Using 3D printing to make air better RF component
Article By : Bill Schweber
Effectively using air in RF circuits may seem like "much ado about nothing" but it's actually a complicated affair amid multiple issues.
For many designers of RF systems, air is viewed simply as a medium for propagation of electromagnetic energy between source and receiver. This usually makes sense, resulting in the bulk of their design effort being focused on the ICs and interconnections which define the physical system along with the software which brings it to life.
But that’s a simplistic view. Air is needed for conventional cooling, of course, and its dielectric properties are often critical to some RF components and their functions.
This is not a new development. Heinrich Hertz’s transmitter in 1888 started “wireless” using a high voltage to energize a millimeter-wide spark gap. This created a wideband pulse which was transmitted by a dipole antenna comprised of two collinear metal rods with capacitive metal plates (Figure 1). The dielectric strength of air at standard atmospheric conditions is about 30 to 70 volts/mil (thousandths of an inch) or 3 to 7 kV/mm.
Figure 1 In Hertz’s 1888 wireless arrangement, the two sides of the antenna were connected to an induction coil powered by a battery (not shown), and the high-voltage spark discharged across the spark gap, causing brief oscillating standing waves of current in the antenna. This energy would be radiated by the antenna as a brief pulse of radio waves. Source: Wikipedia
As wireless grew and matured, RF tuners often used variable capacitors with multiple parallel plates and air gaps to set the capacitance value of a tuning assembly or “tank circuit” (Figure 2). Rotating the shaft adjusted capacitance from near zero to several hundred picofarads in a typical design. You could also get “ganged” versions with two or more distinct units on a single shaft; these would then track changes with each other, a very convenient arrangement in some circuit topologies.
Figure 2 The parallel-plate variable capacitor was the key tuning component for many years, especially in basic consumer AM-band radios; although it’s not suitable for GHz and above designs, it is still viable and used at lower frequencies. Source: Utmel Electronic
But air has many important RF attributes other than just as a simple insulation or basic dielectric. While vacuum is the ideal dielectric with a dielectric constant (Dk) of unity (by definition), air comes pretty close with a value of 1.00058986; by comparison, PTFE comes in at around 2.0, and FR4 at around 4.4. Clearly, air is very close to “perfect” if that’s what you are looking for.
Similarly, the dielectric loss—also called loss tangent or dissipation factor Df—of a vacuum is zero and air’s value is the same, while PTFE’s value is 0.00028 and FR4 is 0.008 (both at 3 GHz). As an added virtue, the characteristics of air are stable into the terahertz range while other dielectrics are not as stable or consistent.
For these and other reasons, air is used for many challenging applications such as high-Q capacitive coupling structures in advanced antenna-system (AAS) panels. It’s also used to enable separate coupled antenna elements and even shaped RF structures such as the Luneburg lens. That’s a spherically symmetric, gradient-index lens having an RF refractive index which decreases radially from the center to its outer surface. It’s used for a compact, high-performance radar reflector and even for an electronically steerable radar antenna where the feed is moved, not the structure.
So, what’s the problem?
It’s obviously hard to maintain a vacuum enclosure in most designs, with issues spanning from pumping out residual air to hermetic seals to other design, production, and maintenance issues. Fortunately, using air yields the same result in most cases but with none of vacuum’s challenges.
So far, so good. But both air and vacuum have the same weakness: neither has any structural strength, so requires some sort of supporting form. The challenge is to build this with as a much air as possible within a dielectric structural medium.
One common solution is to use dielectric foams made from polymers or other dielectric materials. These can be fabricated with considerable effort and skill to have large pores (voids). The problem is that such foam-based structures are hard to shape to the desired configuration. Furthermore, they are weak in the direction of compressive forces, are easily crushed during fabrication and maintenance, and don’t readily support RF plated-through holes.
Consequently, they often need a harder, protective skin layer on the outside to add mechanical rigidity and strength. However, that detracts from RF performance, and applying the lamination can crunch the foam. And, when you are in the high-frequency RF world, even miniscule dimensional changes can have large detrimental effects.
AM plus unique materials to the rescue?
There’s a possible alternative now being used in some projects, which combines adapting additive manufacturing (AM)—often called 3D printing—to work with foam, plus an innovative family of photopolymer materials.
For example, Boston-based 3DFortify has partnered with Rogers Corp., the well-known supplier of specialty RF materials, for its Radix family of high-resolution 3D printable materials. Radix is a highly viscous photo-curable resin with high filler concentration offering good electrical and mechanical properties for high-frequency applications, while Fortify’s FLUX CORE printer is currently the only printer that can effectively print that material.
The material is layered with a thickness of <100 µm and cured with a UV digital light processing (DLP) projector in one flash per layer; both metalized and non-metalized versions are available. Due to the nature of the fillers, they may settle over time, so Fortify adds the Continuous Kinetic Mixing (CKM) technology—a module built onto Fortify’s 3D printers—to maintain the material’s homogeneous and isotropic properties.
With this 3D-printed approach, the material’s structural strength can be varied as needed, providing thicker, stronger structures and surfaces near the outside where there will be physical pressure or where connections are needed (Figure 3).
Figure 3 This 3D printed foam is mostly air, and its structure is thicker and stronger near the top, where it will be subjected to more compression and abuse. Source: 3DFortify
Lensing is another situation for which this approach is well suited. In lensing, a device is designed so that its shape and/or mixed dielectrics change the focus of an antenna beam in some way, such that the wave will be slowed down more in one area than another. Thus, the beam will focus more toward the area that slows it more. The objective in many cases is to fabricate extremely complex gradient-refractive index (GRIN)-style dielectric antennas and lenses, which are a type of dielectric meta-structures with a continuous, spatially graded index of refraction (Figure 4).
Figure 4 AM using the Fortify system with add-on mixer and Rogers’ specialized material can create a dielectric meta-structure with a continuous spatially graded index of refraction. Source: 3DFortify
Using this system, structures which are difficult or impossible to create using conventional methods can be built. So, it’s another case where AM enables new ways of thinking about what can and can’t be fabricated. Alternatively, it reduces the need to do “approximate builds” and instead allows builds which are closer to the ideal with fewer compromises. This effect is not limited to RF, of course, as we have seen it in structures such as specialty valves and nozzles which cannot otherwise be machined, forged, or cast—but can be made using AM.
Have you used AM for any RF parts? If so, was it done primarily for integration of multiple structures into a single component, or was it to achieve a special “something” which otherwise could not be fabricated?
This article was originally published on Planet Analog.
Bill Schweber is an electronics engineer who has written three textbooks on electronic communications systems, as well as hundreds of technical articles, opinion columns, and product features. In past roles, he worked as a technical website manager for multiple EE Times sites and as both Executive Editor and Analog Editor at EDN. At Analog Devices, he was in marketing communications; as a result, he has been on both sides of the technical PR function, presenting company products, stories, and messages to the media and also as the recipient of these. Prior to the marcom role at Analog, Bill was Associate Editor of its respected technical journal, and also worked in its product marketing and applications engineering groups. Before those roles, he was at Instron Corp., doing hands-on analog- and power-circuit design and systems integration for materials-testing machine controls. He has a BSEE from Columbia University and an MSEE from the University of Massachusetts, is a Registered Professional Engineer, and holds an Advanced Class amateur radio license. He has also planned, written, and presented online courses on a variety of engineering topics, including MOSFET basics, ADC selection, and driving LEDs.