Most RF system designers view air simply as a medium for electromagnetic energy propagation from the source to the receiver. This is usually the case, allowing them to focus the bulk of their design effort on interconnections and integrated circuits that define the physical system.
However, that is only a simplistic view, as other properties of air are also important. For instance, air can keep electronics cool with convection, and it has dielectric properties that some RF components find critical.
Heinrich Hertz first demonstrated wireless signals in 1888. He energized a spark gap of 1 millimeter using high voltage, creating a wideband pulse. A dipole antenna transmitted this pulse. The antenna had two collinear metal rods with capacitive metal plates. At standard atmospheric conditions, air has a dielectric strength of about 30-70 volts/mil or 3-7 kV/mm. Discharged through air across the gap, the high voltage spark caused brief standing waves of oscillating current in the antenna, which then radiated this energy as a brief pulse of radio waves.
With the growth and maturing of wireless, RF tuners often had variable capacitors. These consisted of multiple parallel plates with air gaps that decided the capacitance value of the tuning assembly. By rotating a shaft, it was possible to adjust the position of the moving plates with reference to the static ones, thereby changing the capacitance between them from near zero to several hundred picofarads.
Vacuum has the ideal unit dielectric constant, while air is very close, with a value of 1.00058986. In comparison, the dielectric constant of PTFE is 2.0, and for FR4 it is about 4.4.
Another important property of vacuum, is its dielectric loss, dissipation factor, or loss tangent is zero, and so it is for air as well. Moreover, air characteristics are stable well into the terahertz frequency range, but it is not so for other dielectrics.
However, both vacuum and air have a common weakness. Neither has any structural strength. Therefore, they require a supporting form to hold them. Engineers find this a challenge as there must be an adequate amount of air within the structural medium of the dielectric.
The solution to this problem lies in using AM or additive manufacturing, also known as 3D printing, along with foam, and a family of photopolymer materials. Roger’s Corp typically supplies specialty RF materials, such as the Radix family of 3D printable, high-resolution materials. Radix is a photo-curable, highly viscous resin. It is a high filler concentration that offers good mechanical and electrical properties even at high frequencies.
3DFortify, of Boston, makes a particular type of Flux Core 3D printer. This is the only printer in the market that can effectively print using the Radix resin. The two companies are now partnered to produce 3D-printed RF components.
The printer layers the material with a thickness of less than 100 µm and cures it with a UV digital light processing projector in one flash for every layer. They provide both metalized and non-metalized versions. With the 3D-printing approach, the manufacturer can vary the structural strength of the material as necessary. They can give thick and strong structures at places subject to physical pressure or connections.
