennas on the inside of the back cover or onto the inside front facing part of the device. Optomec has demonstrated this ability to many smartphone manufacturers and has produced different antenna types for defense applications as well.
“We have a unique deposition process that enables us to have a standoff height of about 5 mm from the surface of the substrate to the tip of the nozzle that lets us print on complex surfaces,” says Optomec’s O’Reilly. “In the beginning we were asked to print very thin conductive traces for applications such as RF shielding on nose cones, and as we demonstrated this ability it led to opportunities for printing antennas used in smartphones. We can now print a wide variety of materials including conductive metals with silver nanoparticles as small as about 10 µm and features from 10 µm to 2 mm in size. Antenna sizes are generally greater than 100 µm and sometimes 300 or 400 µm.”
O’Reilly says the company has been working with many defense companies and has printed phased array antennas (Figure 3), printed antennas on a cone (Figure 4), and other types. In the case of the cone, “we spin and tilted the cone to keep the print head perpendicular to the surface. It required about two hours and the results were as good as what I believe has been achieved using traditional manufacturing processes. There is no doubt that with further work we could deliver even better performance.”
“With the printed antennas for smartphones we are seeing better performance than what is obtained using the plating process, using a fraction of the material, and printing on polycarbonate for example that allows a manufacturer to save 7 to 8 cents per phone while also eliminating a number of process steps. This doesn’t sound like much cost savings but when you’re manufacturing 500 million phones it can add up.” The company can manufacture these antennas in the time frame and at volumes required for consumer applications.
It’s In the Army Now
Having been working on 3-D printing technologies for several years, researchers at the Armament Research, Development, and Engineering Center at Picatinny Arsenal in New Jersey, are using additive manufacturing and 3-D printing to print electronics, weapons components, and training models (Figure 5). With 3-D printing, items can be made in a matter of minutes or hours depending on the complexity of the design. This makes it well suited for prototyping and low-rate production. One of these efforts is printing antennas on helmets or sensors into clothing as well as on the wing of a UAV. Reactive sensors that change properties in the presence of anthrax or some other toxin could detect and warn the wearer of the chemical’s presence.
The researchers are using an ink-jet printer and silver conductive inks, which allows a communications antenna made of silver nanoparticles to be printed onto a flexible polyimide substrate that could be embedded into a helmet, replacing the antenna that currently attaches to the headgear. Electronics could also be printed on the side of artillery, freeing up space inside the round. The process eliminates the need to machine out groves and place sensors and wires, as well as chemically etching away material.
Wires and printed electronics could also be embedded into the wings of a UAV, which is difficult as the holes for wiring are so intricate. With 3-D printing, the engineers at Picatinny believe they can place all the holes while the piece is being created, forming a single encapsulated system. Different materials can be tried inexpensively and there is no need to seal, package, or glue the structure. The researchers also hope to print and assemble entire weapon systems in one manufacturing cube. For example, an entire claymore mine could be printed and assembled in one machine by using various tools and printing processes.
Integrating Ear and Antenna
Researchers led by Michael McAlpine, an assistant engineering professor at Princeton University, have created a prototype artificial ear from an antenna and 3-D printed cells (Figure 6). The team’s results showed for the first time that 3-D printing can be used to interweave tissue with electronics. McAlpine has worked for years on making electronics that could be integrated with the human body and in 2011 his team built a graphene tattoo that could be stuck on a tooth to detect bacteria.
While it’s possible to reconstruct an ear with cartilage grafts or cultured tissue, printing it allowed researchers to closely duplicate the shape of an ear while integrating an antenna. The team used a hydrogel seeded with calf cells for the structure, adding layers of silver nanoparticles that formed a coil antenna. The cells could then turn into cartilage. The end of the antenna connects to a system meant to simulate the cochlea, which lets it sense sounds.
The ear could pick up radio waves and future versions could pick up acoustic audio with different sensors. McAlpine says that it could theoretically be attached to human nerve endings like some hearing aids, but doing so would require much more testing. McAlpine’s ultimate goal is to help advance “bionic” organs that would seamlessly combine sensors or other electronics with the human body.
“Previously, researchers have suggested strategies to tailor the electronics so that this merger is less awkward. That typically happens between a 2-D sheet of electronics and a surface of the tissue,” he says. “However, our work suggests a new approach: to build and grow the biology up with the electronics synergistically and in a 3-D interwoven format.”
The finished ear consists of a coiled antenna inside a cartilage structure. Two wires lead from the base of the ear and wind around a helical “cochlea” – the part of the ear that senses sound – which can connect to electrodes. Although McAlpine cautions that further work and extensive testing would need to be done before the technology could be used on a patient, he said the ear in principle could be used to restore or enhance human hearing.
Antennas are arguably the largest application or volume laser direct structuring (LDS) 3-D molded interconnect devices (MID) according to SelectConnect Technologies and the company believes it is the lowest-cost approach for rapidly translating prototypes into testable and parts as well as mass-producing the antennas once the designs are finalized. LDS allows rapid iterations of new antenna patterns, which are directly translated from CAD drawings to the antenna using the software and a laser system manufactured by LPKF.
