Written by Senior RF Applications Engineer Colby Hobart
A dielectric resonator antenna (DRA) is a microwave antenna, typically constructed of ceramic for its high Q potential, that contains a radiated wave with its sharp transition from a higher dielectric constant to air. This technology is most impactful at higher frequencies, where the designer has the advantage of not needing a conductor (and its associated loss) in the transmission path. High relative permittivity is desired for miniaturization. This is critical in arrays as there needs to be space for the antennas to fit at the array spacing. Additionally, higher dielectrics define the border between the DRA and the environment better. It is the delta in relative permittivity between the DRA and its surroundings (usually air or 1.0) that creates the boundary of the object to act as a type of waveguide.
This antenna type works by feeding the transmit signal into the body of the dielectric resonator, which is dimensioned to have a cavity resonance for the transmitting frequency, typically one wavelength, derated by the square root of epsilon R at the center frequency of the transmitting band of interest. The dimension of the resonator creates a standing wave at the resonant frequencies and surrounding bands. The re-entry mode for these resonators is at double the main resonant frequency. To determine the physical size of the DRA, the diameter or side length dimension should start at λ0/√εr, where λ0 is the freespace wavelength at center frequency and εr is the relative permittivity of the resonator material.
Figure 1: Typical microstrip-fed DRA
Due to the lack of metallic losses in the structure, a low loss dielectric can greatly influence the total loss of the device, which can be very low compared to metalized antennas. This benefit becomes higher impact at higher frequencies, where total loss savings are more critical. Typical devices have tunable bandwidths, narrow to broad (by εr). DRAs are often used in phased arrays, as their physical size, with high enough εr , is within the optimal array spacing range. DRAs are most often used in mmWave applications (30GHz+). This puts them in the range of military radar bands, automotive radar, and 5G cellular bands. Additive options are viable use cases in all of these instances.
Typically used in the εr = 20+ range of dielectrics, Ceramic rods of cylindrical or rectangular prism shape are extruded into bars. These bars are then roughly cut to length to match the design based on engineering simulations. This cut length is often left intentionally long so that there can be a subtractive tuning range in the length of the resonator. One dimension of the resonator can be ground down until the resonant frequency matches the design exactly.
Figure 2: Additive DRA with plated blind hole fed by a coaxial feed
The feed network for the dielectric resonator antenna can be by a number of different transmission line coupling techniques. A direct microstrip feed is a common method whereby the DRA ceramic slab is placed on top of an open-ended microstrip line. Another method is to extend the microstrip line up the vertical edge of the ceramic slab, causing a taper to high impedance at the launch of the DRA. Coaxial feeds can be used in a similar fashion, but are more conducive to launching the DRA in the vertical axis. Coaxial feeds sometimes require a hole in the dielectric for the center pin to sit inside. This hole needs to be of tight tolerance to the signal pin, so that there is no air discontinuity surrounding the pin. Stripline feeds can capacitively couple to the DRA, often through a matching slot iris in a ground plane. Or a DRA can be excited from a waveguide, which is closer to free space impedance than the TEM or quasi-TEM methods described.
One advantage of additive fabrication is the time it takes between finishing a design and having hardware ready to assemble. With additive technology such as Fortify’s FLUX ONE printer, a set of resonators can be printed in a few hours, then cleaned, sintered, and cooled in about two days total.
A more substantial advantage, though, is the ability to print complex shapes. Complex-shaped resonators can allow for efficient operation using resonators of lower dielectric constant. They can also be used for changes in polarization, beam focusing, and bandwidth. In addition to printing complex shapes, certain regions of the antenna can use latticing techniques to have different effective dielectric constant than the solid regions. Additionally, 3D printed dielectric materials are in development to push the limits of dielectric constant in additive manufacturing. If these materials can breach the 20-40 Er range, then a larger set of the DRA market will be open to additive solutions
Figure 3: DRA in a phased array configuration Integrated microstrip feed substrate is printed as one piece with the resonators.
There are additive advantages to some of the feed structures that will pair with these dielectric resonators as well. In the case of the vertical edge microstrip launch, selective additive copper, as used on Fortify devices, could form the conformal copper trace that spans a plane and transitions into a vertical wall. Selective copper plating would also be favorable for a coaxial feed. The copper could be plated up the vertical side wall of the resonator or plated directly to a blind hole in the middle of the device. This would ensure that there is no air gap, as discussed above. Another option is to combine additive technologies and print metal waveguide antennas and arrays that couple to an array of dielectric resonators.
Since there isn’t a benefit to creating simpler geometries, additive manufacturing brings a higher chance to the integrated solution, where a field of dielectric resonators are printed together in a connected block of the same substrate. This substrate could then be metalized or otherwise integrated with a feed network for simpler assembly.
Figure 4: 3D printed resonator block with integrated superstrate for wide angle matching. All are printed with the same material as one part with the use of laticing for variable effective permittivity. The selective copper plating is a post process.
Due to the laticing capability of additive manufacturing, whereby a mix of air and dielectric material can be printed in different ratios for different areas of a device, there are opportunities to integrate latticed devices into the additive DRAs or arrays. Either a lensing or a gradient superstrate type solution could be integrated directly with a DRA or array of DRAs. Lensing could manipulate the beam width, affecting directivity and gain. It could also steer the boresight of an array to a new zero direction.
Impedance matching superstrates create an environment for the antennas to radiate efficiently into free space over a wider steering angle. Both classes of devices are enabled by latticed printing, which provides a way to use a varying effective εr by mixing different ratios of air to dielectric in a unit cell that is a small enough wavelength ratio to keep behavior linear.
3D printing is becoming a viable option for Dielectric Resonator Antennas (DRAs). It specifically enables:
Furthermore, with latticed shapes, devices like lenses and wide-angle impedance matching superstrates can be integrated into the same device as the DRA using the same additive material. With the addition of selectively plated copper, even further integration is possible, including a feed network, or matching or grounding structures.
Fortify, in collaboration with Rogers Corporation, has developed multiple low-loss dielectric materials for use in high-bandwidth, high-frequency radar and communications systems. Leveraging unique materials capabilities and design approaches such as lattice-based design, Fortify delivers finished part performance that exceeds any traditional process. Learn more about our unique, industry-leading RF materials.