It is particularly well-suited to producing high-performance, electrically-small antennas that have a low Q factor (Figure 7). 3-D LDS antennas can be tuned to a specific frequency by changing the line width and length as well as the total antenna cross-section. The antenna traces conform exactly to the shape of the injection-molded carrier or enclosure.
The process has the advantage of being able to take designs form software onto a carrier, producing a prototype in three days, and producing position patterns at low cost. The company believes that antenna integration is possible into enclosures, clothing, UAVs, or any application requiring an antenna with high performance and low Q.
Researchers and engineers from the Electromagnetic System Group at the University of Denmark’s Department of Electrical Engineering have used 3-D printing to make a prototype of an antenna that would be difficult and very expensive to fabricate any other way (Figure 8). It is an electrically-small spherical wire antenna that exhibits Q very close to a physically fundamental lower bound. The antenna was printed in plastic and subsequently covered with conductive paint.
Google Enters the Game
No one could accuse Google of shying away from massively complex endeavors. Google Maps today covers not only the Earth but the moon, Mars, and the sky while providing huge amounts of information about how long it will take to arrive at a destination by car, mass transit, bicycle, or even walking as well as things to see and do along the way. As of early this year, Google Street View boasts more than 20 petabytes of data comprising photos taken along 5 million miles of roads in 39 countries and 3,000 cities. And of course there’s Google Earth and the company’s driverless cars that have traversed a half million miles without accidents…and so on.
So perhaps it should be no surprise that Google is now taken upon itself the goal of creating an open hardware platform designed to create modular smartphones built to meet the needs of individual owners who can customize, swap out, repair, and upgrade their phones (Figure 9). The program is code-named Project Ara and is being developed by the Advanced Technologies and Projects team within Motorola Mobility. Although Google sold the rest of Motorola Mobility to Lenovo, the sale did not include this team (nor the treasure trove of Motorola’s patents).
As the company puts it, “the smartphone is one of the most empowering and intimate objects in our lives, yet most of us have little say in how the device is made, what it does, and how it looks. And 5 billion of us don’t have one. What if you could make choices about exactly what your phone does, and use it as a creative canvas to tell your own story?” The first Ara Developers’ Conference will be held on April 15 at the Computer History Museum in Mountain View, CA (invitation only) with a live stream and interactive Q&A capability for everyone else. The basic phone is supposed to sell for about $50 (Wi-Fi only), become available in the first quarter of 2015, and be completely customizable to incorporate specific features and be essentially unique to the owner.
3-D printing will play a big role in Project Ara. 3D Systems is developing a high-speed 3-D printer that can produce in high volumes the housings that will enclose the phone’s various modules. It will be able to print 600-dpi color images on module enclosures made out of multiple types of materials that can be selected by the owner. 3D Systems is tasked with integrating in a single platform the 3-D printing tools and various materials that together comprise a smartphone. Key elements in these designs are antennas that would be printed onto the housings (Figure 10).
However, 3D Systems has not focused on the conductive materials required to produce electronic circuits and plans to substantially expand its multi-material printing capabilities to include them. So how will the company do this in keeping with the project’s feverish pace? Motley Fool, always on the lookout for investment opportunities, believes that one way would be to acquire Optomec (assuming it’s for sale), which has formidable capabilities in this area.
It also has a long relationship with 3D Systems’ archrival, Stratasys, which might complicate things. Optomec and Stratysys produced what they called the first 3-D additive and electronic circuit printing structure, effectively making a “smart wing” for a UAV. The plastic wing was first printed using Stratasys’ 3-D technology, fused deposition modeling, and then using an Optomec Aerosol Jet system for printing an antenna, sensor, and other circuitry onto the wing.
Another possibility, according to Motley Fool, is technology developed at Xerox, which has been collaborating with 3D Systems for 15 years resulting in 3D Systems’ popular ProJet line of 3-D printers. In addition, 3D Systems acquired some of Xerox’s solid-ink engineering group along with 100 Xerox engineers. However, the sale did not include Xerographic micro-assembly, which is an interesting technology for printing 3-D electronic circuitry that was developed by the company’s legendary Palo Alto Research Center (PARC).
This technology uses laser printing, which Xerox invented in the 1970s, and involves breaking down silicon wafers into tens of thousands of little “chiplets”, turning them into ink so they can be printed as a circuit onto basically any surface. In addition to producing microprocessors and computer memory, this technology can also be used to produce microelectromechanical systems (MEMS) used for actuating and sensing. The technology is currently in the development stage. One thing is certain: 3-D Systems will rapidly find a way to incorporate these capabilities in order to meet the demands of Project Ara as it is an essential element in the program’s success.
The 3-D printing industry in both its forms, additive and electronic, is advancing so quickly that is not outlandish to consider that someday soon many types of antennas will be producible in both small and large quantities. It’s also not difficult to imagine that additive printing will replace conventional methods to produce the enclosures in which integrated microwave assemblies, amplifiers, and basically every type of passive or active microwave product are contained. While not an enormous marketplace compared to consumer applications, the RF and microwave industry nevertheless has the low-volume requirements to which additive manufacturing is currently well suited. Defense electronics systems in particular, which are generally not produced in very large quantities, ought to be a key target, especially in today’s economic conditions.